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docs: gpio: prefer pread(2) for interrupt reading

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In legacy sysfs GPIO, when using poll(2) on the sysfs GPIO value for
state change awaiting, a subsequent read(2) is required for consuming
the event, which the doc recommends the use of lseek(2) or
close-and-reopen to reset the file offset afterwards.

The recommendations however, require at least 2 syscalls to consume
the event. Gladly, use of pread(2) require only 1 syscall for the
consumption. Let's advertise this usage by prioritizing its placement.

Signed-off-by: Huichun Feng <foxhoundsk.tw@gmail.com>
Link: https://lore.kernel.org/r/20240609173728.2950808-1-foxhoundsk.tw@gmail.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
This commit is contained in:
Huichun Feng 2024-06-10 01:37:28 +08:00 committed by Bartosz Golaszewski
parent 54a687cd49
commit 7f1e45f4ae
1 changed files with 4 additions and 3 deletions
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7
Documentation/userspace-api/gpio/sysfs.rst
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@ -97,9 +97,10 @@ and have the following read/write attributes:
poll(2) will return whenever the interrupt was triggered. If
you use poll(2), set the events POLLPRI and POLLERR. If you
use select(2), set the file descriptor in exceptfds. After
poll(2) returns, either lseek(2) to the beginning of the sysfs
file and read the new value or close the file and re-open it
to read the value.
poll(2) returns, use pread(2) to read the value at offset
zero. Alternatively, either lseek(2) to the beginning of the
sysfs file and read the new value or close the file and
re-open it to read the value.
"edge" ...
reads as either "none", "rising", "falling", or