Documentation: firmware-guide: gpio-properties: Add note to SPI CS case

Historically ACPI has no means of the GPIO polarity and thus
the SPISerialBus() resource defines it on the per-chip basis.
In order to avoid an ambiguity, the GPIO polarity is considered
being always Active High.

Add note about this to the respective documentation file.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

diff --git a/Documentation/firmware-guide/acpi/gpio-properties.rst b/Documentation/firmware-guide/acpi/gpio-properties.rst
index 4e264c16ddff640e29251d5c6892cf9d3472a354..df4b711053ee9860cc8da565c1e373a8ca58c183 100644 (file)
--- a/Documentation/firmware-guide/acpi/gpio-properties.rst
+++ b/Documentation/firmware-guide/acpi/gpio-properties.rst
@@ -99,6 +99,12 @@ native::
       }
   }
 
+Note, that historically ACPI has no means of the GPIO polarity and thus
+the SPISerialBus() resource defines it on the per-chip basis. In order
+to avoid a chain of negations, the GPIO polarity is considered being
+Active High. Even for the cases when _DSD() is involved (see the example
+above) the GPIO CS polarity must be defined Active High to avoid ambiguity.
+
 Other supported properties
 ==========================