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From: Huichun Feng <foxhoundsk.tw@gmail.com>
Date: Mon, 27 May 2024 15:05:26 +0800
Message-ID: <CAFbkdV175LdW_r04DhhG7pkE4xs9W5305Unk6RK06SiudvnzFQ@mail.gmail.com>
Subject: Re: [PATCH] docs: gpio: prefer pread(2) for interrupt reading
To: Linus Walleij <linus.walleij@linaro.org>, Bartosz Golaszewski <brgl@bgdev.pl>, 
	Jonathan Corbet <corbet@lwn.net>, Kent Gibson <warthog618@gmail.com>, linux-gpio@vger.kernel.org, 
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
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Kindly ping.

Sorry that the last ping wasn't sent in plain text.


Huichun Feng <foxhoundsk.tw@gmail.com> =E6=96=BC 2024=E5=B9=B45=E6=9C=8814=
=E6=97=A5 =E9=80=B1=E4=BA=8C =E4=B8=8B=E5=8D=8812:12=E5=AF=AB=E9=81=93=EF=
=BC=9A
>
> 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>
> ---
>  Documentation/driver-api/gpio/legacy.rst   | 6 ++++--
>  Documentation/userspace-api/gpio/sysfs.rst | 7 ++++---
>  2 files changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/driver-api/gpio/legacy.rst b/Documentation/dri=
ver-api/gpio/legacy.rst
> index b6505914791c..c1a083444b0c 100644
> --- a/Documentation/driver-api/gpio/legacy.rst
> +++ b/Documentation/driver-api/gpio/legacy.rst
> @@ -648,8 +648,10 @@ 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. If you use selec=
t(2),
>                 set the file descriptor in exceptfds. After poll(2) retur=
ns,
> -               either lseek(2) to the beginning of the sysfs file and re=
ad the
> -               new value or close the file and re-open it to read the va=
lue.
> +               use pread(2) to read the value at offset zero. Alternativ=
ely,
> +               either lseek(2) to the beginning of the sysfs file and re=
ad
> +               the new value or close the file and re-open it to read th=
e
> +               value.
>
>         "edge" ... reads as either "none", "rising", "falling", or
>                 "both". Write these strings to select the signal edge(s)
> diff --git a/Documentation/userspace-api/gpio/sysfs.rst b/Documentation/u=
serspace-api/gpio/sysfs.rst
> index 116921048b18..bd64896de91a 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 y=
ou
>                 use select(2), set the file descriptor in exceptfds. Afte=
r
> -               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
>