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Subject: Re: [PATCH 0/2] gpiolib: Fix issues introduced by fast bitmap
 processing path
To:     Janusz Krzysztofik <jmkrzyszt@gmail.com>
Cc:     Linus Walleij <linus.walleij@linaro.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Miguel Ojeda Sandonis <miguel.ojeda.sandonis@gmail.com>,
        Peter Korsgaard <peter.korsgaard@barco.com>,
        Peter Rosin <peda@axentia.se>,
        Ulf Hansson <ulf.hansson@linaro.org>,
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        Florian Fainelli <f.fainelli@gmail.com>,
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        Michael Hennerich <Michael.Hennerich@analog.com>,
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        Krzysztof Kozlowski <krzk@kernel.org>,
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From:   Marek Szyprowski <m.szyprowski@samsung.com>
Date:   Mon, 24 Sep 2018 13:38:57 +0200
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Hi Janusz,

On 2018-09-24 13:08, Janusz Krzysztofik wrote:
> 2018-09-24 11:43 GMT+02:00, Marek Szyprowski <m.szyprowski@samsung.com>:
>> On 2018-09-24 01:53, Janusz Krzysztofik wrote:
>>> While investigating possible reasons of GPIO fast bitmap processing
>>> related boot hang on Samsung Snow Chromebook, reported by Marek
>>> Szyprowski (thanks!), I've discovered one coding bug, addressed by
>>> PATCH 1/2 of this series, and one potential regression introduced at
>>> design level of the solution, hopefully fixed by PATCH 2/2.  See
>>> commit messages for details.
>>>
>>> Janusz Krzysztofik (2):
>>>         gpiolib: Fix missing updates of bitmap index
>>>         gpiolib: Fix array members of same chip processed separately
>>>
>>> The fixes should resolve the boot hang observed by Marek, however the
>>> second change excludes that particular case from fast bitmap processing
>>> and restores the old behaviour.
>> I confirm, that the above 2 patches fixes boot issue on Samsung Snow
>> Chromebook with next-20180920.
>>
>> Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
>>
>>> Hence, it is possible still another
>>> issue which have had an influence on that boot hang exists in the code.
>>> In order to fully verify the fix, it would have to be tested on a
>>> platform where an array of GPIO descriptors is used which starts from
>>> at least two consecutive pins of one GPIO chip in hardware order,
>>> starting ftom 0, followed by one or more pins belonging to other
>>> chip(s).
>>>
>>> In order to verify if separate calls to .set() chip callback for each
>>> pin instead of one call to .set_multiple() is actually the reason of
>>> boot hang on Samsung Snow Chromebook, the affected driver -
>>> drivers/mmc/core/pwrseq_simple.c - would have to be temporarily
>>> modified for testing purposes so it calls gpiod_set_value() for each
>>> pin instead of gpiod_set_array_value() for all of them.  If that would
>>> also result in boot hang, we could be sure the issue was really the
>>> one addressed by the second fix.  Marek, could you please try to
>>> perform such test?
>> Yes, I've just tested next-20180920 only with the first patch from this
>> patchset and the mentioned change to drivers/mmc/core/pwrseq_simple.c.
>> It boots fine, so indeed the issue is in handling of arrays of gpios.
>>
>> Just to be sure I did it right, this is my change to the mentioned file:
> Yeah, that's what I had on mind.  However, I'd be more lucky if it didn't work
> for you.  Setting the pins sequentially, not simultaneously as before, was
> exactly what I hoped was the reason of the hang.
>
>> diff --git a/drivers/mmc/core/pwrseq_simple.c
>> b/drivers/mmc/core/pwrseq_simple.c
>> index 7f882a2bb872..9397dc1f2e38 100644
>> --- a/drivers/mmc/core/pwrseq_simple.c
>> +++ b/drivers/mmc/core/pwrseq_simple.c
>> @@ -38,16 +38,11 @@ static void mmc_pwrseq_simple_set_gpios_value(struct
>> mmc_pwrseq_simple *pwrseq,
>>                                                 int value)
>>    {
>>           struct gpio_descs *reset_gpios = pwrseq->reset_gpios;
>> +       int i;
>>
>> -       if (!IS_ERR(reset_gpios)) {
>> -               DECLARE_BITMAP(values, BITS_PER_TYPE(value));
>> -               int nvalues = reset_gpios->ndescs;
>> -
>> -               values[0] = value;
>> -
>> -               gpiod_set_array_value_cansleep(nvalues, reset_gpios->desc,
>> -                                              reset_gpios->info, values);
>> -       }
>> +       if (!IS_ERR(reset_gpios))
>> +               for (i = 0; i < reset_gpios->ndescs; i++)
> The only difference from the behaviour when the hang was occurring is now
> the order the pins are manipulated.  Maybe that matters?
> Could you please retry the same with the order of pins reversed, either in
> the .dts file or here inside this for loop?

I've switched the order of pins in dts and next-20180920 + first patch + 
above
change also boots fine.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland