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

andy-shev · rafaeljw · commit ec3576eac11d · 2021-04-28T19:11:13.000+02:00
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 &lt;andriy.shevchenko@linux.intel.com&gt;
Signed-off-by: Rafael J. Wysocki &lt;rafael.j.wysocki@intel.com&gt;
diff --git 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
 ==========================
 

Original file line number	Diff line number	Diff line change
`@@ -99,6 +99,12 @@ native::`
`99`	`99`	`}`
`100`	`100`	`}`
`101`	`101`
	`102`	`+Note, that historically ACPI has no means of the GPIO polarity and thus`
	`103`	`+the SPISerialBus() resource defines it on the per-chip basis. In order`
	`104`	`+to avoid a chain of negations, the GPIO polarity is considered being`
	`105`	`+Active High. Even for the cases when _DSD() is involved (see the example`
	`106`	`+above) the GPIO CS polarity must be defined Active High to avoid ambiguity.`
	`107`	`+`
`102`	`108`	`Other supported properties`
`103`	`109`	`==========================`
`104`	`110`