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>
Cc: devicetree@vger.kernel.org, alsa-devel@alsa-project.org,
	lgirdwood@gmail.com, linux-kernel@vger.kernel.org,
	broonie@kernel.org, atalambedu@nvidia.com, tiwai@suse.com,
	viswanathl@nvidia.com, linux-tegra@vger.kernel.org,
	robh+dt@kernel.org, thierry.reding@gmail.com, sharadg@nvidia.com,
	rlokhande@nvidia.com, mkumard@nvidia.com, dramesh@nvidia.com
Subject: Re: [alsa-devel] [PATCH 4/9] ASoC: tegra: add Tegra210 based I2S driver
Date: Fri, 24 Jan 2020 17:04:19 +0300	[thread overview]
Message-ID: <075e476a-36bb-5fee-15bc-76af4474a797@gmail.com> (raw)
In-Reply-To: <264d3354-8a2e-ee12-44ae-aff69213d551@nvidia.com>

24.01.2020 12:51, Jon Hunter пишет:
> 
> On 24/01/2020 09:07, Jon Hunter wrote:
>>
>> On 23/01/2020 15:16, Dmitry Osipenko wrote:
>>> 23.01.2020 12:22, Sameer Pujar пишет:
>>>>
>>>>
>>>> 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);
>>>
>>> Maybe this is fine for !RPM, but not really fine in a case of enabled
>>> RPM. Device could be in resumed state after pm_runtime_disable() if it
>>> wasn't suspended before the disabling.
>>
>> I don't see any problem with this for the !RPM case.
> 
> Sorry I meant the RPM case. In other words, I don't see a problem for
> neither the RPM case of the !RPM case.

1. Device shall be in RPM-suspended state at the time of driver's
removal, unless there is a bug in the sound driver. Hence why do you
need the dead code which doesn't bring any practical value?

2. Making changes underneath of RPM is simply error-prone. It may hit
badly in the future once something will change in the RPM core.