linux-kernel - Re: [PATCH] pmdomain: mediatek: fix race conditions with genpd

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Date: Tue, 23 Jan 2024 13:22:12 +0100
From: Ulf Hansson <ulf.hansson@...aro.org>
To: Eugen Hristev <eugen.hristev@...labora.com>
Cc: matthias.bgg@...il.com, angelogioacchino.delregno@...labora.com, 
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-mediatek@...ts.infradead.org, 
	kernel@...labora.com
Subject: Re: [PATCH] pmdomain: mediatek: fix race conditions with genpd

On Mon, 25 Dec 2023 at 14:36, Eugen Hristev <eugen.hristev@...labora.com> wrote:
>
> If the power domains are registered first with genpd and *after that*
> the driver attempts to power them on in the probe sequence, then it is
> possible that a race condition occurs if genpd tries to power them on
> in the same time.
> The same is valid for powering them off before unregistering them
> from genpd.

Right. When the PM domain has been registered with genpd, attempts to
power-on/off the PM domain need to be synchronized with genpd.

> Attempt to fix race conditions by first removing the domains from genpd
> and *after that* powering down domains.
> Also first power up the domains and *after that* register them
> to genpd.

This seems like a reasonable approach to me.

>
> Fixes: 59b644b01cf4 ("soc: mediatek: Add MediaTek SCPSYS power domains")
> Signed-off-by: Eugen Hristev <eugen.hristev@...labora.com>

Applied for fixes and by adding a stable tag, thanks! Although,
please, see some more comments below.

> ---
>
> This comes as another way to fix the problem as described in this thread:
> https://lore.kernel.org/linux-arm-kernel/20231129113120.4907-1-eugen.hristev@collabora.com/
>
> I have not been able to reproduce the problem with either fix anymore
> (so far).
>
> I have a few doubts about this one though, if I really covered the
> way it's supposed to work, and registering the pmdomains in the recursive
> function in the reversed order has any side effect or if it does not
> work correctly.
> Tested on mt8186 where it appears to be fine.

I had a quick look at the code in the driver and a few things caught my eyes.

*) The error path in scpsys_probe() doesn't seem to handle removal of
the link between parent/child-domains (subdomains).
**) An option that might simplify the code and error path too, could
be to convert into using of_genpd_add|remove_subdomain() in favor or
pm_genpd_add|remove_subdomain().

Kind regards
Uffe


>
> Eugen
>
>  drivers/pmdomain/mediatek/mtk-pm-domains.c | 15 +++++++--------
>  1 file changed, 7 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/pmdomain/mediatek/mtk-pm-domains.c b/drivers/pmdomain/mediatek/mtk-pm-domains.c
> index e26dc17d07ad..e274e3315fe7 100644
> --- a/drivers/pmdomain/mediatek/mtk-pm-domains.c
> +++ b/drivers/pmdomain/mediatek/mtk-pm-domains.c
> @@ -561,6 +561,11 @@ static int scpsys_add_subdomain(struct scpsys *scpsys, struct device_node *paren
>                         goto err_put_node;
>                 }
>
> +               /* recursive call to add all subdomains */
> +               ret = scpsys_add_subdomain(scpsys, child);
> +               if (ret)
> +                       goto err_put_node;
> +
>                 ret = pm_genpd_add_subdomain(parent_pd, child_pd);
>                 if (ret) {
>                         dev_err(scpsys->dev, "failed to add %s subdomain to parent %s\n",
> @@ -570,11 +575,6 @@ static int scpsys_add_subdomain(struct scpsys *scpsys, struct device_node *paren
>                         dev_dbg(scpsys->dev, "%s add subdomain: %s\n", parent_pd->name,
>                                 child_pd->name);
>                 }
> -
> -               /* recursive call to add all subdomains */
> -               ret = scpsys_add_subdomain(scpsys, child);
> -               if (ret)
> -                       goto err_put_node;
>         }
>
>         return 0;
> @@ -588,9 +588,6 @@ static void scpsys_remove_one_domain(struct scpsys_domain *pd)
>  {
>         int ret;
>
> -       if (scpsys_domain_is_on(pd))
> -               scpsys_power_off(&pd->genpd);
> -
>         /*
>          * We're in the error cleanup already, so we only complain,
>          * but won't emit another error on top of the original one.
> @@ -600,6 +597,8 @@ static void scpsys_remove_one_domain(struct scpsys_domain *pd)
>                 dev_err(pd->scpsys->dev,
>                         "failed to remove domain '%s' : %d - state may be inconsistent\n",
>                         pd->genpd.name, ret);
> +       if (scpsys_domain_is_on(pd))
> +               scpsys_power_off(&pd->genpd);
>
>         clk_bulk_put(pd->num_clks, pd->clks);
>         clk_bulk_put(pd->num_subsys_clks, pd->subsys_clks);
> --
> 2.34.1
>
>