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| -rw-r--r-- | Documentation/pinctrl.txt | 112 |
1 files changed, 112 insertions, 0 deletions
diff --git a/Documentation/pinctrl.txt b/Documentation/pinctrl.txt index a2b57e0a1db0..447fd4cd54ec 100644 --- a/Documentation/pinctrl.txt +++ b/Documentation/pinctrl.txt @@ -736,6 +736,13 @@ All the above functions are mandatory to implement for a pinmux driver. Pin control interaction with the GPIO subsystem =============================================== +Note that the following implies that the use case is to use a certain pin +from the Linux kernel using the API in <linux/gpio.h> with gpio_request() +and similar functions. There are cases where you may be using something +that your datasheet calls "GPIO mode" but actually is just an electrical +configuration for a certain device. See the section below named +"GPIO mode pitfalls" for more details on this scenario. + The public pinmux API contains two functions named pinctrl_request_gpio() and pinctrl_free_gpio(). These two functions shall *ONLY* be called from gpiolib-based drivers as part of their gpio_request() and @@ -774,6 +781,111 @@ obtain the function "gpioN" where "N" is the global GPIO pin number if no special GPIO-handler is registered. +GPIO mode pitfalls +================== + +Sometime the developer may be confused by a datasheet talking about a pin +being possible to set into "GPIO mode". It appears that what hardware +engineers mean with "GPIO mode" is not necessarily the use case that is +implied in the kernel interface <linux/gpio.h>: a pin that you grab from +kernel code and then either listen for input or drive high/low to +assert/deassert some external line. + +Rather hardware engineers think that "GPIO mode" means that you can +software-control a few electrical properties of the pin that you would +not be able to control if the pin was in some other mode, such as muxed in +for a device. + +Example: a pin is usually muxed in to be used as a UART TX line. But during +system sleep, we need to put this pin into "GPIO mode" and ground it. + +If you make a 1-to-1 map to the GPIO subsystem for this pin, you may start +to think that you need to come up with something real complex, that the +pin shall be used for UART TX and GPIO at the same time, that you will grab +a pin control handle and set it to a certain state to enable UART TX to be +muxed in, then twist it over to GPIO mode and use gpio_direction_output() +to drive it low during sleep, then mux it over to UART TX again when you +wake up and maybe even gpio_request/gpio_free as part of this cycle. This +all gets very complicated. + +The solution is to not think that what the datasheet calls "GPIO mode" +has to be handled by the <linux/gpio.h> interface. Instead view this as +a certain pin config setting. Look in e.g. <linux/pinctrl/pinconf-generic.h> +and you find this in the documentation: + + PIN_CONFIG_OUTPUT: this will configure the pin in output, use argument + 1 to indicate high level, argument 0 to indicate low level. + +So it is perfectly possible to push a pin into "GPIO mode" and drive the +line low as part of the usual pin control map. So for example your UART +driver may look like this: + +#include <linux/pinctrl/consumer.h> + +struct pinctrl *pinctrl; +struct pinctrl_state *pins_default; +struct pinctrl_state *pins_sleep; + +pins_default = pinctrl_lookup_state(uap->pinctrl, PINCTRL_STATE_DEFAULT); +pins_sleep = pinctrl_lookup_state(uap->pinctrl, PINCTRL_STATE_SLEEP); + +/* Normal mode */ +retval = pinctrl_select_state(pinctrl, pins_default); +/* Sleep mode */ +retval = pinctrl_select_state(pinctrl, pins_sleep); + +And your machine configuration may look like this: +-------------------------------------------------- + +static unsigned long uart_default_mode[] = { + PIN_CONF_PACKED(PIN_CONFIG_DRIVE_PUSH_PULL, 0), +}; + +static unsigned long uart_sleep_mode[] = { + PIN_CONF_PACKED(PIN_CONFIG_OUTPUT, 0), +}; + +static struct pinctrl_map __initdata pinmap[] = { + PIN_MAP_MUX_GROUP("uart", PINCTRL_STATE_DEFAULT, "pinctrl-foo", + "u0_group", "u0"), + PIN_MAP_CONFIGS_PIN("uart", PINCTRL_STATE_DEFAULT, "pinctrl-foo", + "UART_TX_PIN", uart_default_mode), + PIN_MAP_MUX_GROUP("uart", PINCTRL_STATE_SLEEP, "pinctrl-foo", + "u0_group", "gpio-mode"), + PIN_MAP_CONFIGS_PIN("uart", PINCTRL_STATE_SLEEP, "pinctrl-foo", + "UART_TX_PIN", uart_sleep_mode), +}; + +foo_init(void) { + pinctrl_register_mappings(pinmap, ARRAY_SIZE(pinmap)); +} + +Here the pins we want to control are in the "u0_group" and there is some +function called "u0" that can be enabled on this group of pins, and then +everything is UART business as usual. But there is also some function +named "gpio-mode" that can be mapped onto the same pins to move them into +GPIO mode. + +This will give the desired effect without any bogus interaction with the +GPIO subsystem. It is just an electrical configuration used by that device +when going to sleep, it might imply that the pin is set into something the +datasheet calls "GPIO mode" but that is not the point: it is still used +by that UART device to control the pins that pertain to that very UART +driver, putting them into modes needed by the UART. GPIO in the Linux +kernel sense are just some 1-bit line, and is a different use case. + +How the registers are poked to attain the push/pull and output low +configuration and the muxing of the "u0" or "gpio-mode" group onto these +pins is a question for the driver. + +Some datasheets will be more helpful and refer to the "GPIO mode" as +"low power mode" rather than anything to do with GPIO. This often means +the same thing electrically speaking, but in this latter case the +software engineers will usually quickly identify that this is some +specific muxing/configuration rather than anything related to the GPIO +API. + + Board/machine configuration ================================== |