Revert "gpio: add a real time compliance checklist"

This reverts commit 677b2ff4af.

Documentation/gpio/driver.txt

@@ -93,37 +93,22 @@ 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.
 
-  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:
+* 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:
 
   static irqreturn_t tc3589x_gpio_irq(int irq, void *data)
       ...
       handle_nested_irq(irq);
-      OR
-      generic_handle_irq(irq);
 
-  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.
+  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.
 
 To help out in handling the set-up and management of GPIO irqchips and the
 associated irqdomain and resource allocation callbacks, the gpiolib has
@@ -140,7 +125,7 @@ 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 threaded/nested irqchip if NULL is passed as handler.
+  to set up a nested irqchip if NULL is passed as handler.
 
 To use the helpers please keep the following in mind:
 
@@ -185,39 +170,6 @@ 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
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