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authorLinus Walleij <linus.walleij@linaro.org>2014-04-09 14:36:32 +0200
committerLinus Walleij <linus.walleij@linaro.org>2014-04-28 12:35:07 -0700
commit90887db8837bb2ea57c3541d56a002e9654082b3 (patch)
treece40d206d01c0abbd016986075e1d4b311b45878 /Documentation/gpio
parentcf42f1cfe419f20425fc0c27b9930b6b51fe77b2 (diff)
gpio: document gpio irqchips a bit more, describe helpers
This adds some documentation about the GPIO irqchips, what types exist etc. Acked-by: Alexandre Courbot <acourbot@nvidia.com> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Diffstat (limited to 'Documentation/gpio')
-rw-r--r--Documentation/gpio/driver.txt59
1 files changed, 59 insertions, 0 deletions
diff --git a/Documentation/gpio/driver.txt b/Documentation/gpio/driver.txt
index f73cc7b5dc85..fa9a0a8b3734 100644
--- a/Documentation/gpio/driver.txt
+++ b/Documentation/gpio/driver.txt
@@ -73,6 +73,65 @@ 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.
+GPIO irqchips usually fall in one of two categories:
+
+* CHAINED GPIO irqchips: these are usually the type that is embedded on
+ an SoC. This means that there is a fast IRQ handler for the GPIOs that
+ gets called in a chain from the parent IRQ handler, most typically the
+ system interrupt controller. This means the GPIO irqchip is registered
+ using irq_set_chained_handler() or the corresponding
+ gpiochip_set_chained_irqchip() helper function, and the GPIO irqchip
+ handler will be called immediately from the parent irqchip, while
+ holding the IRQs disabled. The GPIO irqchip will then end up calling
+ something like this sequence in its interrupt handler:
+
+ static irqreturn_t tc3589x_gpio_irq(int irq, void *data)
+ chained_irq_enter(...);
+ generic_handle_irq(...);
+ chained_irq_exit(...);
+
+ 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:
+
+ static irqreturn_t tc3589x_gpio_irq(int irq, void *data)
+ ...
+ handle_nested_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.
+
+To help out in handling the set-up and management of GPIO irqchips and the
+associated irqdomain and resource allocation callbacks, the gpiolib has
+some helpers that can be enabled by selecting the GPIOLIB_IRQCHIP Kconfig
+symbol:
+
+* gpiochip_irqchip_add(): adds an irqchip to a gpiochip. It will pass
+ the struct gpio_chip* for the chip to all IRQ callbacks, so the callbacks
+ need to embed the gpio_chip in its state container and obtain a pointer
+ to the container using container_of().
+ (See Documentation/driver-model/design-patterns.txt)
+
+* gpiochip_set_chained_irqchip(): sets up a chained irq handler for a
+ 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.
+
+To use the helpers please keep the following in mind:
+
+- Make sure to assign all relevant members of the struct gpio_chip so that
+ the irqchip can initialize. E.g. .dev and .can_sleep shall be set up
+ properly.
+
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

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