linux-kernel - Re: [PATCH] memory: samsung: exynos5422-dmc: propagate error from exynos5_counters

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Date:   Tue, 4 Aug 2020 13:38:11 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Marek Szyprowski <m.szyprowski@...sung.com>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Cc:     Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        linux-samsung-soc@...r.kernel.org,
        Chanwoo Choi <cw00.choi@...sung.com>
Subject: Re: [PATCH] memory: samsung: exynos5422-dmc: propagate error from
 exynos5_counters_get()



On 8/4/20 1:19 PM, Marek Szyprowski wrote:
> Hi Lukasz,
> 
> On 04.08.2020 11:11, Lukasz Luba wrote:
>> Hi Marek,
>>
>> On 8/4/20 7:12 AM, Marek Szyprowski wrote:
>>> exynos5_counters_get() might fail with -EPROBE_DEFER if the driver for
>>> devfreq event counter is not yet probed. Propagate that error value to
>>> the caller to ensure that the exynos5422-dmc driver will be probed again
>>> when devfreq event contuner is available.
>>>
>>> This fixes boot hang if both exynos5422-dmc and exynos-ppmu drivers are
>>> compiled as modules.
>>>
>>> Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
>>> ---
>>>    drivers/memory/samsung/exynos5422-dmc.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/memory/samsung/exynos5422-dmc.c
>>> b/drivers/memory/samsung/exynos5422-dmc.c
>>> index b9c7956e5031..639811a3eecb 100644
>>> --- a/drivers/memory/samsung/exynos5422-dmc.c
>>> +++ b/drivers/memory/samsung/exynos5422-dmc.c
>>> @@ -914,7 +914,7 @@ static int exynos5_dmc_get_status(struct device
>>> *dev,
>>>        } else {
>>>            ret = exynos5_counters_get(dmc, &load, &total);
>>>            if (ret < 0)
>>> -            return -EINVAL;
>>> +            return ret;
>>>              /* To protect from overflow, divide by 1024 */
>>>            stat->busy_time = load >> 10;
>>>
>>
>> Thank you for the patch, LGTM.
>> Some questions are still there, though. The function
>> exynos5_performance_counters_init() should capture that the counters
>> couldn't be enabled or set. So the functions:
>> exynos5_counters_enable_edev() and exynos5_counters_set_event()
>> must pass gently because devfreq device is registered.
>> Then devfreq checks device status, and reaches the state when
>> counters 'get' function returns that they are not ready...
>>
>> If that is a kind of 2-stage initialization, maybe we should add
>> another 'check' in the exynos5_performance_counters_init() and call
>> the devfreq_event_get_event() to make sure that we are ready to go,
>> otherwise return ret from that function (which is probably EPROBE_DEFER)
>> and not register the devfreq device.
> 
> I've finally investigated this further and it turned out that the issue
> is elsewhere. The $subject patch can be discarded, as it doesn't fix
> anything. The -EPROBE_DEFER is properly returned by
> exynos5_performance_counters_init(), which redirects exynos5_dmc_probe()
> to remove_clocks label. This causes disabling mout_bpll/fout_bpll clocks
> what in turn *sometimes* causes boot hang. This random behavior mislead
> me that the $subject patch fixes the issue, but then longer tests
> revealed that it didn't change anything.

Really good investigation, great work Marek!

> 
> It looks that the proper fix would be to keep fout_bpll enabled all the
> time.

Yes, I agree. I am looking for your next patch to test it then.

> 
>>
>> Marek do want to submit such patch or I should bake it and submit on top
>> of this patch?
>>
>> Could you tell me how I can reproduce this? Do you simply load one
>> module after another (exynos-ppmu than exynos5422-dmc) or in parallel?
> 
> I've just boot zImage built from multi_v7_defconfig with modules
> installed. Modules are automatically loaded by udev during boot.

Thank you sharing this test procedure.

Regards,
Lukasz