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

From: Dmitry Osipenko <digetx@gmail.com>
To: Jon Hunter <jonathanh@nvidia.com>,
	Sameer Pujar <spujar@nvidia.com>,
	perex@perex.cz, tiwai@suse.com, robh+dt@kernel.org
Cc: devicetree@vger.kernel.org, alsa-devel@alsa-project.org,
	atalambedu@nvidia.com, linux-kernel@vger.kernel.org,
	lgirdwood@gmail.com, viswanathl@nvidia.com, sharadg@nvidia.com,
	broonie@kernel.org, thierry.reding@gmail.com,
	linux-tegra@vger.kernel.org, rlokhande@nvidia.com,
	mkumard@nvidia.com, dramesh@nvidia.com
Subject: Re: [alsa-devel] [PATCH 4/9] ASoC: tegra: add Tegra210 based I2S driver
Date: Wed, 22 Jan 2020 19:27:11 +0300	[thread overview]
Message-ID: <1a84b393-938f-8bed-d08e-cc3bb6ed4844@gmail.com> (raw)
In-Reply-To: <34ac1fd3-ae0f-07f2-555f-a55087a2c9dc@nvidia.com>

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.