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

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Message-ID: <1a84b393-938f-8bed-d08e-cc3bb6ed4844@gmail.com>
Date:   Wed, 22 Jan 2020 19:27:11 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Jon Hunter <jonathanh@...dia.com>,
        Sameer Pujar <spujar@...dia.com>, perex@...ex.cz,
        tiwai@...e.com, robh+dt@...nel.org
Cc:     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

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);

> 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.