linux-kernel - Re: [PATCH] acpi-cpufreq: Skip initialization if a cpufreq driver exists

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Date:   Mon, 24 May 2021 16:16:13 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Kyle Meyer <kyle.meyer@....com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>, linux-acpi@...r.kernel.org
Subject: Re: [PATCH] acpi-cpufreq: Skip initialization if a cpufreq driver exists

On Sat, May 22, 2021 at 12:19 AM Kyle Meyer <kyle.meyer@....com> wrote:
>
> Revert part of commit 75c0758137c7a
> ("acpi-cpufreq: Fail initialization if driver cannot be registered").
>
> acpi-cpufreq is mutually exclusive with intel_pstate, however,
> acpi-cpufreq is loaded multiple times during startup while intel_pstate is
> enabled. On systems using systemd the kernel triggers one uevent for each
> device as a result of systemd-udev-trigger.service. The service exists to
> retrigger all devices as uevents sent by the kernel before systemd-udevd
> is running are missed. The delay caused by systemd-udevd repeatedly loading
> the driver, getting a fail return, and unloading the driver twice per
> logical CPU has a significant impact on the startup time, and can cause
> some devices to be unavailable after reaching the root login prompt.
>
> Load the driver once but skip initialization if a cpufreq driver exists by
> changing the return value of cpufreq_get_current_driver() from -EEXIST to
> 0.
>
> Signed-off-by: Kyle Meyer <kyle.meyer@....com>
> ---
>  drivers/cpufreq/acpi-cpufreq.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/cpufreq/acpi-cpufreq.c b/drivers/cpufreq/acpi-cpufreq.c
> index 7e7450453714..e79a945369d1 100644
> --- a/drivers/cpufreq/acpi-cpufreq.c
> +++ b/drivers/cpufreq/acpi-cpufreq.c
> @@ -1003,7 +1003,7 @@ static int __init acpi_cpufreq_init(void)
>
>         /* don't keep reloading if cpufreq_driver exists */
>         if (cpufreq_get_current_driver())
> -               return -EEXIST;
> +               return 0;
>
>         pr_debug("%s\n", __func__);
>
> --

Applied as 5.14 material with some edits in the subject and changelog, thanks!