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Message-ID: <ade69c64-db32-4d5a-8ca8-3ae2d6ce1d2b@collabora.com>
Date:   Wed, 29 Nov 2023 16:48:26 +0200
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Subject: Re: [PATCH] pmdomain: mediatek: fix race condition in power on/power
 off sequences
Content-Language: en-US
To:     AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@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>
 <80cbe288-2960-4937-be47-56610946695d@collabora.com>
 <7b759141-ff2d-4cac-803e-120f31d2d556@collabora.com>
 <0bb40af7-6b20-48cb-a05f-4eed1176f5f0@collabora.com>
From:   Eugen Hristev <eugen.hristev@collabora.com>
In-Reply-To: <0bb40af7-6b20-48cb-a05f-4eed1176f5f0@collabora.com>
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On 11/29/23 16:41, AngeloGioacchino Del Regno wrote:
> Il 29/11/23 14:48, Eugen Hristev ha scritto:
>> On 11/29/23 15:37, AngeloGioacchino Del Regno wrote:
>>> 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.
>>
>> You are right, my explanation was bad.
>>
>> It happens during the mtk-pm-domains driver probe.
>>
>> Not all domains can power up, then everything is rolled back. and this happens 
>> multiple times
>> On rare occasions, it happens that another probing sequence starts while the 
>> previous one was not finished .
>> I mentioned devices because I had in mind the fact that each device requires a 
>> power domain, and parallel probing of these devices causes a call to mtk-pm-domains 
>> driver probe to be called from two different places.
>>
>> e.g. device 1 probes -> call mtk-pm-domains probe because it requires X power domain
>>
>> device 2 probes -> call mtk-pm-domains probe because it requires Y power domain.
>>
>> First attempt fails but not completed while second attempt starts.
>>
>> Maybe this is a better explanation of the situation ?
> 
> Yeah, now it's a bit clearer!
> 
> At this point, I think that you can get away with locking just one path (or two):
> 
> /* This is the one giving me lots of suspects */
> static void scpsys_remove_one_domain(struct scpsys_domain *pd)
> {
> 	int ret;
> 
> ***lock***
> 	if (scpsys_domain_is_on(pd))
> 		scpsys_power_off(&pd->genpd);
> ***unlock***
> 
> 	.....
> }
> 
> /* This one as well eventually */
> static struct
> generic_pm_domain *scpsys_add_one_domain(struct scpsys *scpsys, struct device_node 
> *node)
> {
> 	...............
> 
> 	if (MTK_SCPD_CAPS(pd, MTK_SCPD_KEEP_DEFAULT_OFF)) {
> 
> 		if (scpsys_domain_is_on(pd)) /* Maybe LOCK this one too? */
> 
> 			dev_warn(scpsys->dev,
> 				 "%pOF: A default off power domain has been ON\n", node);
> 	} else {
> **		*** lock ***
> 		ret = scpsys_power_on(&pd->genpd);
> **		*** unlock ***
> 		if (ret < 0) {
> 			dev_err(scpsys->dev, "%pOF: failed to power on domain: %d\n", node, ret);
> 			goto err_put_subsys_clocks;
> 		}
> 
> 		if (MTK_SCPD_CAPS(pd, MTK_SCPD_ALWAYS_ON))
> 			pd->genpd.flags |= GENPD_FLAG_ALWAYS_ON;
> 	}
> 
> 	..........
> }
> 
> Can you please try locking only the remove_one_domain() poweroff call before
> trying both?
> 
> Reason is that in the add_one_domain() case, we haven't registered the power
> domain yet, so locking may not be required there to make things ticking right.

We need to have 'critical section' all the access to clocks/registers, so both
paths (power on and power off) must be serialized. So a mutex must be taken before
entering either of those, otherwise it makes no sense. There must be no path left
that leads to power on/ power off that is taken without the mutex.
> 
> Cheers!
> 
>>>
>>>>>
>>>>>> ---
>>>>>>   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;
>>>>>
>>>
>