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Message-ID: <80cbe288-2960-4937-be47-56610946695d@collabora.com>
Date:   Wed, 29 Nov 2023 14:37:28 +0100
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Subject: Re: [PATCH] pmdomain: mediatek: fix race condition in power on/power
 off sequences
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To:     Eugen Hristev <eugen.hristev@collabora.com>,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org, linux-pm@vger.kernel.org
Cc:     eballetbo@kernel.org, ulf.hansson@linaro.org,
        linux-kernel@vger.kernel.org, kernel@collabora.com
References: <20231129113120.4907-1-eugen.hristev@collabora.com>
 <fd645c33-1b16-4825-b9ea-eecb578dd2a0@collabora.com>
 <bf7ac271-1bda-4114-b080-4b20f857dbc2@collabora.com>
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>
In-Reply-To: <bf7ac271-1bda-4114-b080-4b20f857dbc2@collabora.com>
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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@collabora.com>
>>
>> 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;
>>