Re: [PATCH] pmdomain: mediatek: fix race condition in power on/power off sequences

[Eugen Hristev]

On 11/29/23 14:52, AngeloGioacchino Del Regno wrote:
> Il 29/11/23 12:31, Eugen Hristev ha scritto:
> > It can happen that during the power off sequence for a power domain
> > another power on sequence is started, and it can lead to powering on and
> > off in the same time for the similar power domain.
> > This can happen if parallel probing occurs: one device starts probing, 
> > and
> > one power domain is probe deferred, this leads to all power domains being
> > rolled back and powered off, while in the same time another device starts
> > probing and requests powering on the same power domains or similar.
> >
> > This was encountered on MT8186, when the sequence is :
> > Power on SSUSB
> > Power on SSUSB_P1
> > Power on DIS
> >     -> probe deferred
> > Power off DIS
> > Power off SSUSB_P1
> > Power off SSUSB
> >
> > During the sequence of powering off SSUSB, some new similar sequence 
> > starts,
> > and during the power on of SSUSB, clocks are enabled.
> > In this case, powering off SSUSB fails from the first sequence, because
> > power off ACK bit check times out (as clocks are powered back on by 
> > the second
> > sequence). In consequence, powering it on also times out, and it leads to
> > the whole power domain in a bad state.
> >
> > To solve this issue, added a mutex that locks the whole power 
> > off/power on
> > sequence such that it would never happen that multiple sequences try to
> > enable or disable the same power domain in parallel.
> >
> > Fixes: 59b644b01cf4 ("soc: mediatek: Add MediaTek SCPSYS power domains")
> > Signed-off-by: Eugen Hristev <eugen.hristev@xxxxxxxxxxxxx>
>
> I don't think that it's a race between genpd_power_on() and 
> genpd_power_off() calls
> at all, because genpd *does* have locking after all... at least for 
> probe and for
> parents of a power domain (and more anyway).
>
> As far as I remember, what happens when you start .probe()'ing a device is:
> platform_probe() -> dev_pm_domain_attach() -> genpd_dev_pm_attach()
>
> There, you end up with
>
>      if (power_on) {
>          genpd_lock(pd);
>          ret = genpd_power_on(pd, 0);
>          genpd_unlock(pd);
>      }
>
> ...but when you fail probing, you go with genpd_dev_pm_detach(), which 
> then calls
>
>      /* Check if PM domain can be powered off after removing this 
> device. */
>      genpd_queue_power_off_work(pd);
>
> but even then, you end up being in a worker doing
>
>      genpd_lock(genpd);
>      genpd_power_off(genpd, false, 0);
>      genpd_unlock(genpd);
>
> ...so I don't understand why this mutex can resolve the situation here 
> (also: are
> you really sure that the race is solved like that?)
>
> I'd say that this probably needs more justification and a trace of the 
> actual
> situation here.
>
> Besides, if this really resolves the issue, I would prefer seeing 
> variants of
> scpsys_power_{on,off}() functions, because we anyway don't need to lock 
> mutexes
> during this driver's probe (add_subdomain calls scpsys_power_on()).
> In that case, `scpsys_power_on_unlocked()` would be an idea... but 
> still, please
> analyze why your solution works, if it does, because I'm not convinced.

What I see in my tests, is that a power on call for SSUSB domain happens 
while the previous power off sequence did not yet complete, most likely 
while it's waiting in readx_poll_timeout . This leads to inconsistency 
of the power domain, not getting the ACKs next time a power on attempt 
occurs.

I understand what you say about locks, but in this case the powering off 
is not called by the genpd itself, but rather it's called by the 
rollback probe failed mechanism : when the probing fails, 
scpsys_domain_cleanup() is called during the same probing session.
Then it happens that probing begins again and previous cleanup is not 
yet completed. I am not sure whether the lock is still held from the 
previous run, but it's clearly not waiting for a lock to be released to 
be called again.

> Cheers,
> Angelo
>
> > ---
> >   drivers/pmdomain/mediatek/mtk-pm-domains.c | 24 +++++++++++++++++-----
> >   1 file changed, 19 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/pmdomain/mediatek/mtk-pm-domains.c 
> > b/drivers/pmdomain/mediatek/mtk-pm-domains.c
> > index d5f0ee05c794..4f136b47e539 100644
> > --- a/drivers/pmdomain/mediatek/mtk-pm-domains.c
> > +++ b/drivers/pmdomain/mediatek/mtk-pm-domains.c
> > @@ -9,6 +9,7 @@
> >   #include <linux/io.h>
> >   #include <linux/iopoll.h>
> >   #include <linux/mfd/syscon.h>
> > +#include <linux/mutex.h>
> >   #include <linux/of.h>
> >   #include <linux/of_clk.h>
> >   #include <linux/platform_device.h>
> > @@ -56,6 +57,7 @@ struct scpsys {
> >       struct device *dev;
> >       struct regmap *base;
> >       const struct scpsys_soc_data *soc_data;
> > +    struct mutex mutex;
> >       struct genpd_onecell_data pd_data;
> >       struct generic_pm_domain *domains[];
> >   };
> > @@ -238,9 +240,13 @@ static int scpsys_power_on(struct 
> > generic_pm_domain *genpd)
> >       bool tmp;
> >       int ret;
> > +    mutex_lock(&scpsys->mutex);
> > +
> >       ret = scpsys_regulator_enable(pd->supply);
> > -    if (ret)
> > +    if (ret) {
> > +        mutex_unlock(&scpsys->mutex);
> >           return ret;
> > +    }
> >       ret = clk_bulk_prepare_enable(pd->num_clks, pd->clks);
> >       if (ret)
> > @@ -291,6 +297,7 @@ static int scpsys_power_on(struct 
> > generic_pm_domain *genpd)
> >               goto err_enable_bus_protect;
> >       }
> > +    mutex_unlock(&scpsys->mutex);
> >       return 0;
> >   err_enable_bus_protect:
> > @@ -305,6 +312,7 @@ static int scpsys_power_on(struct 
> > generic_pm_domain *genpd)
> >       clk_bulk_disable_unprepare(pd->num_clks, pd->clks);
> >   err_reg:
> >       scpsys_regulator_disable(pd->supply);
> > +    mutex_unlock(&scpsys->mutex);
> >       return ret;
> >   }
> > @@ -315,13 +323,15 @@ static int scpsys_power_off(struct 
> > generic_pm_domain *genpd)
> >       bool tmp;
> >       int ret;
> > +    mutex_lock(&scpsys->mutex);
> > +
> >       ret = scpsys_bus_protect_enable(pd);
> >       if (ret < 0)
> > -        return ret;
> > +        goto err_mutex_unlock;
> >       ret = scpsys_sram_disable(pd);
> >       if (ret < 0)
> > -        return ret;
> > +        goto err_mutex_unlock;
> >       if (pd->data->ext_buck_iso_offs && MTK_SCPD_CAPS(pd, 
> > MTK_SCPD_EXT_BUCK_ISO))
> >           regmap_set_bits(scpsys->base, pd->data->ext_buck_iso_offs,
> > @@ -340,13 +350,15 @@ static int scpsys_power_off(struct 
> > generic_pm_domain *genpd)
> >       ret = readx_poll_timeout(scpsys_domain_is_on, pd, tmp, !tmp, 
> > MTK_POLL_DELAY_US,
> >                    MTK_POLL_TIMEOUT);
> >       if (ret < 0)
> > -        return ret;
> > +        goto err_mutex_unlock;
> >       clk_bulk_disable_unprepare(pd->num_clks, pd->clks);
> >       scpsys_regulator_disable(pd->supply);
> > -    return 0;
> > +err_mutex_unlock:
> > +    mutex_unlock(&scpsys->mutex);
> > +    return ret;
> >   }
> >   static struct
> > @@ -700,6 +712,8 @@ static int scpsys_probe(struct platform_device *pdev)
> >           return PTR_ERR(scpsys->base);
> >       }
> > +    mutex_init(&scpsys->mutex);
> > +
> >       ret = -ENODEV;
> >       for_each_available_child_of_node(np, node) {
> >           struct generic_pm_domain *domain;