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authorLinus Walleij <linus.walleij@linaro.org>2014-01-22 15:00:55 +0100
committerLinus Walleij <linus.walleij@linaro.org>2014-02-03 09:11:57 +0100
commit99adc0594864ebbae4478c5d85d84930894ea098 (patch)
treeeb1a1b0a008a384da7febb03247b92fde8191418 /Documentation/gpio
parent38dbfb59d1175ef458d006556061adeaa8751b72 (diff)
gpio: document how to make combined GPIO+irqchip drivers
Write a few words on how GPIO drivers supplying an irqchip should be written. Cc: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Diffstat (limited to 'Documentation/gpio')
-rw-r--r--Documentation/gpio/driver.txt35
1 files changed, 35 insertions, 0 deletions
diff --git a/Documentation/gpio/driver.txt b/Documentation/gpio/driver.txt
index 9da0bfa74781..f73cc7b5dc85 100644
--- a/Documentation/gpio/driver.txt
+++ b/Documentation/gpio/driver.txt
@@ -62,6 +62,37 @@ 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.
+
+GPIO drivers providing IRQs
+---------------------------
+It is custom that GPIO drivers (GPIO chips) are also providing interrupts,
+most often cascaded off a parent interrupt controller, and in some special
+cases the GPIO logic is melded with a SoC's primary interrupt controller.
+
+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.
+
+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
+other.
+
+gpiod_to_irq() is just a convenience function to figure out the IRQ for a
+certain GPIO line and should not be relied upon to have been called before
+the IRQ is used.
+
+So always prepare the hardware and make it ready for action in respective
+callbacks from the GPIO and irqchip APIs. Do not rely on gpiod_to_irq() having
+been called first.
+
+This orthogonality leads to ambiguities that we need to solve: if there is
+competition inside the subsystem which side is using the resource (a certain
+GPIO line and register for example) it needs to deny certain operations and
+keep track of usage inside of the gpiolib subsystem. This is why the API
+below exists.
+
+
Locking IRQ usage
-----------------
Input GPIOs can be used as IRQ signals. When this happens, a driver is requested
@@ -73,3 +104,7 @@ This will prevent the use of non-irq related GPIO APIs until the GPIO IRQ lock
is released:
void gpiod_unlock_as_irq(struct gpio_desc *desc)
+
+When implementing an irqchip inside a GPIO driver, these two functions should
+typically be called in the .startup() and .shutdown() callbacks from the
+irqchip.

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