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Documentation: firmware-guide: gpio-properties: Clarify initial output state 

GpioIo() doesn't provide an explicit state for an output pin.
Linux tries to be smart and uses a common sense based on other
parameters. Document how it looks like in the code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
This commit is contained in:
Andy Shevchenko 2020-10-29 21:32:43 +02:00 committed by Rafael J. Wysocki
parent 0d6c41cf80
commit 8b31e972f9
1 changed files with 23 additions and 0 deletions
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Documentation/firmware-guide/acpi/gpio-properties.rst
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@ -61,6 +61,29 @@ must be 0. GpioInt() resource has its own means of defining it.
In our Bluetooth example the "reset-gpios" refers to the second GpioIo()
resource, second pin in that resource with the GPIO number of 31.
The GpioIo() resource unfortunately doesn't explicitly provide an initial
state of the output pin which driver should use during its initialization.
Linux tries to use common sense here and derives the state from the bias
and polarity settings. The table below shows the expectations:
========= ============= ==============
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 low, assuming non-active
Up Low as high, assuming non-active
Up High as high, assuming active
========= ============= ==============
That said, for our above example the both GPIOs, since the bias setting
is explicit and _DSD is present, will be treated as active with a high
polarity and Linux will configure the pins in this state until a driver
reprograms them differently.
It is possible to leave holes in the array of GPIOs. This is useful in
cases like with SPI host controllers where some chip selects may be
implemented as GPIOs and some as native signals. For example a SPI host