Documentation: gpio: Clarify effect of active low flag on line edges

The documentation does not make sufficiently clear that edge polarity is
based on changes to the logical line values, and that the physical
polarity of edges is dependent on the active low flag.

Clarify the relationship between the active low flag and edge polarity
for the functions that read edge events.

Suggested-by: David C. Rankin <[email protected]>
Signed-off-by: Kent Gibson <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Bartosz Golaszewski <[email protected]>

Author: warthog618

Documentation/userspace-api/gpio/gpio-lineevent-data-read.rst

@@ -44,6 +44,11 @@ Edge detection must be enabled for the input line using either
 both. Edge events are then generated whenever edge interrupts are detected on
 the input line.
 
+Edges are defined in terms of changes to the logical line value, so an inactive
+to active transition is a rising edge.  If ``GPIOHANDLE_REQUEST_ACTIVE_LOW`` is
+set then logical polarity is the opposite of physical polarity, and
+``GPIOEVENT_REQUEST_RISING_EDGE`` then corresponds to a falling physical edge.
+
 The kernel captures and timestamps edge events as close as possible to their
 occurrence and stores them in a buffer from where they can be read by
 userspace at its convenience using `read()`.

Documentation/userspace-api/gpio/gpio-v2-line-event-read.rst

@@ -40,6 +40,11 @@ Edge detection must be enabled for the input line using either
 both. Edge events are then generated whenever edge interrupts are detected on
 the input line.
 
+Edges are defined in terms of changes to the logical line value, so an inactive
+to active transition is a rising edge.  If ``GPIO_V2_LINE_FLAG_ACTIVE_LOW`` is
+set then logical polarity is the opposite of physical polarity, and
+``GPIO_V2_LINE_FLAG_EDGE_RISING`` then corresponds to a falling physical edge.
+
 The kernel captures and timestamps edge events as close as possible to their
 occurrence and stores them in a buffer from where they can be read by
 userspace at its convenience using `read()`.