linux-kernel - Re: [PATCH v3] thermal: sysfs: Return ENODATA instead of EAGAIN for reads

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Message-ID: <CAJZ5v0gUzFGxbR+en2JUaFvGt5vGY8z3LAJOba9f8RHoPrTvWQ@mail.gmail.com>
Date: Thu, 3 Jul 2025 16:44:54 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Hsin-Te Yuan <yuanhsinte@...omium.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Daniel Lezcano <daniel.lezcano@...aro.org>, 
	Zhang Rui <rui.zhang@...el.com>, Lukasz Luba <lukasz.luba@....com>, linux-pm@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] thermal: sysfs: Return ENODATA instead of EAGAIN for reads

On Fri, Jun 20, 2025 at 12:41 PM Hsin-Te Yuan <yuanhsinte@...omium.org> wrote:
>
> According to POSIX spec, EAGAIN returned by read with O_NONBLOCK set
> means the read would block. Hence, the common implementation in
> nonblocking model will poll the file when the nonblocking read returns
> EAGAIN. However, when the target file is thermal zone, this mechanism
> will totally malfunction because thermal zone doesn't implement sysfs
> notification and thus the poll will never return.
>
> For example, the read in Golang implemnts such method and sometimes
> hangs at reading some thermal zones via sysfs.
>
> Change to throw ENODATA instead of EAGAIN to userspace.
>
> Signed-off-by: Hsin-Te Yuan <yuanhsinte@...omium.org>
> ---
> Changes in v3:
> - Refine the control flow and optimize the success case.
> - Link to v2: https://lore.kernel.org/r/20250512-temp-v2-1-048be58eaaa5@chromium.org
>
> Changes in v2:
> - Modify commit message to make it clear
> - Link to v1: https://lore.kernel.org/r/20250409-temp-v1-1-9a391d8c60fd@chromium.org
> ---
>  drivers/thermal/thermal_sysfs.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/thermal/thermal_sysfs.c b/drivers/thermal/thermal_sysfs.c
> index 24b9055a0b6c515b865e0d7e2db1d0de176ff767..d80612506a334ab739e7545cdfe984ab4dffab7c 100644
> --- a/drivers/thermal/thermal_sysfs.c
> +++ b/drivers/thermal/thermal_sysfs.c
> @@ -40,10 +40,13 @@ temp_show(struct device *dev, struct device_attribute *attr, char *buf)
>
>         ret = thermal_zone_get_temp(tz, &temperature);
>
> -       if (ret)
> -               return ret;
> +       if (!ret)
> +               return sprintf(buf, "%d\n", temperature);
>
> -       return sprintf(buf, "%d\n", temperature);
> +       if (ret == -EAGAIN)
> +               return -ENODATA;
> +
> +       return ret;
>  }
>
>  static ssize_t
>
> ---

Applied as 6.17 material, thanks!