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

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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 <drankinatty@gmail.com>
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Link: https://lore.kernel.org/r/20240610092157.9147-3-warthog618@gmail.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
This commit is contained in:
Kent Gibson 2024-06-10 17:21:57 +08:00 committed by Bartosz Golaszewski
parent 5ca84d416e
commit 08d94c7428
2 changed files with 10 additions and 0 deletions
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5
Documentation/userspace-api/gpio/gpio-lineevent-data-read.rst
View File

@ -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()`.

5
Documentation/userspace-api/gpio/gpio-v2-line-event-read.rst
View File

@ -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()`.