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@xxxxxxxxx> --- Changes in v2: - rebase against the latest tree. v1 link: https://lore.kernel.org/linux-gpio/20240514041223.800811-1-foxhoundsk.tw@xxxxxxxxx/T/#mfa19d877e6056936d3a2025cb64ca8c5b8718644 --- Documentation/userspace-api/gpio/sysfs.rst | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/Documentation/userspace-api/gpio/sysfs.rst b/Documentation/userspace-api/gpio/sysfs.rst index 116921048..bd64896de 100644 --- a/Documentation/userspace-api/gpio/sysfs.rst +++ b/Documentation/userspace-api/gpio/sysfs.rst @@ -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 -- 2.34.1
