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Diffstat (limited to 'Documentation/gpio/driver.txt')
| -rw-r--r-- | Documentation/gpio/driver.txt | 64 |
1 files changed, 51 insertions, 13 deletions
diff --git a/Documentation/gpio/driver.txt b/Documentation/gpio/driver.txt index 747c721776ed..fc1d2f83564d 100644 --- a/Documentation/gpio/driver.txt +++ b/Documentation/gpio/driver.txt @@ -41,34 +41,71 @@ In the gpiolib framework each GPIO controller is packaged as a "struct gpio_chip" (see linux/gpio/driver.h for its complete definition) with members common to each controller of that type: - - methods to establish GPIO direction - - methods used to access GPIO values - - method to return the IRQ number associated to a given GPIO + - methods to establish GPIO line direction + - methods used to access GPIO line values + - method to set electrical configuration to a a given GPIO line + - method to return the IRQ number associated to a given GPIO line - flag saying whether calls to its methods may sleep + - optional line names array to identify lines - optional debugfs dump method (showing extra state like pullup config) - optional base number (will be automatically assigned if omitted) - - label for diagnostics and GPIOs mapping using platform data + - optional label for diagnostics and GPIO chip mapping using platform data The code implementing a gpio_chip should support multiple instances of the controller, possibly using the driver model. That code will configure each -gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be rare; -use gpiochip_remove() when it is unavoidable. +gpio_chip and issue gpiochip_add[_data]() or devm_gpiochip_add_data(). +Removing a GPIO controller should be rare; use [devm_]gpiochip_remove() when +it is unavoidable. -Most often a gpio_chip is part of an instance-specific structure with state not +Often a gpio_chip is part of an instance-specific structure with states not exposed by the GPIO interfaces, such as addressing, power management, and more. -Chips such as codecs will have complex non-GPIO state. +Chips such as audio codecs will have complex non-GPIO states. 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. -RT_FULL: GPIO driver should not use spinlock_t or any sleepable APIs +RT_FULL: the GPIO driver should not use spinlock_t or any sleepable APIs (like PM runtime) in its gpio_chip implementation (.get/.set and direction control callbacks) if it is expected to call GPIO APIs from atomic context on -RT (inside hard IRQ handlers and similar contexts). Normally this should not be required. +GPIO electrical configuration +----------------------------- + +GPIOs can be configured for several electrical modes of operation by using the +.set_config() callback. Currently this API supports setting debouncing and +single-ended modes (open drain/open source). These settings are described +below. + +The .set_config() callback uses the same enumerators and configuration +semantics as the generic pin control drivers. This is not a coincidence: it is +possible to assign the .set_config() to the function gpiochip_generic_config() +which will result in pinctrl_gpio_set_config() being called and eventually +ending up in the pin control back-end "behind" the GPIO controller, usually +closer to the actual pins. This way the pin controller can manage the below +listed GPIO configurations. + + +GPIOs with debounce support +--------------------------- + +Debouncing is a configuration set to a pin indicating that it is connected to +a mechanical switch or button, or similar that may bounce. Bouncing means the +line is pulled high/low quickly at very short intervals for mechanical +reasons. This can result in the value being unstable or irqs fireing repeatedly +unless the line is debounced. + +Debouncing in practice involves setting up a timer when something happens on +the line, wait a little while and then sample the line again, so see if it +still has the same value (low or high). This could also be repeated by a clever +state machine, waiting for a line to become stable. In either case, it sets +a certain number of milliseconds for debouncing, or just "on/off" if that time +is not configurable. + + GPIOs with open drain/source support ------------------------------------ @@ -146,10 +183,11 @@ a pull-up resistor is needed on the outgoing rail to complete the circuit, and in the second case, a pull-down resistor is needed on the rail. Hardware that supports open drain or open source or both, can implement a -special callback in the gpio_chip: .set_single_ended() that takes an enum flag -telling whether to configure the line as open drain, open source or push-pull. -This will happen in response to the GPIO_OPEN_DRAIN or GPIO_OPEN_SOURCE flag -set in the machine file, or coming from other hardware descriptions. +special callback in the gpio_chip: .set_config() that takes a generic +pinconf packed value telling whether to configure the line as open drain, +open source or push-pull. This will happen in response to the +GPIO_OPEN_DRAIN or GPIO_OPEN_SOURCE flag set in the machine file, or coming +from other hardware descriptions. If this state can not be configured in hardware, i.e. if the GPIO hardware does not support open drain/open source in hardware, the GPIO library will instead |