Merge remote-tracking branch 'h8300/h8300-next'

[karo-tx-linux.git] / Documentation / gpio / driver.txt

diff --git a/Documentation/gpio/driver.txt b/Documentation/gpio/driver.txt
index 90d0f6aba7a6848a3043ff7a7de3913118a98461..12a61948ec917ff2af9787a59f82336994acfde7 100644 (file)
--- 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.
 
 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
 ---------------------------
 
 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.
 
 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
 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.
 
   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,
 * 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.
 
   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
 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.
 
 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
 -------------------------------
 
 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.
 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