linux-kernel - [PATCH v2] docs: gpio: prefer pread(2) for interrupt reading

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Date: Mon, 10 Jun 2024 01:37:28 +0800
From: Huichun Feng <foxhoundsk.tw@...il.com>
To: brgl@...ev.pl
Cc: corbet@....net,
	foxhoundsk.tw@...il.com,
	linus.walleij@...aro.org,
	linux-doc@...r.kernel.org,
	linux-gpio@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	warthog618@...il.com
Subject: [PATCH v2] docs: gpio: prefer pread(2) for interrupt reading

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@...il.com>
---
Changes in v2:
- rebase against the latest tree.

v1 link:
https://lore.kernel.org/linux-gpio/20240514041223.800811-1-foxhoundsk.tw@gmail.com/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