@@ -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