76 lines
3.3 KiB
Plaintext
76 lines
3.3 KiB
Plaintext
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GPIO Descriptor Driver Interface
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================================
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This document serves as a guide for GPIO chip drivers writers. Note that it
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describes the new descriptor-based interface. For a description of the
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deprecated integer-based GPIO interface please refer to gpio-legacy.txt.
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Each GPIO controller driver needs to include the following header, which defines
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the structures used to define a GPIO driver:
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#include <linux/gpio/driver.h>
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Internal Representation of GPIOs
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================================
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Inside a GPIO driver, individual GPIOs are identified by their hardware number,
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which is a unique number between 0 and n, n being the number of GPIOs managed by
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the chip. This number is purely internal: the hardware number of a particular
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GPIO descriptor is never made visible outside of the driver.
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On top of this internal number, each GPIO also need to have a global number in
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the integer GPIO namespace so that it can be used with the legacy GPIO
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interface. Each chip must thus have a "base" number (which can be automatically
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assigned), and for each GPIO the global number will be (base + hardware number).
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Although the integer representation is considered deprecated, it still has many
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users and thus needs to be maintained.
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So for example one platform could use numbers 32-159 for GPIOs, with a
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controller defining 128 GPIOs at a "base" of 32 ; while another platform uses
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numbers 0..63 with one set of GPIO controllers, 64-79 with another type of GPIO
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controller, and on one particular board 80-95 with an FPGA. The numbers need not
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be contiguous; either of those platforms could also use numbers 2000-2063 to
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identify GPIOs in a bank of I2C GPIO expanders.
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Controller Drivers: gpio_chip
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=============================
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In the gpiolib framework each GPIO controller is packaged as a "struct
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gpio_chip" (see linux/gpio/driver.h for its complete definition) with members
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common to each controller of that type:
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- methods to establish GPIO direction
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- methods used to access GPIO values
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- method to return the IRQ number associated to a given GPIO
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- flag saying whether calls to its methods may sleep
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- optional debugfs dump method (showing extra state like pullup config)
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- optional base number (will be automatically assigned if omitted)
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- label for diagnostics and GPIOs mapping using platform data
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The code implementing a gpio_chip should support multiple instances of the
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controller, possibly using the driver model. That code will configure each
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gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be rare;
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use gpiochip_remove() when it is unavoidable.
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Most often a gpio_chip is part of an instance-specific structure with state not
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exposed by the GPIO interfaces, such as addressing, power management, and more.
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Chips such as codecs will have complex non-GPIO state.
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Any debugfs dump method should normally ignore signals which haven't been
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requested as GPIOs. They can use gpiochip_is_requested(), which returns either
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NULL or the label associated with that GPIO when it was requested.
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Locking IRQ usage
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-----------------
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Input GPIOs can be used as IRQ signals. When this happens, a driver is requested
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to mark the GPIO as being used as an IRQ:
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int gpiod_lock_as_irq(struct gpio_desc *desc)
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This will prevent the use of non-irq related GPIO APIs until the GPIO IRQ lock
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is released:
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void gpiod_unlock_as_irq(struct gpio_desc *desc)
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