linux-kernel - Re: [PATCH v1] ACPI: reboot: Increase the delay to avoid racing after writing to ACPI RESET

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Message-ID: <CAJZ5v0hTsEzFKvRf-MHpUxyJdFDUqc2ZL63s6BkyJyFtEzxvhw@mail.gmail.com>
Date:   Wed, 7 Jun 2023 13:19:42 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     James Liu <james.liu@....com>
Cc:     rafael@...nel.org, lenb@...nel.org, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org, hpelk@...ts.osp.hpe.com
Subject: Re: [PATCH v1] ACPI: reboot: Increase the delay to avoid racing after
 writing to ACPI RESET_REG on AMD Milan platforms.

On Wed, Jun 7, 2023 at 5:44 AM James Liu <james.liu@....com> wrote:
>
> For AMD Milan platforms, the delay of 15ms is insufficient to avoid racing
> of reboot mechanisms. That said, the AMD Milan processors don't reboot
> in 15ms after invoking acpi_reset().
>
> The proposed 50ms delay can effectively work around this issue.
> This extended delay aligns better with ACPI v6.4 (i.e., sec. 4.8.4.6),
> which indicates that ideally OSPM should execute spin loops on the CPUs
> in the system following a write to this register.
>
> Signed-off-by: James Liu <james.liu@....com>

Why do you want to affect everyone (including guest kernels running in
virtual machines AFAICS) in order to address a problem specific to one
platform?

Wouldn't it be better to quirk that platform and document the quirk properly?

> ---
>  drivers/acpi/reboot.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/acpi/reboot.c b/drivers/acpi/reboot.c
> index b79b7c99c237..002f7c7814a1 100644
> --- a/drivers/acpi/reboot.c
> +++ b/drivers/acpi/reboot.c
> @@ -78,5 +78,5 @@ void acpi_reboot(void)
>          * The 15ms delay has been found to be long enough for the system
>          * to reboot on the affected platforms.
>          */
> -       mdelay(15);
> +       mdelay(50);
>  }
> --
> 2.40.1
>