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diff --git a/Documentation/driver-api/pin-control.rst b/Documentation/driver-api/pin-control.rst new file mode 100644 index 000000000..71eefe5a0 --- /dev/null +++ b/Documentation/driver-api/pin-control.rst @@ -0,0 +1,1467 @@ +=============================== +PINCTRL (PIN CONTROL) subsystem +=============================== + +This document outlines the pin control subsystem in Linux + +This subsystem deals with: + +- Enumerating and naming controllable pins + +- Multiplexing of pins, pads, fingers (etc) see below for details + +- Configuration of pins, pads, fingers (etc), such as software-controlled + biasing and driving mode specific pins, such as pull-up/down, open drain, + load capacitance etc. + +Top-level interface +=================== + +Definition of PIN CONTROLLER: + +- A pin controller is a piece of hardware, usually a set of registers, that + can control PINs. It may be able to multiplex, bias, set load capacitance, + set drive strength, etc. for individual pins or groups of pins. + +Definition of PIN: + +- PINS are equal to pads, fingers, balls or whatever packaging input or + output line you want to control and these are denoted by unsigned integers + in the range 0..maxpin. This numberspace is local to each PIN CONTROLLER, so + there may be several such number spaces in a system. This pin space may + be sparse - i.e. there may be gaps in the space with numbers where no + pin exists. + +When a PIN CONTROLLER is instantiated, it will register a descriptor to the +pin control framework, and this descriptor contains an array of pin descriptors +describing the pins handled by this specific pin controller. + +Here is an example of a PGA (Pin Grid Array) chip seen from underneath:: + + A B C D E F G H + + 8 o o o o o o o o + + 7 o o o o o o o o + + 6 o o o o o o o o + + 5 o o o o o o o o + + 4 o o o o o o o o + + 3 o o o o o o o o + + 2 o o o o o o o o + + 1 o o o o o o o o + +To register a pin controller and name all the pins on this package we can do +this in our driver:: + + #include <linux/pinctrl/pinctrl.h> + + const struct pinctrl_pin_desc foo_pins[] = { + PINCTRL_PIN(0, "A8"), + PINCTRL_PIN(1, "B8"), + PINCTRL_PIN(2, "C8"), + ... + PINCTRL_PIN(61, "F1"), + PINCTRL_PIN(62, "G1"), + PINCTRL_PIN(63, "H1"), + }; + + static struct pinctrl_desc foo_desc = { + .name = "foo", + .pins = foo_pins, + .npins = ARRAY_SIZE(foo_pins), + .owner = THIS_MODULE, + }; + + int __init foo_probe(void) + { + int error; + + struct pinctrl_dev *pctl; + + error = pinctrl_register_and_init(&foo_desc, <PARENT>, + NULL, &pctl); + if (error) + return error; + + return pinctrl_enable(pctl); + } + +To enable the pinctrl subsystem and the subgroups for PINMUX and PINCONF and +selected drivers, you need to select them from your machine's Kconfig entry, +since these are so tightly integrated with the machines they are used on. +See for example arch/arm/mach-ux500/Kconfig for an example. + +Pins usually have fancier names than this. You can find these in the datasheet +for your chip. Notice that the core pinctrl.h file provides a fancy macro +called PINCTRL_PIN() to create the struct entries. As you can see I enumerated +the pins from 0 in the upper left corner to 63 in the lower right corner. +This enumeration was arbitrarily chosen, in practice you need to think +through your numbering system so that it matches the layout of registers +and such things in your driver, or the code may become complicated. You must +also consider matching of offsets to the GPIO ranges that may be handled by +the pin controller. + +For a padring with 467 pads, as opposed to actual pins, I used an enumeration +like this, walking around the edge of the chip, which seems to be industry +standard too (all these pads had names, too):: + + + 0 ..... 104 + 466 105 + . . + . . + 358 224 + 357 .... 225 + + +Pin groups +========== + +Many controllers need to deal with groups of pins, so the pin controller +subsystem has a mechanism for enumerating groups of pins and retrieving the +actual enumerated pins that are part of a certain group. + +For example, say that we have a group of pins dealing with an SPI interface +on { 0, 8, 16, 24 }, and a group of pins dealing with an I2C interface on pins +on { 24, 25 }. + +These two groups are presented to the pin control subsystem by implementing +some generic pinctrl_ops like this:: + + #include <linux/pinctrl/pinctrl.h> + + struct foo_group { + const char *name; + const unsigned int *pins; + const unsigned num_pins; + }; + + static const unsigned int spi0_pins[] = { 0, 8, 16, 24 }; + static const unsigned int i2c0_pins[] = { 24, 25 }; + + static const struct foo_group foo_groups[] = { + { + .name = "spi0_grp", + .pins = spi0_pins, + .num_pins = ARRAY_SIZE(spi0_pins), + }, + { + .name = "i2c0_grp", + .pins = i2c0_pins, + .num_pins = ARRAY_SIZE(i2c0_pins), + }, + }; + + + static int foo_get_groups_count(struct pinctrl_dev *pctldev) + { + return ARRAY_SIZE(foo_groups); + } + + static const char *foo_get_group_name(struct pinctrl_dev *pctldev, + unsigned selector) + { + return foo_groups[selector].name; + } + + static int foo_get_group_pins(struct pinctrl_dev *pctldev, unsigned selector, + const unsigned **pins, + unsigned *num_pins) + { + *pins = (unsigned *) foo_groups[selector].pins; + *num_pins = foo_groups[selector].num_pins; + return 0; + } + + static struct pinctrl_ops foo_pctrl_ops = { + .get_groups_count = foo_get_groups_count, + .get_group_name = foo_get_group_name, + .get_group_pins = foo_get_group_pins, + }; + + + static struct pinctrl_desc foo_desc = { + ... + .pctlops = &foo_pctrl_ops, + }; + +The pin control subsystem will call the .get_groups_count() function to +determine the total number of legal selectors, then it will call the other functions +to retrieve the name and pins of the group. Maintaining the data structure of +the groups is up to the driver, this is just a simple example - in practice you +may need more entries in your group structure, for example specific register +ranges associated with each group and so on. + + +Pin configuration +================= + +Pins can sometimes be software-configured in various ways, mostly related +to their electronic properties when used as inputs or outputs. For example you +may be able to make an output pin high impedance, or "tristate" meaning it is +effectively disconnected. You may be able to connect an input pin to VDD or GND +using a certain resistor value - pull up and pull down - so that the pin has a +stable value when nothing is driving the rail it is connected to, or when it's +unconnected. + +Pin configuration can be programmed by adding configuration entries into the +mapping table; see section "Board/machine configuration" below. + +The format and meaning of the configuration parameter, PLATFORM_X_PULL_UP +above, is entirely defined by the pin controller driver. + +The pin configuration driver implements callbacks for changing pin +configuration in the pin controller ops like this:: + + #include <linux/pinctrl/pinctrl.h> + #include <linux/pinctrl/pinconf.h> + #include "platform_x_pindefs.h" + + static int foo_pin_config_get(struct pinctrl_dev *pctldev, + unsigned offset, + unsigned long *config) + { + struct my_conftype conf; + + ... Find setting for pin @ offset ... + + *config = (unsigned long) conf; + } + + static int foo_pin_config_set(struct pinctrl_dev *pctldev, + unsigned offset, + unsigned long config) + { + struct my_conftype *conf = (struct my_conftype *) config; + + switch (conf) { + case PLATFORM_X_PULL_UP: + ... + } + } + } + + static int foo_pin_config_group_get (struct pinctrl_dev *pctldev, + unsigned selector, + unsigned long *config) + { + ... + } + + static int foo_pin_config_group_set (struct pinctrl_dev *pctldev, + unsigned selector, + unsigned long config) + { + ... + } + + static struct pinconf_ops foo_pconf_ops = { + .pin_config_get = foo_pin_config_get, + .pin_config_set = foo_pin_config_set, + .pin_config_group_get = foo_pin_config_group_get, + .pin_config_group_set = foo_pin_config_group_set, + }; + + /* Pin config operations are handled by some pin controller */ + static struct pinctrl_desc foo_desc = { + ... + .confops = &foo_pconf_ops, + }; + +Interaction with the GPIO subsystem +=================================== + +The GPIO drivers may want to perform operations of various types on the same +physical pins that are also registered as pin controller pins. + +First and foremost, the two subsystems can be used as completely orthogonal, +see the section named "pin control requests from drivers" and +"drivers needing both pin control and GPIOs" below for details. But in some +situations a cross-subsystem mapping between pins and GPIOs is needed. + +Since the pin controller subsystem has its pinspace local to the pin controller +we need a mapping so that the pin control subsystem can figure out which pin +controller handles control of a certain GPIO pin. Since a single pin controller +may be muxing several GPIO ranges (typically SoCs that have one set of pins, +but internally several GPIO silicon blocks, each modelled as a struct +gpio_chip) any number of GPIO ranges can be added to a pin controller instance +like this:: + + struct gpio_chip chip_a; + struct gpio_chip chip_b; + + static struct pinctrl_gpio_range gpio_range_a = { + .name = "chip a", + .id = 0, + .base = 32, + .pin_base = 32, + .npins = 16, + .gc = &chip_a; + }; + + static struct pinctrl_gpio_range gpio_range_b = { + .name = "chip b", + .id = 0, + .base = 48, + .pin_base = 64, + .npins = 8, + .gc = &chip_b; + }; + + { + struct pinctrl_dev *pctl; + ... + pinctrl_add_gpio_range(pctl, &gpio_range_a); + pinctrl_add_gpio_range(pctl, &gpio_range_b); + } + +So this complex system has one pin controller handling two different +GPIO chips. "chip a" has 16 pins and "chip b" has 8 pins. The "chip a" and +"chip b" have different .pin_base, which means a start pin number of the +GPIO range. + +The GPIO range of "chip a" starts from the GPIO base of 32 and actual +pin range also starts from 32. However "chip b" has different starting +offset for the GPIO range and pin range. The GPIO range of "chip b" starts +from GPIO number 48, while the pin range of "chip b" starts from 64. + +We can convert a gpio number to actual pin number using this "pin_base". +They are mapped in the global GPIO pin space at: + +chip a: + - GPIO range : [32 .. 47] + - pin range : [32 .. 47] +chip b: + - GPIO range : [48 .. 55] + - pin range : [64 .. 71] + +The above examples assume the mapping between the GPIOs and pins is +linear. If the mapping is sparse or haphazard, an array of arbitrary pin +numbers can be encoded in the range like this:: + + static const unsigned range_pins[] = { 14, 1, 22, 17, 10, 8, 6, 2 }; + + static struct pinctrl_gpio_range gpio_range = { + .name = "chip", + .id = 0, + .base = 32, + .pins = &range_pins, + .npins = ARRAY_SIZE(range_pins), + .gc = &chip; + }; + +In this case the pin_base property will be ignored. If the name of a pin +group is known, the pins and npins elements of the above structure can be +initialised using the function pinctrl_get_group_pins(), e.g. for pin +group "foo":: + + pinctrl_get_group_pins(pctl, "foo", &gpio_range.pins, + &gpio_range.npins); + +When GPIO-specific functions in the pin control subsystem are called, these +ranges will be used to look up the appropriate pin controller by inspecting +and matching the pin to the pin ranges across all controllers. When a +pin controller handling the matching range is found, GPIO-specific functions +will be called on that specific pin controller. + +For all functionalities dealing with pin biasing, pin muxing etc, the pin +controller subsystem will look up the corresponding pin number from the passed +in gpio number, and use the range's internals to retrieve a pin number. After +that, the subsystem passes it on to the pin control driver, so the driver +will get a pin number into its handled number range. Further it is also passed +the range ID value, so that the pin controller knows which range it should +deal with. + +Calling pinctrl_add_gpio_range from pinctrl driver is DEPRECATED. Please see +section 2.1 of Documentation/devicetree/bindings/gpio/gpio.txt on how to bind +pinctrl and gpio drivers. + + +PINMUX interfaces +================= + +These calls use the pinmux_* naming prefix. No other calls should use that +prefix. + + +What is pinmuxing? +================== + +PINMUX, also known as padmux, ballmux, alternate functions or mission modes +is a way for chip vendors producing some kind of electrical packages to use +a certain physical pin (ball, pad, finger, etc) for multiple mutually exclusive +functions, depending on the application. By "application" in this context +we usually mean a way of soldering or wiring the package into an electronic +system, even though the framework makes it possible to also change the function +at runtime. + +Here is an example of a PGA (Pin Grid Array) chip seen from underneath:: + + A B C D E F G H + +---+ + 8 | o | o o o o o o o + | | + 7 | o | o o o o o o o + | | + 6 | o | o o o o o o o + +---+---+ + 5 | o | o | o o o o o o + +---+---+ +---+ + 4 o o o o o o | o | o + | | + 3 o o o o o o | o | o + | | + 2 o o o o o o | o | o + +-------+-------+-------+---+---+ + 1 | o o | o o | o o | o | o | + +-------+-------+-------+---+---+ + +This is not tetris. The game to think of is chess. Not all PGA/BGA packages +are chessboard-like, big ones have "holes" in some arrangement according to +different design patterns, but we're using this as a simple example. Of the +pins you see some will be taken by things like a few VCC and GND to feed power +to the chip, and quite a few will be taken by large ports like an external +memory interface. The remaining pins will often be subject to pin multiplexing. + +The example 8x8 PGA package above will have pin numbers 0 through 63 assigned +to its physical pins. It will name the pins { A1, A2, A3 ... H6, H7, H8 } using +pinctrl_register_pins() and a suitable data set as shown earlier. + +In this 8x8 BGA package the pins { A8, A7, A6, A5 } can be used as an SPI port +(these are four pins: CLK, RXD, TXD, FRM). In that case, pin B5 can be used as +some general-purpose GPIO pin. However, in another setting, pins { A5, B5 } can +be used as an I2C port (these are just two pins: SCL, SDA). Needless to say, +we cannot use the SPI port and I2C port at the same time. However in the inside +of the package the silicon performing the SPI logic can alternatively be routed +out on pins { G4, G3, G2, G1 }. + +On the bottom row at { A1, B1, C1, D1, E1, F1, G1, H1 } we have something +special - it's an external MMC bus that can be 2, 4 or 8 bits wide, and it will +consume 2, 4 or 8 pins respectively, so either { A1, B1 } are taken or +{ A1, B1, C1, D1 } or all of them. If we use all 8 bits, we cannot use the SPI +port on pins { G4, G3, G2, G1 } of course. + +This way the silicon blocks present inside the chip can be multiplexed "muxed" +out on different pin ranges. Often contemporary SoC (systems on chip) will +contain several I2C, SPI, SDIO/MMC, etc silicon blocks that can be routed to +different pins by pinmux settings. + +Since general-purpose I/O pins (GPIO) are typically always in shortage, it is +common to be able to use almost any pin as a GPIO pin if it is not currently +in use by some other I/O port. + + +Pinmux conventions +================== + +The purpose of the pinmux functionality in the pin controller subsystem is to +abstract and provide pinmux settings to the devices you choose to instantiate +in your machine configuration. It is inspired by the clk, GPIO and regulator +subsystems, so devices will request their mux setting, but it's also possible +to request a single pin for e.g. GPIO. + +Definitions: + +- FUNCTIONS can be switched in and out by a driver residing with the pin + control subsystem in the drivers/pinctrl/* directory of the kernel. The + pin control driver knows the possible functions. In the example above you can + identify three pinmux functions, one for spi, one for i2c and one for mmc. + +- FUNCTIONS are assumed to be enumerable from zero in a one-dimensional array. + In this case the array could be something like: { spi0, i2c0, mmc0 } + for the three available functions. + +- FUNCTIONS have PIN GROUPS as defined on the generic level - so a certain + function is *always* associated with a certain set of pin groups, could + be just a single one, but could also be many. In the example above the + function i2c is associated with the pins { A5, B5 }, enumerated as + { 24, 25 } in the controller pin space. + + The Function spi is associated with pin groups { A8, A7, A6, A5 } + and { G4, G3, G2, G1 }, which are enumerated as { 0, 8, 16, 24 } and + { 38, 46, 54, 62 } respectively. + + Group names must be unique per pin controller, no two groups on the same + controller may have the same name. + +- The combination of a FUNCTION and a PIN GROUP determine a certain function + for a certain set of pins. The knowledge of the functions and pin groups + and their machine-specific particulars are kept inside the pinmux driver, + from the outside only the enumerators are known, and the driver core can + request: + + - The name of a function with a certain selector (>= 0) + - A list of groups associated with a certain function + - That a certain group in that list to be activated for a certain function + + As already described above, pin groups are in turn self-descriptive, so + the core will retrieve the actual pin range in a certain group from the + driver. + +- FUNCTIONS and GROUPS on a certain PIN CONTROLLER are MAPPED to a certain + device by the board file, device tree or similar machine setup configuration + mechanism, similar to how regulators are connected to devices, usually by + name. Defining a pin controller, function and group thus uniquely identify + the set of pins to be used by a certain device. (If only one possible group + of pins is available for the function, no group name need to be supplied - + the core will simply select the first and only group available.) + + In the example case we can define that this particular machine shall + use device spi0 with pinmux function fspi0 group gspi0 and i2c0 on function + fi2c0 group gi2c0, on the primary pin controller, we get mappings + like these:: + + { + {"map-spi0", spi0, pinctrl0, fspi0, gspi0}, + {"map-i2c0", i2c0, pinctrl0, fi2c0, gi2c0} + } + + Every map must be assigned a state name, pin controller, device and + function. The group is not compulsory - if it is omitted the first group + presented by the driver as applicable for the function will be selected, + which is useful for simple cases. + + It is possible to map several groups to the same combination of device, + pin controller and function. This is for cases where a certain function on + a certain pin controller may use different sets of pins in different + configurations. + +- PINS for a certain FUNCTION using a certain PIN GROUP on a certain + PIN CONTROLLER are provided on a first-come first-serve basis, so if some + other device mux setting or GPIO pin request has already taken your physical + pin, you will be denied the use of it. To get (activate) a new setting, the + old one has to be put (deactivated) first. + +Sometimes the documentation and hardware registers will be oriented around +pads (or "fingers") rather than pins - these are the soldering surfaces on the +silicon inside the package, and may or may not match the actual number of +pins/balls underneath the capsule. Pick some enumeration that makes sense to +you. Define enumerators only for the pins you can control if that makes sense. + +Assumptions: + +We assume that the number of possible function maps to pin groups is limited by +the hardware. I.e. we assume that there is no system where any function can be +mapped to any pin, like in a phone exchange. So the available pin groups for +a certain function will be limited to a few choices (say up to eight or so), +not hundreds or any amount of choices. This is the characteristic we have found +by inspecting available pinmux hardware, and a necessary assumption since we +expect pinmux drivers to present *all* possible function vs pin group mappings +to the subsystem. + + +Pinmux drivers +============== + +The pinmux core takes care of preventing conflicts on pins and calling +the pin controller driver to execute different settings. + +It is the responsibility of the pinmux driver to impose further restrictions +(say for example infer electronic limitations due to load, etc.) to determine +whether or not the requested function can actually be allowed, and in case it +is possible to perform the requested mux setting, poke the hardware so that +this happens. + +Pinmux drivers are required to supply a few callback functions, some are +optional. Usually the set_mux() function is implemented, writing values into +some certain registers to activate a certain mux setting for a certain pin. + +A simple driver for the above example will work by setting bits 0, 1, 2, 3 or 4 +into some register named MUX to select a certain function with a certain +group of pins would work something like this:: + + #include <linux/pinctrl/pinctrl.h> + #include <linux/pinctrl/pinmux.h> + + struct foo_group { + const char *name; + const unsigned int *pins; + const unsigned num_pins; + }; + + static const unsigned spi0_0_pins[] = { 0, 8, 16, 24 }; + static const unsigned spi0_1_pins[] = { 38, 46, 54, 62 }; + static const unsigned i2c0_pins[] = { 24, 25 }; + static const unsigned mmc0_1_pins[] = { 56, 57 }; + static const unsigned mmc0_2_pins[] = { 58, 59 }; + static const unsigned mmc0_3_pins[] = { 60, 61, 62, 63 }; + + static const struct foo_group foo_groups[] = { + { + .name = "spi0_0_grp", + .pins = spi0_0_pins, + .num_pins = ARRAY_SIZE(spi0_0_pins), + }, + { + .name = "spi0_1_grp", + .pins = spi0_1_pins, + .num_pins = ARRAY_SIZE(spi0_1_pins), + }, + { + .name = "i2c0_grp", + .pins = i2c0_pins, + .num_pins = ARRAY_SIZE(i2c0_pins), + }, + { + .name = "mmc0_1_grp", + .pins = mmc0_1_pins, + .num_pins = ARRAY_SIZE(mmc0_1_pins), + }, + { + .name = "mmc0_2_grp", + .pins = mmc0_2_pins, + .num_pins = ARRAY_SIZE(mmc0_2_pins), + }, + { + .name = "mmc0_3_grp", + .pins = mmc0_3_pins, + .num_pins = ARRAY_SIZE(mmc0_3_pins), + }, + }; + + + static int foo_get_groups_count(struct pinctrl_dev *pctldev) + { + return ARRAY_SIZE(foo_groups); + } + + static const char *foo_get_group_name(struct pinctrl_dev *pctldev, + unsigned selector) + { + return foo_groups[selector].name; + } + + static int foo_get_group_pins(struct pinctrl_dev *pctldev, unsigned selector, + const unsigned ** pins, + unsigned * num_pins) + { + *pins = (unsigned *) foo_groups[selector].pins; + *num_pins = foo_groups[selector].num_pins; + return 0; + } + + static struct pinctrl_ops foo_pctrl_ops = { + .get_groups_count = foo_get_groups_count, + .get_group_name = foo_get_group_name, + .get_group_pins = foo_get_group_pins, + }; + + struct foo_pmx_func { + const char *name; + const char * const *groups; + const unsigned num_groups; + }; + + static const char * const spi0_groups[] = { "spi0_0_grp", "spi0_1_grp" }; + static const char * const i2c0_groups[] = { "i2c0_grp" }; + static const char * const mmc0_groups[] = { "mmc0_1_grp", "mmc0_2_grp", + "mmc0_3_grp" }; + + static const struct foo_pmx_func foo_functions[] = { + { + .name = "spi0", + .groups = spi0_groups, + .num_groups = ARRAY_SIZE(spi0_groups), + }, + { + .name = "i2c0", + .groups = i2c0_groups, + .num_groups = ARRAY_SIZE(i2c0_groups), + }, + { + .name = "mmc0", + .groups = mmc0_groups, + .num_groups = ARRAY_SIZE(mmc0_groups), + }, + }; + + static int foo_get_functions_count(struct pinctrl_dev *pctldev) + { + return ARRAY_SIZE(foo_functions); + } + + static const char *foo_get_fname(struct pinctrl_dev *pctldev, unsigned selector) + { + return foo_functions[selector].name; + } + + static int foo_get_groups(struct pinctrl_dev *pctldev, unsigned selector, + const char * const **groups, + unsigned * const num_groups) + { + *groups = foo_functions[selector].groups; + *num_groups = foo_functions[selector].num_groups; + return 0; + } + + static int foo_set_mux(struct pinctrl_dev *pctldev, unsigned selector, + unsigned group) + { + u8 regbit = (1 << selector + group); + + writeb((readb(MUX)|regbit), MUX); + return 0; + } + + static struct pinmux_ops foo_pmxops = { + .get_functions_count = foo_get_functions_count, + .get_function_name = foo_get_fname, + .get_function_groups = foo_get_groups, + .set_mux = foo_set_mux, + .strict = true, + }; + + /* Pinmux operations are handled by some pin controller */ + static struct pinctrl_desc foo_desc = { + ... + .pctlops = &foo_pctrl_ops, + .pmxops = &foo_pmxops, + }; + +In the example activating muxing 0 and 1 at the same time setting bits +0 and 1, uses one pin in common so they would collide. + +The beauty of the pinmux subsystem is that since it keeps track of all +pins and who is using them, it will already have denied an impossible +request like that, so the driver does not need to worry about such +things - when it gets a selector passed in, the pinmux subsystem makes +sure no other device or GPIO assignment is already using the selected +pins. Thus bits 0 and 1 in the control register will never be set at the +same time. + +All the above functions are mandatory to implement for a pinmux driver. + + +Pin control interaction with the GPIO subsystem +=============================================== + +Note that the following implies that the use case is to use a certain pin +from the Linux kernel using the API in <linux/gpio.h> with gpio_request() +and similar functions. There are cases where you may be using something +that your datasheet calls "GPIO mode", but actually is just an electrical +configuration for a certain device. See the section below named +"GPIO mode pitfalls" for more details on this scenario. + +The public pinmux API contains two functions named pinctrl_gpio_request() +and pinctrl_gpio_free(). These two functions shall *ONLY* be called from +gpiolib-based drivers as part of their gpio_request() and +gpio_free() semantics. Likewise the pinctrl_gpio_direction_[input|output] +shall only be called from within respective gpio_direction_[input|output] +gpiolib implementation. + +NOTE that platforms and individual drivers shall *NOT* request GPIO pins to be +controlled e.g. muxed in. Instead, implement a proper gpiolib driver and have +that driver request proper muxing and other control for its pins. + +The function list could become long, especially if you can convert every +individual pin into a GPIO pin independent of any other pins, and then try +the approach to define every pin as a function. + +In this case, the function array would become 64 entries for each GPIO +setting and then the device functions. + +For this reason there are two functions a pin control driver can implement +to enable only GPIO on an individual pin: .gpio_request_enable() and +.gpio_disable_free(). + +This function will pass in the affected GPIO range identified by the pin +controller core, so you know which GPIO pins are being affected by the request +operation. + +If your driver needs to have an indication from the framework of whether the +GPIO pin shall be used for input or output you can implement the +.gpio_set_direction() function. As described this shall be called from the +gpiolib driver and the affected GPIO range, pin offset and desired direction +will be passed along to this function. + +Alternatively to using these special functions, it is fully allowed to use +named functions for each GPIO pin, the pinctrl_gpio_request() will attempt to +obtain the function "gpioN" where "N" is the global GPIO pin number if no +special GPIO-handler is registered. + + +GPIO mode pitfalls +================== + +Due to the naming conventions used by hardware engineers, where "GPIO" +is taken to mean different things than what the kernel does, the developer +may be confused by a datasheet talking about a pin being possible to set +into "GPIO mode". It appears that what hardware engineers mean with +"GPIO mode" is not necessarily the use case that is implied in the kernel +interface <linux/gpio.h>: a pin that you grab from kernel code and then +either listen for input or drive high/low to assert/deassert some +external line. + +Rather hardware engineers think that "GPIO mode" means that you can +software-control a few electrical properties of the pin that you would +not be able to control if the pin was in some other mode, such as muxed in +for a device. + +The GPIO portions of a pin and its relation to a certain pin controller +configuration and muxing logic can be constructed in several ways. Here +are two examples:: + + (A) + pin config + logic regs + | +- SPI + Physical pins --- pad --- pinmux -+- I2C + | +- mmc + | +- GPIO + pin + multiplex + logic regs + +Here some electrical properties of the pin can be configured no matter +whether the pin is used for GPIO or not. If you multiplex a GPIO onto a +pin, you can also drive it high/low from "GPIO" registers. +Alternatively, the pin can be controlled by a certain peripheral, while +still applying desired pin config properties. GPIO functionality is thus +orthogonal to any other device using the pin. + +In this arrangement the registers for the GPIO portions of the pin controller, +or the registers for the GPIO hardware module are likely to reside in a +separate memory range only intended for GPIO driving, and the register +range dealing with pin config and pin multiplexing get placed into a +different memory range and a separate section of the data sheet. + +A flag "strict" in struct pinmux_ops is available to check and deny +simultaneous access to the same pin from GPIO and pin multiplexing +consumers on hardware of this type. The pinctrl driver should set this flag +accordingly. + +:: + + (B) + + pin config + logic regs + | +- SPI + Physical pins --- pad --- pinmux -+- I2C + | | +- mmc + | | + GPIO pin + multiplex + logic regs + +In this arrangement, the GPIO functionality can always be enabled, such that +e.g. a GPIO input can be used to "spy" on the SPI/I2C/MMC signal while it is +pulsed out. It is likely possible to disrupt the traffic on the pin by doing +wrong things on the GPIO block, as it is never really disconnected. It is +possible that the GPIO, pin config and pin multiplex registers are placed into +the same memory range and the same section of the data sheet, although that +need not be the case. + +In some pin controllers, although the physical pins are designed in the same +way as (B), the GPIO function still can't be enabled at the same time as the +peripheral functions. So again the "strict" flag should be set, denying +simultaneous activation by GPIO and other muxed in devices. + +From a kernel point of view, however, these are different aspects of the +hardware and shall be put into different subsystems: + +- Registers (or fields within registers) that control electrical + properties of the pin such as biasing and drive strength should be + exposed through the pinctrl subsystem, as "pin configuration" settings. + +- Registers (or fields within registers) that control muxing of signals + from various other HW blocks (e.g. I2C, MMC, or GPIO) onto pins should + be exposed through the pinctrl subsystem, as mux functions. + +- Registers (or fields within registers) that control GPIO functionality + such as setting a GPIO's output value, reading a GPIO's input value, or + setting GPIO pin direction should be exposed through the GPIO subsystem, + and if they also support interrupt capabilities, through the irqchip + abstraction. + +Depending on the exact HW register design, some functions exposed by the +GPIO subsystem may call into the pinctrl subsystem in order to +co-ordinate register settings across HW modules. In particular, this may +be needed for HW with separate GPIO and pin controller HW modules, where +e.g. GPIO direction is determined by a register in the pin controller HW +module rather than the GPIO HW module. + +Electrical properties of the pin such as biasing and drive strength +may be placed at some pin-specific register in all cases or as part +of the GPIO register in case (B) especially. This doesn't mean that such +properties necessarily pertain to what the Linux kernel calls "GPIO". + +Example: a pin is usually muxed in to be used as a UART TX line. But during +system sleep, we need to put this pin into "GPIO mode" and ground it. + +If you make a 1-to-1 map to the GPIO subsystem for this pin, you may start +to think that you need to come up with something really complex, that the +pin shall be used for UART TX and GPIO at the same time, that you will grab +a pin control handle and set it to a certain state to enable UART TX to be +muxed in, then twist it over to GPIO mode and use gpio_direction_output() +to drive it low during sleep, then mux it over to UART TX again when you +wake up and maybe even gpio_request/gpio_free as part of this cycle. This +all gets very complicated. + +The solution is to not think that what the datasheet calls "GPIO mode" +has to be handled by the <linux/gpio.h> interface. Instead view this as +a certain pin config setting. Look in e.g. <linux/pinctrl/pinconf-generic.h> +and you find this in the documentation: + + PIN_CONFIG_OUTPUT: + this will configure the pin in output, use argument + 1 to indicate high level, argument 0 to indicate low level. + +So it is perfectly possible to push a pin into "GPIO mode" and drive the +line low as part of the usual pin control map. So for example your UART +driver may look like this:: + + #include <linux/pinctrl/consumer.h> + + struct pinctrl *pinctrl; + struct pinctrl_state *pins_default; + struct pinctrl_state *pins_sleep; + + pins_default = pinctrl_lookup_state(uap->pinctrl, PINCTRL_STATE_DEFAULT); + pins_sleep = pinctrl_lookup_state(uap->pinctrl, PINCTRL_STATE_SLEEP); + + /* Normal mode */ + retval = pinctrl_select_state(pinctrl, pins_default); + /* Sleep mode */ + retval = pinctrl_select_state(pinctrl, pins_sleep); + +And your machine configuration may look like this: +-------------------------------------------------- + +:: + + static unsigned long uart_default_mode[] = { + PIN_CONF_PACKED(PIN_CONFIG_DRIVE_PUSH_PULL, 0), + }; + + static unsigned long uart_sleep_mode[] = { + PIN_CONF_PACKED(PIN_CONFIG_OUTPUT, 0), + }; + + static struct pinctrl_map pinmap[] __initdata = { + PIN_MAP_MUX_GROUP("uart", PINCTRL_STATE_DEFAULT, "pinctrl-foo", + "u0_group", "u0"), + PIN_MAP_CONFIGS_PIN("uart", PINCTRL_STATE_DEFAULT, "pinctrl-foo", + "UART_TX_PIN", uart_default_mode), + PIN_MAP_MUX_GROUP("uart", PINCTRL_STATE_SLEEP, "pinctrl-foo", + "u0_group", "gpio-mode"), + PIN_MAP_CONFIGS_PIN("uart", PINCTRL_STATE_SLEEP, "pinctrl-foo", + "UART_TX_PIN", uart_sleep_mode), + }; + + foo_init(void) { + pinctrl_register_mappings(pinmap, ARRAY_SIZE(pinmap)); + } + +Here the pins we want to control are in the "u0_group" and there is some +function called "u0" that can be enabled on this group of pins, and then +everything is UART business as usual. But there is also some function +named "gpio-mode" that can be mapped onto the same pins to move them into +GPIO mode. + +This will give the desired effect without any bogus interaction with the +GPIO subsystem. It is just an electrical configuration used by that device +when going to sleep, it might imply that the pin is set into something the +datasheet calls "GPIO mode", but that is not the point: it is still used +by that UART device to control the pins that pertain to that very UART +driver, putting them into modes needed by the UART. GPIO in the Linux +kernel sense are just some 1-bit line, and is a different use case. + +How the registers are poked to attain the push or pull, and output low +configuration and the muxing of the "u0" or "gpio-mode" group onto these +pins is a question for the driver. + +Some datasheets will be more helpful and refer to the "GPIO mode" as +"low power mode" rather than anything to do with GPIO. This often means +the same thing electrically speaking, but in this latter case the +software engineers will usually quickly identify that this is some +specific muxing or configuration rather than anything related to the GPIO +API. + + +Board/machine configuration +=========================== + +Boards and machines define how a certain complete running system is put +together, including how GPIOs and devices are muxed, how regulators are +constrained and how the clock tree looks. Of course pinmux settings are also +part of this. + +A pin controller configuration for a machine looks pretty much like a simple +regulator configuration, so for the example array above we want to enable i2c +and spi on the second function mapping:: + + #include <linux/pinctrl/machine.h> + + static const struct pinctrl_map mapping[] __initconst = { + { + .dev_name = "foo-spi.0", + .name = PINCTRL_STATE_DEFAULT, + .type = PIN_MAP_TYPE_MUX_GROUP, + .ctrl_dev_name = "pinctrl-foo", + .data.mux.function = "spi0", + }, + { + .dev_name = "foo-i2c.0", + .name = PINCTRL_STATE_DEFAULT, + .type = PIN_MAP_TYPE_MUX_GROUP, + .ctrl_dev_name = "pinctrl-foo", + .data.mux.function = "i2c0", + }, + { + .dev_name = "foo-mmc.0", + .name = PINCTRL_STATE_DEFAULT, + .type = PIN_MAP_TYPE_MUX_GROUP, + .ctrl_dev_name = "pinctrl-foo", + .data.mux.function = "mmc0", + }, + }; + +The dev_name here matches to the unique device name that can be used to look +up the device struct (just like with clockdev or regulators). The function name +must match a function provided by the pinmux driver handling this pin range. + +As you can see we may have several pin controllers on the system and thus +we need to specify which one of them contains the functions we wish to map. + +You register this pinmux mapping to the pinmux subsystem by simply:: + + ret = pinctrl_register_mappings(mapping, ARRAY_SIZE(mapping)); + +Since the above construct is pretty common there is a helper macro to make +it even more compact which assumes you want to use pinctrl-foo and position +0 for mapping, for example:: + + static struct pinctrl_map mapping[] __initdata = { + PIN_MAP_MUX_GROUP("foo-i2c.o", PINCTRL_STATE_DEFAULT, + "pinctrl-foo", NULL, "i2c0"), + }; + +The mapping table may also contain pin configuration entries. It's common for +each pin/group to have a number of configuration entries that affect it, so +the table entries for configuration reference an array of config parameters +and values. An example using the convenience macros is shown below:: + + static unsigned long i2c_grp_configs[] = { + FOO_PIN_DRIVEN, + FOO_PIN_PULLUP, + }; + + static unsigned long i2c_pin_configs[] = { + FOO_OPEN_COLLECTOR, + FOO_SLEW_RATE_SLOW, + }; + + static struct pinctrl_map mapping[] __initdata = { + PIN_MAP_MUX_GROUP("foo-i2c.0", PINCTRL_STATE_DEFAULT, + "pinctrl-foo", "i2c0", "i2c0"), + PIN_MAP_CONFIGS_GROUP("foo-i2c.0", PINCTRL_STATE_DEFAULT, + "pinctrl-foo", "i2c0", i2c_grp_configs), + PIN_MAP_CONFIGS_PIN("foo-i2c.0", PINCTRL_STATE_DEFAULT, + "pinctrl-foo", "i2c0scl", i2c_pin_configs), + PIN_MAP_CONFIGS_PIN("foo-i2c.0", PINCTRL_STATE_DEFAULT, + "pinctrl-foo", "i2c0sda", i2c_pin_configs), + }; + +Finally, some devices expect the mapping table to contain certain specific +named states. When running on hardware that doesn't need any pin controller +configuration, the mapping table must still contain those named states, in +order to explicitly indicate that the states were provided and intended to +be empty. Table entry macro PIN_MAP_DUMMY_STATE serves the purpose of defining +a named state without causing any pin controller to be programmed:: + + static struct pinctrl_map mapping[] __initdata = { + PIN_MAP_DUMMY_STATE("foo-i2c.0", PINCTRL_STATE_DEFAULT), + }; + + +Complex mappings +================ + +As it is possible to map a function to different groups of pins an optional +.group can be specified like this:: + + ... + { + .dev_name = "foo-spi.0", + .name = "spi0-pos-A", + .type = PIN_MAP_TYPE_MUX_GROUP, + .ctrl_dev_name = "pinctrl-foo", + .function = "spi0", + .group = "spi0_0_grp", + }, + { + .dev_name = "foo-spi.0", + .name = "spi0-pos-B", + .type = PIN_MAP_TYPE_MUX_GROUP, + .ctrl_dev_name = "pinctrl-foo", + .function = "spi0", + .group = "spi0_1_grp", + }, + ... + +This example mapping is used to switch between two positions for spi0 at +runtime, as described further below under the heading "Runtime pinmuxing". + +Further it is possible for one named state to affect the muxing of several +groups of pins, say for example in the mmc0 example above, where you can +additively expand the mmc0 bus from 2 to 4 to 8 pins. If we want to use all +three groups for a total of 2+2+4 = 8 pins (for an 8-bit MMC bus as is the +case), we define a mapping like this:: + + ... + { + .dev_name = "foo-mmc.0", + .name = "2bit" + .type = PIN_MAP_TYPE_MUX_GROUP, + .ctrl_dev_name = "pinctrl-foo", + .function = "mmc0", + .group = "mmc0_1_grp", + }, + { + .dev_name = "foo-mmc.0", + .name = "4bit" + .type = PIN_MAP_TYPE_MUX_GROUP, + .ctrl_dev_name = "pinctrl-foo", + .function = "mmc0", + .group = "mmc0_1_grp", + }, + { + .dev_name = "foo-mmc.0", + .name = "4bit" + .type = PIN_MAP_TYPE_MUX_GROUP, + .ctrl_dev_name = "pinctrl-foo", + .function = "mmc0", + .group = "mmc0_2_grp", + }, + { + .dev_name = "foo-mmc.0", + .name = "8bit" + .type = PIN_MAP_TYPE_MUX_GROUP, + .ctrl_dev_name = "pinctrl-foo", + .function = "mmc0", + .group = "mmc0_1_grp", + }, + { + .dev_name = "foo-mmc.0", + .name = "8bit" + .type = PIN_MAP_TYPE_MUX_GROUP, + .ctrl_dev_name = "pinctrl-foo", + .function = "mmc0", + .group = "mmc0_2_grp", + }, + { + .dev_name = "foo-mmc.0", + .name = "8bit" + .type = PIN_MAP_TYPE_MUX_GROUP, + .ctrl_dev_name = "pinctrl-foo", + .function = "mmc0", + .group = "mmc0_3_grp", + }, + ... + +The result of grabbing this mapping from the device with something like +this (see next paragraph):: + + p = devm_pinctrl_get(dev); + s = pinctrl_lookup_state(p, "8bit"); + ret = pinctrl_select_state(p, s); + +or more simply:: + + p = devm_pinctrl_get_select(dev, "8bit"); + +Will be that you activate all the three bottom records in the mapping at +once. Since they share the same name, pin controller device, function and +device, and since we allow multiple groups to match to a single device, they +all get selected, and they all get enabled and disable simultaneously by the +pinmux core. + + +Pin control requests from drivers +================================= + +When a device driver is about to probe the device core will automatically +attempt to issue pinctrl_get_select_default() on these devices. +This way driver writers do not need to add any of the boilerplate code +of the type found below. However when doing fine-grained state selection +and not using the "default" state, you may have to do some device driver +handling of the pinctrl handles and states. + +So if you just want to put the pins for a certain device into the default +state and be done with it, there is nothing you need to do besides +providing the proper mapping table. The device core will take care of +the rest. + +Generally it is discouraged to let individual drivers get and enable pin +control. So if possible, handle the pin control in platform code or some other +place where you have access to all the affected struct device * pointers. In +some cases where a driver needs to e.g. switch between different mux mappings +at runtime this is not possible. + +A typical case is if a driver needs to switch bias of pins from normal +operation and going to sleep, moving from the PINCTRL_STATE_DEFAULT to +PINCTRL_STATE_SLEEP at runtime, re-biasing or even re-muxing pins to save +current in sleep mode. + +A driver may request a certain control state to be activated, usually just the +default state like this:: + + #include <linux/pinctrl/consumer.h> + + struct foo_state { + struct pinctrl *p; + struct pinctrl_state *s; + ... + }; + + foo_probe() + { + /* Allocate a state holder named "foo" etc */ + struct foo_state *foo = ...; + + foo->p = devm_pinctrl_get(&device); + if (IS_ERR(foo->p)) { + /* FIXME: clean up "foo" here */ + return PTR_ERR(foo->p); + } + + foo->s = pinctrl_lookup_state(foo->p, PINCTRL_STATE_DEFAULT); + if (IS_ERR(foo->s)) { + /* FIXME: clean up "foo" here */ + return PTR_ERR(foo->s); + } + + ret = pinctrl_select_state(foo->s); + if (ret < 0) { + /* FIXME: clean up "foo" here */ + return ret; + } + } + +This get/lookup/select/put sequence can just as well be handled by bus drivers +if you don't want each and every driver to handle it and you know the +arrangement on your bus. + +The semantics of the pinctrl APIs are: + +- pinctrl_get() is called in process context to obtain a handle to all pinctrl + information for a given client device. It will allocate a struct from the + kernel memory to hold the pinmux state. All mapping table parsing or similar + slow operations take place within this API. + +- devm_pinctrl_get() is a variant of pinctrl_get() that causes pinctrl_put() + to be called automatically on the retrieved pointer when the associated + device is removed. It is recommended to use this function over plain + pinctrl_get(). + +- pinctrl_lookup_state() is called in process context to obtain a handle to a + specific state for a client device. This operation may be slow, too. + +- pinctrl_select_state() programs pin controller hardware according to the + definition of the state as given by the mapping table. In theory, this is a + fast-path operation, since it only involved blasting some register settings + into hardware. However, note that some pin controllers may have their + registers on a slow/IRQ-based bus, so client devices should not assume they + can call pinctrl_select_state() from non-blocking contexts. + +- pinctrl_put() frees all information associated with a pinctrl handle. + +- devm_pinctrl_put() is a variant of pinctrl_put() that may be used to + explicitly destroy a pinctrl object returned by devm_pinctrl_get(). + However, use of this function will be rare, due to the automatic cleanup + that will occur even without calling it. + + pinctrl_get() must be paired with a plain pinctrl_put(). + pinctrl_get() may not be paired with devm_pinctrl_put(). + devm_pinctrl_get() can optionally be paired with devm_pinctrl_put(). + devm_pinctrl_get() may not be paired with plain pinctrl_put(). + +Usually the pin control core handled the get/put pair and call out to the +device drivers bookkeeping operations, like checking available functions and +the associated pins, whereas select_state pass on to the pin controller +driver which takes care of activating and/or deactivating the mux setting by +quickly poking some registers. + +The pins are allocated for your device when you issue the devm_pinctrl_get() +call, after this you should be able to see this in the debugfs listing of all +pins. + +NOTE: the pinctrl system will return -EPROBE_DEFER if it cannot find the +requested pinctrl handles, for example if the pinctrl driver has not yet +registered. Thus make sure that the error path in your driver gracefully +cleans up and is ready to retry the probing later in the startup process. + + +Drivers needing both pin control and GPIOs +========================================== + +Again, it is discouraged to let drivers lookup and select pin control states +themselves, but again sometimes this is unavoidable. + +So say that your driver is fetching its resources like this:: + + #include <linux/pinctrl/consumer.h> + #include <linux/gpio.h> + + struct pinctrl *pinctrl; + int gpio; + + pinctrl = devm_pinctrl_get_select_default(&dev); + gpio = devm_gpio_request(&dev, 14, "foo"); + +Here we first request a certain pin state and then request GPIO 14 to be +used. If you're using the subsystems orthogonally like this, you should +nominally always get your pinctrl handle and select the desired pinctrl +state BEFORE requesting the GPIO. This is a semantic convention to avoid +situations that can be electrically unpleasant, you will certainly want to +mux in and bias pins in a certain way before the GPIO subsystems starts to +deal with them. + +The above can be hidden: using the device core, the pinctrl core may be +setting up the config and muxing for the pins right before the device is +probing, nevertheless orthogonal to the GPIO subsystem. + +But there are also situations where it makes sense for the GPIO subsystem +to communicate directly with the pinctrl subsystem, using the latter as a +back-end. This is when the GPIO driver may call out to the functions +described in the section "Pin control interaction with the GPIO subsystem" +above. This only involves per-pin multiplexing, and will be completely +hidden behind the gpio_*() function namespace. In this case, the driver +need not interact with the pin control subsystem at all. + +If a pin control driver and a GPIO driver is dealing with the same pins +and the use cases involve multiplexing, you MUST implement the pin controller +as a back-end for the GPIO driver like this, unless your hardware design +is such that the GPIO controller can override the pin controller's +multiplexing state through hardware without the need to interact with the +pin control system. + + +System pin control hogging +========================== + +Pin control map entries can be hogged by the core when the pin controller +is registered. This means that the core will attempt to call pinctrl_get(), +lookup_state() and select_state() on it immediately after the pin control +device has been registered. + +This occurs for mapping table entries where the client device name is equal +to the pin controller device name, and the state name is PINCTRL_STATE_DEFAULT:: + + { + .dev_name = "pinctrl-foo", + .name = PINCTRL_STATE_DEFAULT, + .type = PIN_MAP_TYPE_MUX_GROUP, + .ctrl_dev_name = "pinctrl-foo", + .function = "power_func", + }, + +Since it may be common to request the core to hog a few always-applicable +mux settings on the primary pin controller, there is a convenience macro for +this:: + + PIN_MAP_MUX_GROUP_HOG_DEFAULT("pinctrl-foo", NULL /* group */, + "power_func") + +This gives the exact same result as the above construction. + + +Runtime pinmuxing +================= + +It is possible to mux a certain function in and out at runtime, say to move +an SPI port from one set of pins to another set of pins. Say for example for +spi0 in the example above, we expose two different groups of pins for the same +function, but with different named in the mapping as described under +"Advanced mapping" above. So that for an SPI device, we have two states named +"pos-A" and "pos-B". + +This snippet first initializes a state object for both groups (in foo_probe()), +then muxes the function in the pins defined by group A, and finally muxes it in +on the pins defined by group B:: + + #include <linux/pinctrl/consumer.h> + + struct pinctrl *p; + struct pinctrl_state *s1, *s2; + + foo_probe() + { + /* Setup */ + p = devm_pinctrl_get(&device); + if (IS_ERR(p)) + ... + + s1 = pinctrl_lookup_state(foo->p, "pos-A"); + if (IS_ERR(s1)) + ... + + s2 = pinctrl_lookup_state(foo->p, "pos-B"); + if (IS_ERR(s2)) + ... + } + + foo_switch() + { + /* Enable on position A */ + ret = pinctrl_select_state(s1); + if (ret < 0) + ... + + ... + + /* Enable on position B */ + ret = pinctrl_select_state(s2); + if (ret < 0) + ... + + ... + } + +The above has to be done from process context. The reservation of the pins +will be done when the state is activated, so in effect one specific pin +can be used by different functions at different times on a running system. + + +Debugfs files +============= +These files are created in ``/sys/kernel/debug/pinctrl``: + +- ``pinctrl-devices``: prints each pin controller device along with columns to + indicate support for pinmux and pinconf + +- ``pinctrl-handles``: prints each configured pin controller handle and the + corresponding pinmux maps + +- ``pinctrl-maps``: print all pinctrl maps + +A sub-directory is created inside of ``/sys/kernel/debug/pinctrl`` for each pin +controller device containing these files: + +- ``pins``: prints a line for each pin registered on the pin controller. The + pinctrl driver may add additional information such as register contents. + +- ``gpio-ranges``: print ranges that map gpio lines to pins on the controller + +- ``pingroups``: print all pin groups registered on the pin controller + +- ``pinconf-pins``: print pin config settings for each pin + +- ``pinconf-groups``: print pin config settings per pin group + +- ``pinmux-functions``: print each pin function along with the pin groups that + map to the pin function + +- ``pinmux-pins``: iterate through all pins and print mux owner, gpio owner + and if the pin is a hog + +- ``pinmux-select``: write to this file to activate a pin function for a group:: + + echo "<group-name function-name>" > pinmux-select |