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diff --git a/Documentation/driver-api/gpio/driver.rst b/Documentation/driver-api/gpio/driver.rst new file mode 100644 index 000000000..6baaeab79 --- /dev/null +++ b/Documentation/driver-api/gpio/driver.rst @@ -0,0 +1,778 @@ +===================== +GPIO Driver Interface +===================== + +This document serves as a guide for writers of GPIO chip drivers. + +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 +================================ + +A GPIO chip handles one or more GPIO lines. To be considered a GPIO chip, the +lines must conform to the definition: General Purpose Input/Output. If the +line is not general purpose, it is not GPIO and should not be handled by a +GPIO chip. The use case is the indicative: certain lines in a system may be +called GPIO but serve a very particular purpose thus not meeting the criteria +of a general purpose I/O. On the other hand a LED driver line may be used as a +GPIO and should therefore still be handled by a GPIO chip driver. + +Inside a GPIO driver, individual GPIO lines are identified by their hardware +number, sometime also referred to as ``offset``, which is a unique number +between 0 and n-1, n being the number of GPIOs managed by the chip. + +The hardware GPIO number should be something intuitive to the hardware, for +example if a system uses a memory-mapped set of I/O-registers where 32 GPIO +lines are handled by one bit per line in a 32-bit register, it makes sense to +use hardware offsets 0..31 for these, corresponding to bits 0..31 in the +register. + +This number is purely internal: the hardware number of a particular GPIO +line is never made visible outside of the driver. + +On top of this internal number, each GPIO line also needs 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 line 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 global numbers 32-159 for GPIOs, with a +controller defining 128 GPIOs at a "base" of 32 ; while another platform uses +global 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 legacy +numbers need not be contiguous; either of those platforms could also use numbers +2000-2063 to identify GPIO lines 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, these should be assigned by the +driver code: + + - 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 information) + - 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, preferably using the driver model. That code will configure each +gpio_chip and issue gpiochip_add(), gpiochip_add_data(), or +devm_gpiochip_add_data(). Removing a GPIO controller should be rare; use +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 lines which haven't been +requested. They can use gpiochip_is_requested(), which returns either +NULL or the label associated with that GPIO line when it was requested. + +Realtime considerations: 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 realtime kernels (inside hard IRQ handlers and similar +contexts). Normally this should not be required. + + +GPIO electrical configuration +----------------------------- + +GPIO lines can be configured for several electrical modes of operation by using +the .set_config() callback. Currently this API supports setting: + +- Debouncing +- Single-ended modes (open drain/open source) +- Pull up and pull down resistor enablement + +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. + + +GPIO lines 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 firing 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. + + +GPIO lines 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 SDA lines of an I2C bus, which is by definition a + wire-OR bus. + +Both use cases 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 +achieving 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 lines with pull up/down resistor support +--------------------------------------------- + +A GPIO line can support pull-up/down using the .set_config() callback. This +means that a pull up or pull-down resistor is available on the output of the +GPIO line, and this resistor is software controlled. + +In discrete designs, a pull-up or pull-down resistor is simply soldered on +the circuit board. This is not something we deal with or model in software. The +most you will think about these lines is that they will very likely be +configured as open drain or open source (see the section above). + +The .set_config() callback can only turn pull up or down on and off, and will +no have any semantic knowledge about the resistance used. It will only say +switch a bit in a register enabling or disabling pull-up or pull-down. + +If the GPIO line supports shunting in different resistance values for the +pull-up or pull-down resistor, the GPIO chip callback .set_config() will not +suffice. For these complex use cases, a combined GPIO chip and pin controller +need to be implemented, as the pin config interface of a pin controller +supports more versatile control over electrical properties and can handle +different pull-up or pull-down resistance values. + + +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 irq_chip, using +the header <linux/irq.h>. So this combined driver is utilizing two sub- +systems simultaneously: gpio and irq. + +It is legal for any IRQ consumer to request an IRQ from any irqchip even if it +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. + +Always prepare the hardware and make it ready for action in respective +callbacks from the GPIO and irq_chip APIs. Do not rely on gpiod_to_irq() having +been called first. + +We can divide GPIO irqchips in two broad categories: + +- CASCADED INTERRUPT CHIPS: this means that the GPIO chip has one common + interrupt output line, which is triggered by any enabled GPIO line on that + chip. The interrupt output line will then be routed to an parent interrupt + controller one level up, in the most simple case the systems primary + interrupt controller. This is modeled by an irqchip that will inspect bits + inside the GPIO controller to figure out which line fired it. The irqchip + part of the driver needs to inspect registers to figure this out and it + will likely also need to acknowledge that it is handling the interrupt + by clearing some bit (sometime implicitly, by just reading a status + register) and it will often need to set up the configuration such as + edge sensitivity (rising or falling edge, or high/low level interrupt for + example). + +- HIERARCHICAL INTERRUPT CHIPS: this means that each GPIO line has a dedicated + irq line to a parent interrupt controller one level up. There is no need + to inquire the GPIO hardware to figure out which line has fired, but it + may still be necessary to acknowledge the interrupt and set up configuration + such as edge sensitivity. + +Realtime considerations: a realtime compliant GPIO driver should not use +spinlock_t or any sleepable APIs (like PM runtime) as part of its irqchip +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] + + +Cascaded GPIO irqchips +---------------------- + +Cascaded GPIO irqchips usually fall in one of three categories: + +- CHAINED CASCADED 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. + + Realtime considerations: Note that chained IRQ handlers will not be forced + threaded on -RT. As a result, spinlock_t or any sleepable APIs (like PM + runtime) can't be used in a chained IRQ handler. + + If required (and if it can't be converted to the nested threaded GPIO irqchip, + see below) a chained IRQ handler can be converted to generic irq handler and + this way it will become a threaded IRQ handler on -RT and a hard IRQ handler + on non-RT (for example, see [3]). + + 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 work around 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(...); + + Realtime considerations: this kind of handlers will be forced threaded on -RT, + and as result the IRQ core will complain that generic_handle_irq() is called + with IRQ enabled and the same work-around 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 such as I2C + or SPI. + + 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. + + These kinds of irqchips are inherently realtime tolerant as they are + already set up to handle sleeping contexts. + + +Infrastructure helpers for GPIO irqchips +---------------------------------------- + +To help out in handling the set-up and management of GPIO irqchips and the +associated irqdomain and resource allocation callbacks. These are activated +by selecting the Kconfig symbol GPIOLIB_IRQCHIP. If the symbol +IRQ_DOMAIN_HIERARCHY is also selected, hierarchical helpers will also be +provided. A big portion of overhead code will be managed by gpiolib, +under the assumption that your interrupts are 1-to-1-mapped to the +GPIO line index: + +.. csv-table:: + :header: GPIO line offset, Hardware IRQ + + 0,0 + 1,1 + 2,2 + ...,... + ngpio-1, ngpio-1 + + +If some GPIO lines do not have corresponding IRQs, the bitmask valid_mask +and the flag need_valid_mask in gpio_irq_chip can be used to mask off some +lines as invalid for associating with IRQs. + +The preferred way to set up the helpers is to fill in the +struct gpio_irq_chip inside struct gpio_chip before adding the gpio_chip. +If you do this, the additional irq_chip will be set up by gpiolib at the +same time as setting up the rest of the GPIO functionality. The following +is a typical example of a chained cascaded interrupt handler using +the gpio_irq_chip. Note how the mask/unmask (or disable/enable) functions +call into the core gpiolib code: + +.. code-block:: c + + /* Typical state container */ + struct my_gpio { + struct gpio_chip gc; + }; + + static void my_gpio_mask_irq(struct irq_data *d) + { + struct gpio_chip *gc = irq_data_get_irq_chip_data(d); + irq_hw_number_t hwirq = irqd_to_hwirq(d); + + /* + * Perform any necessary action to mask the interrupt, + * and then call into the core code to synchronise the + * state. + */ + + gpiochip_disable_irq(gc, hwirq); + } + + static void my_gpio_unmask_irq(struct irq_data *d) + { + struct gpio_chip *gc = irq_data_get_irq_chip_data(d); + irq_hw_number_t hwirq = irqd_to_hwirq(d); + + gpiochip_enable_irq(gc, hwirq); + + /* + * Perform any necessary action to unmask the interrupt, + * after having called into the core code to synchronise + * the state. + */ + } + + /* + * Statically populate the irqchip. Note that it is made const + * (further indicated by the IRQCHIP_IMMUTABLE flag), and that + * the GPIOCHIP_IRQ_RESOURCE_HELPER macro adds some extra + * callbacks to the structure. + */ + static const struct irq_chip my_gpio_irq_chip = { + .name = "my_gpio_irq", + .irq_ack = my_gpio_ack_irq, + .irq_mask = my_gpio_mask_irq, + .irq_unmask = my_gpio_unmask_irq, + .irq_set_type = my_gpio_set_irq_type, + .flags = IRQCHIP_IMMUTABLE, + /* Provide the gpio resource callbacks */ + GPIOCHIP_IRQ_RESOURCE_HELPERS, + }; + + int irq; /* from platform etc */ + struct my_gpio *g; + struct gpio_irq_chip *girq; + + /* Get a pointer to the gpio_irq_chip */ + girq = &g->gc.irq; + gpio_irq_chip_set_chip(girq, &my_gpio_irq_chip); + girq->parent_handler = ftgpio_gpio_irq_handler; + girq->num_parents = 1; + girq->parents = devm_kcalloc(dev, 1, sizeof(*girq->parents), + GFP_KERNEL); + if (!girq->parents) + return -ENOMEM; + girq->default_type = IRQ_TYPE_NONE; + girq->handler = handle_bad_irq; + girq->parents[0] = irq; + + return devm_gpiochip_add_data(dev, &g->gc, g); + +The helper supports using threaded interrupts as well. Then you just request +the interrupt separately and go with it: + +.. code-block:: c + + /* Typical state container */ + struct my_gpio { + struct gpio_chip gc; + }; + + static void my_gpio_mask_irq(struct irq_data *d) + { + struct gpio_chip *gc = irq_data_get_irq_chip_data(d); + irq_hw_number_t hwirq = irqd_to_hwirq(d); + + /* + * Perform any necessary action to mask the interrupt, + * and then call into the core code to synchronise the + * state. + */ + + gpiochip_disable_irq(gc, hwirq); + } + + static void my_gpio_unmask_irq(struct irq_data *d) + { + struct gpio_chip *gc = irq_data_get_irq_chip_data(d); + irq_hw_number_t hwirq = irqd_to_hwirq(d); + + gpiochip_enable_irq(gc, hwirq); + + /* + * Perform any necessary action to unmask the interrupt, + * after having called into the core code to synchronise + * the state. + */ + } + + /* + * Statically populate the irqchip. Note that it is made const + * (further indicated by the IRQCHIP_IMMUTABLE flag), and that + * the GPIOCHIP_IRQ_RESOURCE_HELPER macro adds some extra + * callbacks to the structure. + */ + static const struct irq_chip my_gpio_irq_chip = { + .name = "my_gpio_irq", + .irq_ack = my_gpio_ack_irq, + .irq_mask = my_gpio_mask_irq, + .irq_unmask = my_gpio_unmask_irq, + .irq_set_type = my_gpio_set_irq_type, + .flags = IRQCHIP_IMMUTABLE, + /* Provide the gpio resource callbacks */ + GPIOCHIP_IRQ_RESOURCE_HELPERS, + }; + + int irq; /* from platform etc */ + struct my_gpio *g; + struct gpio_irq_chip *girq; + + ret = devm_request_threaded_irq(dev, irq, NULL, + irq_thread_fn, IRQF_ONESHOT, "my-chip", g); + if (ret < 0) + return ret; + + /* Get a pointer to the gpio_irq_chip */ + girq = &g->gc.irq; + gpio_irq_chip_set_chip(girq, &my_gpio_irq_chip); + /* This will let us handle the parent IRQ in the driver */ + girq->parent_handler = NULL; + girq->num_parents = 0; + girq->parents = NULL; + girq->default_type = IRQ_TYPE_NONE; + girq->handler = handle_bad_irq; + + return devm_gpiochip_add_data(dev, &g->gc, g); + +The helper supports using hierarchical interrupt controllers as well. +In this case the typical set-up will look like this: + +.. code-block:: c + + /* Typical state container with dynamic irqchip */ + struct my_gpio { + struct gpio_chip gc; + struct fwnode_handle *fwnode; + }; + + static void my_gpio_mask_irq(struct irq_data *d) + { + struct gpio_chip *gc = irq_data_get_irq_chip_data(d); + irq_hw_number_t hwirq = irqd_to_hwirq(d); + + /* + * Perform any necessary action to mask the interrupt, + * and then call into the core code to synchronise the + * state. + */ + + gpiochip_disable_irq(gc, hwirq); + irq_mask_mask_parent(d); + } + + static void my_gpio_unmask_irq(struct irq_data *d) + { + struct gpio_chip *gc = irq_data_get_irq_chip_data(d); + irq_hw_number_t hwirq = irqd_to_hwirq(d); + + gpiochip_enable_irq(gc, hwirq); + + /* + * Perform any necessary action to unmask the interrupt, + * after having called into the core code to synchronise + * the state. + */ + + irq_mask_unmask_parent(d); + } + + /* + * Statically populate the irqchip. Note that it is made const + * (further indicated by the IRQCHIP_IMMUTABLE flag), and that + * the GPIOCHIP_IRQ_RESOURCE_HELPER macro adds some extra + * callbacks to the structure. + */ + static const struct irq_chip my_gpio_irq_chip = { + .name = "my_gpio_irq", + .irq_ack = my_gpio_ack_irq, + .irq_mask = my_gpio_mask_irq, + .irq_unmask = my_gpio_unmask_irq, + .irq_set_type = my_gpio_set_irq_type, + .flags = IRQCHIP_IMMUTABLE, + /* Provide the gpio resource callbacks */ + GPIOCHIP_IRQ_RESOURCE_HELPERS, + }; + + struct my_gpio *g; + struct gpio_irq_chip *girq; + + /* Get a pointer to the gpio_irq_chip */ + girq = &g->gc.irq; + gpio_irq_chip_set_chip(girq, &my_gpio_irq_chip); + girq->default_type = IRQ_TYPE_NONE; + girq->handler = handle_bad_irq; + girq->fwnode = g->fwnode; + girq->parent_domain = parent; + girq->child_to_parent_hwirq = my_gpio_child_to_parent_hwirq; + + return devm_gpiochip_add_data(dev, &g->gc, g); + +As you can see pretty similar, but you do not supply a parent handler for +the IRQ, instead a parent irqdomain, an fwnode for the hardware and +a function .child_to_parent_hwirq() that has the purpose of looking up +the parent hardware irq from a child (i.e. this gpio chip) hardware irq. +As always it is good to look at examples in the kernel tree for advice +on how to find the required pieces. + +If there is a need to exclude certain GPIO lines from the IRQ domain handled by +these helpers, we can set .irq.need_valid_mask of the gpiochip before +devm_gpiochip_add_data() or gpiochip_add_data() is called. This allocates an +.irq.valid_mask with as many bits set as there are GPIO lines in the chip, each +bit representing line 0..n-1. Drivers can exclude GPIO lines by clearing bits +from this mask. The mask can be filled in the init_valid_mask() callback +that is part of the struct gpio_irq_chip. + +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 gpio_irq_chip.handler to handle_bad_irq. Then, if your irqchip + is cascaded, set the handler to handle_level_irq() and/or handle_edge_irq() + in the irqchip .set_type() callback depending on what your controller + supports and what is requested by the consumer. + + +Locking IRQ usage +----------------- + +Since GPIO and irq_chip are orthogonal, we can get conflicts between different +use cases. For example a GPIO line used for IRQs should be an input line, +it does not make sense to fire interrupts on an output GPIO. + +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. + +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 callbacks are automatically +assigned. + + +Disabling and enabling IRQs +--------------------------- + +In some (fringe) use cases, a driver may be using a GPIO line as input for IRQs, +but occasionally switch that line over to drive output and then back to being +an input with interrupts again. This happens on things like CEC (Consumer +Electronics Control). + +When a GPIO is used as an IRQ signal, then gpiolib also needs to know if +the IRQ is enabled or disabled. In order to inform gpiolib about this, +the irqchip driver should call:: + + void gpiochip_disable_irq(struct gpio_chip *chip, unsigned int offset) + +This allows drivers to drive the GPIO as an output while the IRQ is +disabled. When the IRQ is enabled again, a driver should call:: + + void gpiochip_enable_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 .irq_disable() and .irq_enable() callbacks from the +irqchip. + +When IRQCHIP_IMMUTABLE is not advertised by the irqchip, these callbacks +are automatically assigned. This behaviour is deprecated and on its way +to be removed from the kernel. + + +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 realtime considerations in the documentation. + +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 the chained IRQ handler +- Generic chained GPIO irqchips: take care about generic_handle_irq() calls and + apply corresponding work-around +- Chained GPIO irqchips: get rid of the chained IRQ handler and use generic irq + handler if possible +- regmap_mmio: it is possible to disable internal locking in regmap by setting + .disable_locking and handling the locking in the GPIO driver +- Test your driver with the appropriate in-kernel real-time test cases for both + level and edge IRQs + +* [1] http://www.spinics.net/lists/linux-omap/msg120425.html +* [2] https://lore.kernel.org/r/1443209283-20781-2-git-send-email-grygorii.strashko@ti.com +* [3] https://lore.kernel.org/r/1443209283-20781-3-git-send-email-grygorii.strashko@ti.com + + +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. A GPIO driver can use the following +functions to request and free descriptors:: + + struct gpio_desc *gpiochip_request_own_desc(struct gpio_desc *desc, + u16 hwnum, + const char *label, + enum gpiod_flags flags) + + 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. |