diff options
Diffstat (limited to 'Documentation/driver-api/gpio/driver.rst')
| -rw-r--r-- | Documentation/driver-api/gpio/driver.rst | 429 |
1 files changed, 429 insertions, 0 deletions
diff --git a/Documentation/driver-api/gpio/driver.rst b/Documentation/driver-api/gpio/driver.rst new file mode 100644 index 000000000..cbe024284 --- /dev/null +++ b/Documentation/driver-api/gpio/driver.rst @@ -0,0 +1,429 @@ +================================ +GPIO Descriptor Driver Interface +================================ + +This document serves as a guide for GPIO chip drivers writers. Note that it +describes the new descriptor-based interface. For a description of the +deprecated integer-based GPIO interface please refer to gpio-legacy.txt. + +Each GPIO controller driver needs to include the following header, which defines +the structures used to define a GPIO driver: + + #include <linux/gpio/driver.h> + + +Internal Representation of GPIOs +================================ + +Inside a GPIO driver, individual GPIOs are identified by their hardware number, +which is a unique number between 0 and n, n being the number of GPIOs managed by +the chip. This number is purely internal: the hardware number of a particular +GPIO descriptor is never made visible outside of the driver. + +On top of this internal number, each GPIO also need to have a global number in +the integer GPIO namespace so that it can be used with the legacy GPIO +interface. Each chip must thus have a "base" number (which can be automatically +assigned), and for each GPIO the global number will be (base + hardware number). +Although the integer representation is considered deprecated, it still has many +users and thus needs to be maintained. + +So for example one platform could use numbers 32-159 for GPIOs, with a +controller defining 128 GPIOs at a "base" of 32 ; while another platform uses +numbers 0..63 with one set of GPIO controllers, 64-79 with another type of GPIO +controller, and on one particular board 80-95 with an FPGA. The numbers need not +be contiguous; either of those platforms could also use numbers 2000-2063 to +identify GPIOs in a bank of I2C GPIO expanders. + + +Controller Drivers: gpio_chip +============================= + +In the gpiolib framework each GPIO controller is packaged as a "struct +gpio_chip" (see linux/gpio/driver.h for its complete definition) with members +common to each controller of that type: + + - methods to establish GPIO line direction + - methods used to access GPIO line values + - method to set electrical configuration for a given GPIO line + - method to return the IRQ number associated to a given GPIO line + - flag saying whether calls to its methods may sleep + - optional line names array to identify lines + - optional debugfs dump method (showing extra state like pullup config) + - optional base number (will be automatically assigned if omitted) + - optional label for diagnostics and GPIO chip mapping using platform data + +The code implementing a gpio_chip should support multiple instances of the +controller, possibly using the driver model. That code will configure each +gpio_chip and issue ``gpiochip_add[_data]()`` or ``devm_gpiochip_add_data()``. +Removing a GPIO controller should be rare; use ``[devm_]gpiochip_remove()`` +when it is unavoidable. + +Often a gpio_chip is part of an instance-specific structure with states not +exposed by the GPIO interfaces, such as addressing, power management, and more. +Chips such as audio codecs will have complex non-GPIO states. + +Any debugfs dump method should normally ignore signals which haven't been +requested as GPIOs. They can use gpiochip_is_requested(), which returns either +NULL or the label associated with that GPIO when it was requested. + +RT_FULL: the GPIO driver should not use spinlock_t or any sleepable APIs +(like PM runtime) in its gpio_chip implementation (.get/.set and direction +control callbacks) if it is expected to call GPIO APIs from atomic context +on -RT (inside hard IRQ handlers and similar contexts). Normally this should +not be required. + + +GPIO electrical configuration +----------------------------- + +GPIOs can be configured for several electrical modes of operation by using the +.set_config() callback. Currently this API supports setting debouncing and +single-ended modes (open drain/open source). These settings are described +below. + +The .set_config() callback uses the same enumerators and configuration +semantics as the generic pin control drivers. This is not a coincidence: it is +possible to assign the .set_config() to the function gpiochip_generic_config() +which will result in pinctrl_gpio_set_config() being called and eventually +ending up in the pin control back-end "behind" the GPIO controller, usually +closer to the actual pins. This way the pin controller can manage the below +listed GPIO configurations. + +If a pin controller back-end is used, the GPIO controller or hardware +description needs to provide "GPIO ranges" mapping the GPIO line offsets to pin +numbers on the pin controller so they can properly cross-reference each other. + + +GPIOs with debounce support +--------------------------- + +Debouncing is a configuration set to a pin indicating that it is connected to +a mechanical switch or button, or similar that may bounce. Bouncing means the +line is pulled high/low quickly at very short intervals for mechanical +reasons. This can result in the value being unstable or irqs fireing repeatedly +unless the line is debounced. + +Debouncing in practice involves setting up a timer when something happens on +the line, wait a little while and then sample the line again, so see if it +still has the same value (low or high). This could also be repeated by a clever +state machine, waiting for a line to become stable. In either case, it sets +a certain number of milliseconds for debouncing, or just "on/off" if that time +is not configurable. + + +GPIOs with open drain/source support +------------------------------------ + +Open drain (CMOS) or open collector (TTL) means the line is not actively driven +high: instead you provide the drain/collector as output, so when the transistor +is not open, it will present a high-impedance (tristate) to the external rail:: + + + CMOS CONFIGURATION TTL CONFIGURATION + + ||--- out +--- out + in ----|| |/ + ||--+ in ----| + | |\ + GND GND + +This configuration is normally used as a way to achieve one of two things: + +- Level-shifting: to reach a logical level higher than that of the silicon + where the output resides. + +- inverse wire-OR on an I/O line, for example a GPIO line, making it possible + for any driving stage on the line to drive it low even if any other output + to the same line is simultaneously driving it high. A special case of this + is driving the SCL and SCA lines of an I2C bus, which is by definition a + wire-OR bus. + +Both usecases require that the line be equipped with a pull-up resistor. This +resistor will make the line tend to high level unless one of the transistors on +the rail actively pulls it down. + +The level on the line will go as high as the VDD on the pull-up resistor, which +may be higher than the level supported by the transistor, achieving a +level-shift to the higher VDD. + +Integrated electronics often have an output driver stage in the form of a CMOS +"totem-pole" with one N-MOS and one P-MOS transistor where one of them drives +the line high and one of them drives the line low. This is called a push-pull +output. The "totem-pole" looks like so:: + + VDD + | + OD ||--+ + +--/ ---o|| P-MOS-FET + | ||--+ + IN --+ +----- out + | ||--+ + +--/ ----|| N-MOS-FET + OS ||--+ + | + GND + +The desired output signal (e.g. coming directly from some GPIO output register) +arrives at IN. The switches named "OD" and "OS" are normally closed, creating +a push-pull circuit. + +Consider the little "switches" named "OD" and "OS" that enable/disable the +P-MOS or N-MOS transistor right after the split of the input. As you can see, +either transistor will go totally numb if this switch is open. The totem-pole +is then halved and give high impedance instead of actively driving the line +high or low respectively. That is usually how software-controlled open +drain/source works. + +Some GPIO hardware come in open drain / open source configuration. Some are +hard-wired lines that will only support open drain or open source no matter +what: there is only one transistor there. Some are software-configurable: +by flipping a bit in a register the output can be configured as open drain +or open source, in practice by flicking open the switches labeled "OD" and "OS" +in the drawing above. + +By disabling the P-MOS transistor, the output can be driven between GND and +high impedance (open drain), and by disabling the N-MOS transistor, the output +can be driven between VDD and high impedance (open source). In the first case, +a pull-up resistor is needed on the outgoing rail to complete the circuit, and +in the second case, a pull-down resistor is needed on the rail. + +Hardware that supports open drain or open source or both, can implement a +special callback in the gpio_chip: .set_config() that takes a generic +pinconf packed value telling whether to configure the line as open drain, +open source or push-pull. This will happen in response to the +GPIO_OPEN_DRAIN or GPIO_OPEN_SOURCE flag set in the machine file, or coming +from other hardware descriptions. + +If this state can not be configured in hardware, i.e. if the GPIO hardware does +not support open drain/open source in hardware, the GPIO library will instead +use a trick: when a line is set as output, if the line is flagged as open +drain, and the IN output value is low, it will be driven low as usual. But +if the IN output value is set to high, it will instead *NOT* be driven high, +instead it will be switched to input, as input mode is high impedance, thus +achieveing an "open drain emulation" of sorts: electrically the behaviour will +be identical, with the exception of possible hardware glitches when switching +the mode of the line. + +For open source configuration the same principle is used, just that instead +of actively driving the line low, it is set to input. + + +GPIO drivers providing IRQs +--------------------------- +It is custom that GPIO drivers (GPIO chips) are also providing interrupts, +most often cascaded off a parent interrupt controller, and in some special +cases the GPIO logic is melded with a SoC's primary interrupt controller. + +The IRQ portions of the GPIO block are implemented using an irqchip, using +the header <linux/irq.h>. So basically such a driver is utilizing two sub- +systems simultaneously: gpio and irq. + +RT_FULL: a realtime compliant GPIO driver should not use spinlock_t or any +sleepable APIs (like PM runtime) as part of its irq_chip implementation. + +* spinlock_t should be replaced with raw_spinlock_t [1]. +* If sleepable APIs have to be used, these can be done from the .irq_bus_lock() + and .irq_bus_unlock() callbacks, as these are the only slowpath callbacks + on an irqchip. Create the callbacks if needed [2]. + +GPIO irqchips usually fall in one of two categories: + +* CHAINED GPIO irqchips: these are usually the type that is embedded on + an SoC. This means that there is a fast IRQ flow handler for the GPIOs that + gets called in a chain from the parent IRQ handler, most typically the + system interrupt controller. This means that the GPIO irqchip handler will + be called immediately from the parent irqchip, while holding the IRQs + disabled. The GPIO irqchip will then end up calling something like this + sequence in its interrupt handler:: + + static irqreturn_t foo_gpio_irq(int irq, void *data) + chained_irq_enter(...); + generic_handle_irq(...); + chained_irq_exit(...); + + Chained GPIO irqchips typically can NOT set the .can_sleep flag on + struct gpio_chip, as everything happens directly in the callbacks: no + slow bus traffic like I2C can be used. + + RT_FULL: Note, chained IRQ handlers will not be forced threaded on -RT. + As result, spinlock_t or any sleepable APIs (like PM runtime) can't be used + in chained IRQ handler. + If required (and if it can't be converted to the nested threaded GPIO irqchip) + a chained IRQ handler can be converted to generic irq handler and this way + it will be a threaded IRQ handler on -RT and a hard IRQ handler on non-RT + (for example, see [3]). + Know W/A: The generic_handle_irq() is expected to be called with IRQ disabled, + so the IRQ core will complain if it is called from an IRQ handler which is + forced to a thread. The "fake?" raw lock can be used to W/A this problem:: + + raw_spinlock_t wa_lock; + static irqreturn_t omap_gpio_irq_handler(int irq, void *gpiobank) + unsigned long wa_lock_flags; + raw_spin_lock_irqsave(&bank->wa_lock, wa_lock_flags); + generic_handle_irq(irq_find_mapping(bank->chip.irq.domain, bit)); + raw_spin_unlock_irqrestore(&bank->wa_lock, wa_lock_flags); + +* GENERIC CHAINED GPIO irqchips: these are the same as "CHAINED GPIO irqchips", + but chained IRQ handlers are not used. Instead GPIO IRQs dispatching is + performed by generic IRQ handler which is configured using request_irq(). + The GPIO irqchip will then end up calling something like this sequence in + its interrupt handler:: + + static irqreturn_t gpio_rcar_irq_handler(int irq, void *dev_id) + for each detected GPIO IRQ + generic_handle_irq(...); + + RT_FULL: Such kind of handlers will be forced threaded on -RT, as result IRQ + core will complain that generic_handle_irq() is called with IRQ enabled and + the same W/A as for "CHAINED GPIO irqchips" can be applied. + +* NESTED THREADED GPIO irqchips: these are off-chip GPIO expanders and any + other GPIO irqchip residing on the other side of a sleeping bus. Of course + such drivers that need slow bus traffic to read out IRQ status and similar, + traffic which may in turn incur other IRQs to happen, cannot be handled + in a quick IRQ handler with IRQs disabled. Instead they need to spawn a + thread and then mask the parent IRQ line until the interrupt is handled + by the driver. The hallmark of this driver is to call something like + this in its interrupt handler:: + + static irqreturn_t foo_gpio_irq(int irq, void *data) + ... + handle_nested_irq(irq); + + The hallmark of threaded GPIO irqchips is that they set the .can_sleep + flag on struct gpio_chip to true, indicating that this chip may sleep + when accessing the GPIOs. + +To help out in handling the set-up and management of GPIO irqchips and the +associated irqdomain and resource allocation callbacks, the gpiolib has +some helpers that can be enabled by selecting the GPIOLIB_IRQCHIP Kconfig +symbol: + +* gpiochip_irqchip_add(): adds a chained irqchip to a gpiochip. It will pass + the struct gpio_chip* for the chip to all IRQ callbacks, so the callbacks + need to embed the gpio_chip in its state container and obtain a pointer + to the container using container_of(). + (See Documentation/driver-model/design-patterns.txt) + +* gpiochip_irqchip_add_nested(): adds a nested irqchip to a gpiochip. + Apart from that it works exactly like the chained irqchip. + +* gpiochip_set_chained_irqchip(): sets up a chained irq handler for a + gpio_chip from a parent IRQ and passes the struct gpio_chip* as handler + data. (Notice handler data, since the irqchip data is likely used by the + parent irqchip!). + +* gpiochip_set_nested_irqchip(): sets up a nested irq handler for a + gpio_chip from a parent IRQ. As the parent IRQ has usually been + explicitly requested by the driver, this does very little more than + mark all the child IRQs as having the other IRQ as parent. + +If there is a need to exclude certain GPIOs from the IRQ domain, you can +set .irq.need_valid_mask of the gpiochip before gpiochip_add_data() is +called. This allocates an .irq.valid_mask with as many bits set as there +are GPIOs in the chip. Drivers can exclude GPIOs by clearing bits from this +mask. The mask must be filled in before gpiochip_irqchip_add() or +gpiochip_irqchip_add_nested() is called. + +To use the helpers please keep the following in mind: + +- Make sure to assign all relevant members of the struct gpio_chip so that + the irqchip can initialize. E.g. .dev and .can_sleep shall be set up + properly. + +- Nominally set all handlers to handle_bad_irq() in the setup call and pass + handle_bad_irq() as flow handler parameter in gpiochip_irqchip_add() if it is + expected for GPIO driver that irqchip .set_type() callback have to be called + before using/enabling GPIO IRQ. Then set the handler to handle_level_irq() + and/or handle_edge_irq() in the irqchip .set_type() callback depending on + what your controller supports. + +It is legal for any IRQ consumer to request an IRQ from any irqchip no matter +if that is a combined GPIO+IRQ driver. The basic premise is that gpio_chip and +irq_chip are orthogonal, and offering their services independent of each +other. + +gpiod_to_irq() is just a convenience function to figure out the IRQ for a +certain GPIO line and should not be relied upon to have been called before +the IRQ is used. + +So always prepare the hardware and make it ready for action in respective +callbacks from the GPIO and irqchip APIs. Do not rely on gpiod_to_irq() having +been called first. + +This orthogonality leads to ambiguities that we need to solve: if there is +competition inside the subsystem which side is using the resource (a certain +GPIO line and register for example) it needs to deny certain operations and +keep track of usage inside of the gpiolib subsystem. This is why the API +below exists. + + +Locking IRQ usage +----------------- +Input GPIOs can be used as IRQ signals. When this happens, a driver is requested +to mark the GPIO as being used as an IRQ:: + + int gpiochip_lock_as_irq(struct gpio_chip *chip, unsigned int offset) + +This will prevent the use of non-irq related GPIO APIs until the GPIO IRQ lock +is released:: + + void gpiochip_unlock_as_irq(struct gpio_chip *chip, unsigned int offset) + +When implementing an irqchip inside a GPIO driver, these two functions should +typically be called in the .startup() and .shutdown() callbacks from the +irqchip. + +When using the gpiolib irqchip helpers, these callback are automatically +assigned. + +Real-Time compliance for GPIO IRQ chips +--------------------------------------- + +Any provider of irqchips needs to be carefully tailored to support Real Time +preemption. It is desirable that all irqchips in the GPIO subsystem keep this +in mind and do the proper testing to assure they are real time-enabled. +So, pay attention on above " RT_FULL:" notes, please. +The following is a checklist to follow when preparing a driver for real +time-compliance: + +- ensure spinlock_t is not used as part irq_chip implementation; +- ensure that sleepable APIs are not used as part irq_chip implementation. + If sleepable APIs have to be used, these can be done from the .irq_bus_lock() + and .irq_bus_unlock() callbacks; +- Chained GPIO irqchips: ensure spinlock_t or any sleepable APIs are not used + from chained IRQ handler; +- Generic chained GPIO irqchips: take care about generic_handle_irq() calls and + apply corresponding W/A; +- Chained GPIO irqchips: get rid of chained IRQ handler and use generic irq + handler if possible :) +- regmap_mmio: Sry, but you are in trouble :( if MMIO regmap is used as for + GPIO IRQ chip implementation; +- Test your driver with the appropriate in-kernel real time test cases for both + level and edge IRQs. + + +Requesting self-owned GPIO pins +------------------------------- + +Sometimes it is useful to allow a GPIO chip driver to request its own GPIO +descriptors through the gpiolib API. Using gpio_request() for this purpose +does not help since it pins the module to the kernel forever (it calls +try_module_get()). A GPIO driver can use the following functions instead +to request and free descriptors without being pinned to the kernel forever:: + + struct gpio_desc *gpiochip_request_own_desc(struct gpio_desc *desc, + const char *label) + + void gpiochip_free_own_desc(struct gpio_desc *desc) + +Descriptors requested with gpiochip_request_own_desc() must be released with +gpiochip_free_own_desc(). + +These functions must be used with care since they do not affect module use +count. Do not use the functions to request gpio descriptors not owned by the +calling driver. + +* [1] http://www.spinics.net/lists/linux-omap/msg120425.html +* [2] https://lkml.org/lkml/2015/9/25/494 +* [3] https://lkml.org/lkml/2015/9/25/495 |