@@ -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
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> --- .../firmware-guide/acpi/gpio-properties.rst | 23 +++++++++++++++++++ 1 file changed, 23 insertions(+)