Commit 08d94c74 authored by Kent Gibson's avatar Kent Gibson Committed by Bartosz Golaszewski
Browse files

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: default avatarDavid C. Rankin <drankinatty@gmail.com>
Signed-off-by: default avatarKent Gibson <warthog618@gmail.com>
Link: https://lore.kernel.org/r/20240610092157.9147-3-warthog618@gmail.com


Signed-off-by: default avatarBartosz Golaszewski <bartosz.golaszewski@linaro.org>
parent 5ca84d41

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

+5 −0
Original line number Diff line number Diff line
@@ -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

+5 −0
Original line number Diff line number Diff line
@@ -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()`.