Message-ID: <5628A00C.5020804@osg.samsung.com>
Date:	Thu, 22 Oct 2015 10:36:28 +0200
From:	Javier Martinez Canillas <javier@....samsung.com>
To:	Anand Moon <linux.amoon@...il.com>
Cc:	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Markus Reichl <m.reichl@...etechno.de>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>,
	Alim Akhtar <alim.akhtar@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	linux-mmc@...r.kernel.org, Alexandre Courbot <acourbot@...dia.com>,
	Ulf Hansson <ulf.hansson@...aro.org>
Subject: Re: [PATCH] mmc: pwrseq: Use highest priority for eMMC restart
 handler

Hello Anand,

On 10/22/2015 07:03 AM, Anand Moon wrote:
> Hi Javier,
> 
> On 22 October 2015 at 08:22, Javier Martinez Canillas
> <javier@....samsung.com> wrote:
>> Hello Krzysztof,
>>
>> On 10/22/2015 03:43 AM, Krzysztof Kozlowski wrote:
>>> On 22.10.2015 10:20, Javier Martinez Canillas wrote:> Hello Krzysztof,
>>>>
>>>> Thanks for your feedback.
>>>>
>>>> On 10/22/2015 02:36 AM, Krzysztof Kozlowski wrote:
>>>>> On 22.10.2015 00:15, Javier Martinez Canillas wrote:
>>>>>> The pwrseq_emmc driver does a eMMC card reset before a system reboot to
>>>>>> allow broken or limited ROM boot-loaders (that don't have an eMMC reset
>>>>>> logic) to be able to read the second stage from the eMMC.
>>>>>>
>>>>>> But this has to be called before a system reboot handler and while most
>>>>>> of them use the priority 128, there are other restart handlers (such as
>>>>>> the syscon-reboot one) that use a higher priority. So, use the highest
>>>>>> priority to make sure that the eMMC hw is reset before a system reboot.
>>>>>>
>>>>>> Signed-off-by: Javier Martinez Canillas <javier@....samsung.com>
>>>>>> Tested-by: Markus Reichl <m.reichl@...etechno.de>
>>>>>> Tested-by: Anand Moon <linux.amoon@...il.com>
>>>>>> Reviewed-by: Alim Akhtar <alim.akhtar@...sung.com>
>>>>>>
>>>>>> ---
>>>>>> Hello,
>>>>>>
>>>>>> This patch was needed since a recent series from Alim [0] added
>>>>>> syscon reboot and poweroff support to Exynos SoCs and removed
>>>>>> the reset handler in the Exynos Power Management Unit (PMU) code.
>>>>>>
>>>>>> But the PMU and syscon-reboot restart handler have a different
>>>>>> priority so [0] breaks restart when eMMC is used on these boards.
>>>>>>
>>>>>> [0]: http://www.spinics.net/lists/arm-kernel/msg454396.html
>>>>>>
>>>>>> So this patch must be merged before [0] to avoid regressions.
>>>>>>
>>>>>> Best regards,
>>>>>> Javier
>>>>>>
>>>>>>  drivers/mmc/core/pwrseq_emmc.c | 6 +++---
>>>>>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/mmc/core/pwrseq_emmc.c b/drivers/mmc/core/pwrseq_emmc.c
>>>>>> index 137c97fb7aa8..ad4f94ec7e8d 100644
>>>>>> --- a/drivers/mmc/core/pwrseq_emmc.c
>>>>>> +++ b/drivers/mmc/core/pwrseq_emmc.c
>>>>>> @@ -84,11 +84,11 @@ struct mmc_pwrseq *mmc_pwrseq_emmc_alloc(struct mmc_host *host,
>>>>>>
>>>>>>    /*
>>>>>>     * register reset handler to ensure emmc reset also from
>>>>>> -   * emergency_reboot(), priority 129 schedules it just before
>>>>>> -   * system reboot
>>>>>> +   * emergency_reboot(), priority 255 is the highest priority
>>>>>> +   * so it will be executed before any system reboot handler.
>>>>>>     */
>>>>>>    pwrseq->reset_nb.notifier_call = mmc_pwrseq_emmc_reset_nb;
>>>>>> -  pwrseq->reset_nb.priority = 129;
>>>>>> +  pwrseq->reset_nb.priority = 255;
>>>>>
>>>>> I see the problem which you are trying to solve but this may be tricker
>>>>> then just kicking the number. Some of restart handlers are registered
>>>>> with priority 192. I found few of such, like: at91_restart_nb,
>>>>> zynq_slcr_restart_nb, rmobile_reset_nb (maybe more, I did not grep too
>>>>> much).
>>>>>
>>>>
>>>> Yes, the syscon-reboot restart handler also uses a priority 192 and that
>>>> is why reboot with eMMC broke with Alim's patches since the PMU restart
>>>> handler priority is 128.
>>>>
>>>>> I guess they chose the "192" priority on purpose.
>>>>>
>>>>
>>>> I tried to understand what's the policy w.r.t priority numbering for
>>>> restart handlers but only found this in the register_restart_handler
>>>> kernel-doc [0]:
>>>>
>>>> /**
>>>>  *   register_restart_handler - Register function to be called to reset
>>>>  *                              the system
>>>>  *   @nb: Info about handler function to be called
>>>>  *   @nb->priority:  Handler priority. Handlers should follow the
>>>>  *                   following guidelines for setting priorities.
>>>>  *                   0:      Restart handler of last resort,
>>>>  *                           with limited restart capabilities
>>>>  *                   128:    Default restart handler; use if no other
>>>>  *                           restart handler is expected to be available,
>>>>  *                           and/or if restart functionality is
>>>>  *                           sufficient to restart the entire system
>>>>  *                   255:    Highest priority restart handler, will
>>>>  *                           preempt all other restart handlers
>>>>
>>>> So, reading that is not clear to me if only the values 0, 128 and 255
>>>> should be used or any value from 0-255.
>>>>
>>>> What's clear to me is that restart handlers to reset a specific hw block
>>>> should be called before the restart handler that resets the whole system.
>>>>
>>>> The 192 seems to be used because there are other default restart handlers
>>>> that are using a prio of 128. See for example the commit that changed the
>>>> syscon-reboot prio from 128 to 192:
>>>>
>>>> b81180b3fd48 power: reset: adjust priority of simple syscon reboot driver
>>>
>>> But were are here not talking about syscon handler but the others. Now
>>> you will be ahead of them.
>>>
>>
>> Yes, I know that. My point was that the platforms were either not using the
>> mmc-pwrseq-emmc or their system restart handler already had a lower priority
>> but that is not true for at least rk3288-veyron as you said.
>>
>>>>
>>>> So probably the 192 value was chosen because is in the middle of 128 and
>>>> 255 but it seems to me a rather arbitrary value and I would prefer it to
>>>> be documented in some place.
>>>>
>>>>> Effectively, now the emmc handler will be executed before their
>>>>> handlers... is it an issue? Maybe some testing on these platforms is
>>>>> necessary?
>>>>>
>>>>
>>>> I don't think is an issue, the reason why I chose 255 is that it is
>>>> a documented value in the kernel-doc and since is the highest prio,
>>>> it makes sure the eMMC will be reset before any system restart handler.
>>>>
>>>> Also, the pwrseq_emmc driver is only used in platforms whose SoC ROM
>>>> can either leave the eMMC in an unknown state so the kernel needs to
>>>> hw reset the eMMC or does not have a reset logic so it can only read
>>>> from an eMMC if is in a known state (i.e: after a reset from kernel).
>>>
>>> I think at least one platform may be affected because it used
>>> mmc-pwrseq-emmc and gpio-restart.
>>>
>>> Look at rk3288-veyron.dtsi.
>>>
>>> Both of restart handlers had the priority of 129 which means that the
>>> order of execution depends on probing sequence. Now you will make the
>>> sequence strict - first mmc then gpio.
>>>
>>
>> The behavior is going to change indeed in that board but no due probe
>> order but because the gpio-restart handler dev node has priority = <200>
>> which overrides the default 129 in the gpio-restart driver.
>>
>> So before $SUBJECT the eMMC restart handler was not executed but now it
>> will be after this change.
>>
>>> You seems convinced that this is not a problem... I don't know. I would
>>> prefer test this on affected platforms before risking to break them.
>>> It's annoying if fix for one SoC breaks another.
>>>
>>
>> Agreed.
>>
>>>>
>>>> Since the current mmc_pwrseq_emmc_reset_nb notifier priority is 129,
>>>> eMMC reset will not work if one of the platforms you mentioned needs
>>>> this since the system restart handler with prio 192 will be executed
>>>> before the eMMC one, leaving the eMMC in an unknown state on reboot.
>>>
>>> And now you will "fix this" by making eMMC working correctly. So let's
>>> make it straight:
>>> 1. Previously the eMMC could be left on these platforms in an unknown
>>> state (because emmc handler was not executed).
>>> 2. No one complained! Which could mean that in fact this was working fine...
>>> 3. Now you will change it.
>>> 4. Maybe someone will complain?
>>>
>>> Just test it (or get an ack/tested tag). That's all what is needed.
>>>
>>
>> Yes, I never meant that the patch should be merged without testing...
>>
>>>
>>>> And $SUBJECT should not cause any regressions for the platforms that
>>>> are currently using the pwrseq_emmc, since the restart handler was
>>>> already being called before the system restart handler so bumping
>>>> the priority should not cause any effect.
>>>
>>> I found at least one platform where the sequence *might* change. There
>>> could be more of them.
>>>
>>
>> Agreed, I missed that rk3288-veyron is using a restart handler with higher
>> priority and could be other boards too as you said.
>>
>> Let's see what is Marek's opinion since he added the pwrseq_emmc support
>> and also what Ulf thinks about always doing a eMMC reset before reboot.
>>
>> I can't think how doing a eMMC card reset before reboot could affect a
>> board but you are right that we don't know without testing.
>>
>>> Best regards,
>>> Krzysztof
>>>
>>
> 
> Well I have tested with
> 
> pwrseq->reset_nb.priority = 192;
> 

I'm not sure why you are testing that case to be honest.

> But it did not resolve the issue of reboot.
>

That's expected since you are using the same priority for both
the mmc-pwrseq-emmc and syscon-reboot restart handlers so it
will only work if the eMMC restart handler is registered before
the syscon one and I don't if that's the case.

The eMMC restart handler priority should be higher than the one
used by syscon-reboot (or any system restart handler) to work.

> will early rest of emmc will that not affect the sync of data before reboot.
>

Is this a question? It shouldn't since the restart handlers are
the last things that are executed before a system is rebooted.

> -Anand Moon
> 

Best regards,
-- 
Javier Martinez Canillas
Open Source Group
Samsung Research America
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