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gpio: document how to make combined GPIO+irqchip drivers

Message ID 1390399390-9414-1-git-send-email-linus.walleij@linaro.org
State Accepted
Commit 99adc0594864ebbae4478c5d85d84930894ea098
Headers show

Commit Message

Linus Walleij Jan. 22, 2014, 2:03 p.m. UTC
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>
 Documentation/gpio/driver.txt | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)


Linus Walleij Jan. 31, 2014, 8:15 a.m. UTC | #1
On Wed, Jan 22, 2014 at 7:51 PM, Gerhard Sittig <gsi@denx.de> wrote:

> ... with a SoC's primary ... (apostroph instead of colon)


> The two paragraphs refer to different routines (gpiod_to_irq()
> and gpio_to_irq()) -- is this intentional?

No :-)

Fixed up both in the commit in my tree, THANKS!

Linus Walleij
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diff mbox


diff --git a/Documentation/gpio/driver.txt b/Documentation/gpio/driver.txt
index 9da0bfa74781..9818b2055197 100644
--- a/Documentation/gpio/driver.txt
+++ b/Documentation/gpio/driver.txt
@@ -62,6 +62,36 @@  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
+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 gpio_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 +103,7 @@  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