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

[AngeloGioacchino Del Regno]

Il 29/11/23 14:28, Eugen Hristev ha scritto:
> 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.

Sorry but I'm a bit lost now: is the problem about probe deferrals of the USB
driver, or about probe deferrals of the mtk-pm-domains driver?

scpsys_domain_cleanup() is only called upon scpsys_probe() failure.

> > > ---
> > >   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;