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| -rw-r--r-- | Documentation/gpio/driver.txt | 80 |
1 files changed, 80 insertions, 0 deletions
diff --git a/Documentation/gpio/driver.txt b/Documentation/gpio/driver.txt index 90d0f6aba7a6..12a61948ec91 100644 --- a/Documentation/gpio/driver.txt +++ b/Documentation/gpio/driver.txt @@ -62,6 +62,11 @@ 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: 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 drivers providing IRQs --------------------------- @@ -73,6 +78,13 @@ 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: GPIO driver should not use spinlock_t or any sleepable APIs +(like PM runtime) as part of its irq_chip implementation on -RT. +- 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 @@ -93,6 +105,38 @@ GPIO irqchips usually fall in one of two categories: Chained GPIO irqchips typically can NOT set the .can_sleep flag on struct gpio_chip, as everything happens directly in the callbacks. + 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) + - chained IRQ handler can be converted to generic irq handler and this way + it will be threaded IRQ handler on -RT and 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 IRQ core will complain if it will be called from IRQ handler wich is forced + 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.irqdomain, 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, @@ -133,6 +177,13 @@ To use the helpers please keep the following in mind: 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 @@ -169,6 +220,31 @@ When implementing an irqchip inside a GPIO driver, these two functions should typically be called in the .startup() and .shutdown() callbacks from the irqchip. +Real-Time compliance for GPIO IRQ chips +--------------------------------------- + +Any provider of irqchips needs to be carefully tailored to support Real Time +preemption. It is desireable that all irqchips in the GPIO subsystem keep this +in mind and does 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 ------------------------------- @@ -190,3 +266,7 @@ 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 |