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-rw-r--r--Documentation/gpio/driver.txt72
1 files changed, 60 insertions, 12 deletions
diff --git a/Documentation/gpio/driver.txt b/Documentation/gpio/driver.txt
index 90d0f6aba7a6..9d7985171f07 100644
--- a/Documentation/gpio/driver.txt
+++ b/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
-------------------------------