Add some documentation provided by Andy Shevchenko to describe how to use struct acpi_gpio.
Signed-off-by: Simon Glass sjg@chromium.org ---
Changes in v4: - Add Andy's documentation to struct acpi_gpio
include/acpi/acpi_device.h | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+)
diff --git a/include/acpi/acpi_device.h b/include/acpi/acpi_device.h index 1b838fcb857..007b7e7caf1 100644 --- a/include/acpi/acpi_device.h +++ b/include/acpi/acpi_device.h @@ -170,6 +170,28 @@ enum acpi_gpio_polarity { * @io_shared; true if GPIO is shared * @io_restrict: I/O restriction setting * @polarity: GPIO polarity + * + * Note that GpioIo doesn't have any means of Active Low / High setting, so a + * _DSD must be provided to mitigate this. + * + * GpioIo doesn't properly communicate the initial state of the output pin, + * thus Linux assumes the simple rule: + * + * Pull Bias Polarity Requested... + * + * Implicit x AS IS (assumed firmware configured for us) + * Explicit x (no _DSD) as Pull Bias (Up == High, Down == Low), + * assuming non-active (Polarity = !Pull Bias) + * + * Down Low as low, assuming active + * Down High as high, assuming non-active + * Up Low as high, assuming non-active + * Up High as high, assuming active + * + * GpioIo() can be used as interrupt and in this case the IoRestriction mustn't + * be OutputOnly. It also requires active_low flag from _DSD in cases where it's + * needed (better to always provide than rely on above assumption made on OS + * level). */ struct acpi_gpio { int pin_count;