gpio: add a real time compliance checklist

Add some information about real time compliance to the driver document.
Inspired by Grygorii Strashko's real time compliance patches.

Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/gpio/driver.txt

@@ -93,22 +93,37 @@ 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.
 
-* 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:
+  NOTE: chained IRQ handlers are usually not good for real time. If you
+  are submitting a new driver or refactoring a driver for real time compliance,
+  consider using creating a nested/threaded irqchip instead, see below.
+
+* NESTED THREADED GPIO irqchips: these are traditionally 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.
+
+  With the introduction of real time support in the Linux kernel, also other
+  GPIO irqchips are encouraged to use a nested and threaded IRQ handler.
+  Doing so makes the interrupts naturally preemptible on a real time
+  setup, which means the system can easily be configured for real time with
+  a (usually negligable) performance loss.
+
+  These drivers 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 tc3589x_gpio_irq(int irq, void *data)
       ...
       handle_nested_irq(irq);
+      OR
+      generic_handle_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.
+  Threaded GPIO irqchips should set the .can_sleep flag on struct gpio_chip
+  to true if they are e.g. accessing the chip over I2C or SPI, indicating that
+  this chip may sleep when accessing the GPIOs. irqchips that are just made
+  threaded to be preemptible and thus real time compliant need not do this:
+  preemption is not sleeping.
 
 To help out in handling the set-up and management of GPIO irqchips and the
 associated irqdomain and resource allocation callbacks, the gpiolib has
@@ -125,7 +140,7 @@ symbol:
   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!) This is for the chained type of chip. This is also used
-  to set up a nested irqchip if NULL is passed as handler.
+  to set up a threaded/nested irqchip if NULL is passed as handler.
 
 To use the helpers please keep the following in mind:
 
@@ -170,6 +185,39 @@ 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. The
+following is a checklist to follow when preparing a driver for real
+time-compliance:
+
+- Nominally use raw_spinlock_t in the IRQ context path of the IRQ handler as
+  we do not want these sections to be preempted.
+
+- Do NOT use chained_irq_enter() or chained_irq_exit() in the IRQ handler,
+  as we want the hotpath to be preemptible.
+
+- Instead use nested IRQs and generic handlers such as handle_bad_irq(),
+  handle_level_irq() and handle_edge_irq(). Consequentally the handler
+  argument of gpiochip_set_chained_irqchip() should be NULL when using the
+  gpiolib irqchip helpers.
+
+- Nominally set all handlers to handle_bad_irq() in the setup call, 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.
+
+- If you need to use the pm_runtime_get*()/pm_runtime_put*() callbacks in some
+  of the irqchip callbacks, these should be moved to the .irq_bus_lock()
+  and .irq_bus_unlock() callbacks respectively, as these are the only
+  slowpath callbacks on an irqchip. Create the callbacks if need be.
+
+- Test your driver with the apropriate in-kernel real time test cases for both
+  level and edge IRQs.
+
+
 Requesting self-owned GPIO pins
 -------------------------------