203 lines
8.4 KiB
Plaintext
203 lines
8.4 KiB
Plaintext
GPIO Descriptor Consumer Interface
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==================================
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This document describes the consumer interface of the GPIO framework. Note that
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it 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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Guidelines for GPIOs consumers
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==============================
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Drivers that can't work without standard GPIO calls should have Kconfig entries
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that depend on GPIOLIB. The functions that allow a driver to obtain and use
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GPIOs are available by including the following file:
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#include <linux/gpio/consumer.h>
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All the functions that work with the descriptor-based GPIO interface are
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prefixed with gpiod_. The gpio_ prefix is used for the legacy interface. No
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other function in the kernel should use these prefixes.
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Obtaining and Disposing GPIOs
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=============================
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With the descriptor-based interface, GPIOs are identified with an opaque,
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non-forgeable handler that must be obtained through a call to one of the
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gpiod_get() functions. Like many other kernel subsystems, gpiod_get() takes the
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device that will use the GPIO and the function the requested GPIO is supposed to
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fulfill:
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struct gpio_desc *gpiod_get(struct device *dev, const char *con_id)
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If a function is implemented by using several GPIOs together (e.g. a simple LED
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device that displays digits), an additional index argument can be specified:
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struct gpio_desc *gpiod_get_index(struct device *dev,
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const char *con_id, unsigned int idx)
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Both functions return either a valid GPIO descriptor, or an error code checkable
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with IS_ERR() (they will never return a NULL pointer). -ENOENT will be returned
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if and only if no GPIO has been assigned to the device/function/index triplet,
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other error codes are used for cases where a GPIO has been assigned but an error
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occurred while trying to acquire it. This is useful to discriminate between mere
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errors and an absence of GPIO for optional GPIO parameters.
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Device-managed variants of these functions are also defined:
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struct gpio_desc *devm_gpiod_get(struct device *dev, const char *con_id)
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struct gpio_desc *devm_gpiod_get_index(struct device *dev,
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const char *con_id,
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unsigned int idx)
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A GPIO descriptor can be disposed of using the gpiod_put() function:
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void gpiod_put(struct gpio_desc *desc)
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It is strictly forbidden to use a descriptor after calling this function. The
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device-managed variant is, unsurprisingly:
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void devm_gpiod_put(struct device *dev, struct gpio_desc *desc)
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Using GPIOs
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===========
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Setting Direction
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-----------------
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The first thing a driver must do with a GPIO is setting its direction. This is
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done by invoking one of the gpiod_direction_*() functions:
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int gpiod_direction_input(struct gpio_desc *desc)
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int gpiod_direction_output(struct gpio_desc *desc, int value)
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The return value is zero for success, else a negative errno. It should be
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checked, since the get/set calls don't return errors and since misconfiguration
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is possible. You should normally issue these calls from a task context. However,
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for spinlock-safe GPIOs it is OK to use them before tasking is enabled, as part
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of early board setup.
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For output GPIOs, the value provided becomes the initial output value. This
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helps avoid signal glitching during system startup.
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A driver can also query the current direction of a GPIO:
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int gpiod_get_direction(const struct gpio_desc *desc)
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This function will return either GPIOF_DIR_IN or GPIOF_DIR_OUT.
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Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO
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without setting its direction first is illegal and will result in undefined
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behavior!**
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Spinlock-Safe GPIO Access
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-------------------------
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Most GPIO controllers can be accessed with memory read/write instructions. Those
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don't need to sleep, and can safely be done from inside hard (non-threaded) IRQ
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handlers and similar contexts.
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Use the following calls to access GPIOs from an atomic context:
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int gpiod_get_value(const struct gpio_desc *desc);
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void gpiod_set_value(struct gpio_desc *desc, int value);
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The values are boolean, zero for low, nonzero for high. When reading the value
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of an output pin, the value returned should be what's seen on the pin. That
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won't always match the specified output value, because of issues including
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open-drain signaling and output latencies.
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The get/set calls do not return errors because "invalid GPIO" should have been
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reported earlier from gpiod_direction_*(). However, note that not all platforms
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can read the value of output pins; those that can't should always return zero.
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Also, using these calls for GPIOs that can't safely be accessed without sleeping
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(see below) is an error.
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GPIO Access That May Sleep
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--------------------------
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Some GPIO controllers must be accessed using message based buses like I2C or
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SPI. Commands to read or write those GPIO values require waiting to get to the
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head of a queue to transmit a command and get its response. This requires
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sleeping, which can't be done from inside IRQ handlers.
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Platforms that support this type of GPIO distinguish them from other GPIOs by
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returning nonzero from this call:
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int gpiod_cansleep(const struct gpio_desc *desc)
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To access such GPIOs, a different set of accessors is defined:
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int gpiod_get_value_cansleep(const struct gpio_desc *desc)
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void gpiod_set_value_cansleep(struct gpio_desc *desc, int value)
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Accessing such GPIOs requires a context which may sleep, for example a threaded
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IRQ handler, and those accessors must be used instead of spinlock-safe
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accessors without the cansleep() name suffix.
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Other than the fact that these accessors might sleep, and will work on GPIOs
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that can't be accessed from hardIRQ handlers, these calls act the same as the
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spinlock-safe calls.
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Active-low State and Raw GPIO Values
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------------------------------------
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Device drivers like to manage the logical state of a GPIO, i.e. the value their
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device will actually receive, no matter what lies between it and the GPIO line.
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In some cases, it might make sense to control the actual GPIO line value. The
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following set of calls ignore the active-low property of a GPIO and work on the
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raw line value:
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int gpiod_get_raw_value(const struct gpio_desc *desc)
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void gpiod_set_raw_value(struct gpio_desc *desc, int value)
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int gpiod_get_raw_value_cansleep(const struct gpio_desc *desc)
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void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value)
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int gpiod_direction_output_raw(struct gpio_desc *desc, int value)
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The active-low state of a GPIO can also be queried using the following call:
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int gpiod_is_active_low(const struct gpio_desc *desc)
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Note that these functions should only be used with great moderation ; a driver
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should not have to care about the physical line level.
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GPIOs mapped to IRQs
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--------------------
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GPIO lines can quite often be used as IRQs. You can get the IRQ number
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corresponding to a given GPIO using the following call:
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int gpiod_to_irq(const struct gpio_desc *desc)
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It will return an IRQ number, or an negative errno code if the mapping can't be
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done (most likely because that particular GPIO cannot be used as IRQ). It is an
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unchecked error to use a GPIO that wasn't set up as an input using
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gpiod_direction_input(), or to use an IRQ number that didn't originally come
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from gpiod_to_irq(). gpiod_to_irq() is not allowed to sleep.
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Non-error values returned from gpiod_to_irq() can be passed to request_irq() or
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free_irq(). They will often be stored into IRQ resources for platform devices,
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by the board-specific initialization code. Note that IRQ trigger options are
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part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are system wakeup
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capabilities.
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Interacting With the Legacy GPIO Subsystem
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==========================================
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Many kernel subsystems still handle GPIOs using the legacy integer-based
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interface. Although it is strongly encouraged to upgrade them to the safer
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descriptor-based API, the following two functions allow you to convert a GPIO
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descriptor into the GPIO integer namespace and vice-versa:
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int desc_to_gpio(const struct gpio_desc *desc)
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struct gpio_desc *gpio_to_desc(unsigned gpio)
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The GPIO number returned by desc_to_gpio() can be safely used as long as the
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GPIO descriptor has not been freed. All the same, a GPIO number passed to
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gpio_to_desc() must have been properly acquired, and usage of the returned GPIO
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descriptor is only possible after the GPIO number has been released.
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Freeing a GPIO obtained by one API with the other API is forbidden and an
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unchecked error.
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