From ec3576eac11d66a388b6cba6a7cfb3b45039a712 Mon Sep 17 00:00:00 2001 From: Andy Shevchenko Date: Sat, 24 Apr 2021 17:39:35 +0300 Subject: [PATCH] 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 Signed-off-by: Rafael J. Wysocki --- Documentation/firmware-guide/acpi/gpio-properties.rst | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/Documentation/firmware-guide/acpi/gpio-properties.rst b/Documentation/firmware-guide/acpi/gpio-properties.rst index 4e264c16ddff..df4b711053ee 100644 --- 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 ==========================