linux-kernel - Re: [alsa-devel] [PATCH 4/9] ASoC: tegra: add Tegra210 based I2S driver

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Message-ID: <0fc814c2-0dc6-7741-b954-463381ff7fb9@nvidia.com>
Date:   Thu, 23 Jan 2020 14:52:14 +0530
From:   Sameer Pujar <spujar@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>,
        Jon Hunter <jonathanh@...dia.com>, <perex@...ex.cz>,
        <tiwai@...e.com>, <robh+dt@...nel.org>
CC:     <spujar@...dia.com>, <devicetree@...r.kernel.org>,
        <alsa-devel@...a-project.org>, <atalambedu@...dia.com>,
        <linux-kernel@...r.kernel.org>, <lgirdwood@...il.com>,
        <viswanathl@...dia.com>, <sharadg@...dia.com>,
        <broonie@...nel.org>, <thierry.reding@...il.com>,
        <linux-tegra@...r.kernel.org>, <rlokhande@...dia.com>,
        <mkumard@...dia.com>, <dramesh@...dia.com>
Subject: Re: [alsa-devel] [PATCH 4/9] ASoC: tegra: add Tegra210 based I2S
 driver



On 1/22/2020 9:57 PM, Dmitry Osipenko wrote:
> External email: Use caution opening links or attachments
>
>
> 22.01.2020 14:52, Jon Hunter пишет:
>> On 22/01/2020 07:16, Sameer Pujar wrote:
>>
>> ...
>>
>>>>>>>>> +static int tegra210_i2s_remove(struct platform_device *pdev)
>>>>>>>>> +{
>>>>>>>>> +     pm_runtime_disable(&pdev->dev);
>>>>>>>>> +     if (!pm_runtime_status_suspended(&pdev->dev))
>>>>>>>>> +             tegra210_i2s_runtime_suspend(&pdev->dev);
>>>>>>>> This breaks device's RPM refcounting if it was disabled in the active
>>>>>>>> state. This code should be removed. At most you could warn about the
>>>>>>>> unxpected RPM state here, but it shouldn't be necessary.
>>>>>>> I guess this was added for safety and explicit suspend keeps clock
>>>>>>> disabled.
>>>>>>> Not sure if ref-counting of the device matters when runtime PM is
>>>>>>> disabled and device is removed.
>>>>>>> I see few drivers using this way.
>>>>>> It should matter (if I'm not missing something) because RPM should
>>>>>> be in
>>>>>> a wrecked state once you'll try to re-load the driver's module. Likely
>>>>>> that those few other drivers are wrong.
>>>>>>
>>>>>> [snip]
>>>>> Once the driver is re-loaded and RPM is enabled, I don't think it
>>>>> would use
>>>>> the same 'dev' and the corresponding ref count. Doesn't it use the new
>>>>> counters?
>>>>> If RPM is not working for some reason, most likely it would be the case
>>>>> for other
>>>>> devices. What best driver can do is probably do a force suspend during
>>>>> removal if
>>>>> already not done. I would prefer to keep, since multiple drivers still
>>>>> have it,
>>>>> unless there is a real harm in doing so.
>>>> I took a closer look and looks like the counter actually should be
>>>> reset. Still I don't think that it's a good practice to make changes
>>>> underneath of RPM, it may strike back.
>>> If RPM is broken, it probably would have been caught during device usage.
>>> I will remove explicit suspend here if no any concerns from other folks.
>>> Thanks.
>> I recall that this was the preferred way of doing this from the RPM
>> folks. Tegra30 I2S driver does the same and Stephen had pointed me to
>> this as a reference.
>> I believe that this is meant to ensure that the
>> device is always powered-off regardless of it RPM is enabled or not and
>> what the current state is.
> Yes, it was kinda actual for the case of unavailable RPM.

> Anyways, /I think/ variant like this should have been more preferred:
>
> if (!pm_runtime_enabled(&pdev->dev))
>          tegra210_i2s_runtime_suspend(&pdev->dev);
> else
>          pm_runtime_disable(&pdev->dev);

I think it looks to be similar to what is there already.

pm_runtime_disable(&pdev->dev); // it would turn out to be a dummy call 
if !RPM
if (!pm_runtime_status_suspended(&pdev->dev)) // it is true always if !RPM
         tegra210_i2s_runtime_suspend(&pdev->dev);
>> Now for Tegra210 (or actually 64-bit Tegra) RPM is always enabled and so
>> we don't need to worry about the !RPM case. However, I still don't see
>> the harm in this.
> There is no real harm today, but:
>
> 1. I'd prefer to be very careful with RPM in general, based on
>     previous experience.
>
> 2. It should be a bug if device isn't RPM-suspended during
>     of driver's removal. Thus the real problem needs to be fixed
>     rather than worked around.