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Subject: Re: [RFC PATCH v2 11/18] media: tegra-video: Add support for external
 sensor capture
To:     Hans Verkuil <hverkuil@xs4all.nl>, <thierry.reding@gmail.com>,
        <jonathanh@nvidia.com>, <frankc@nvidia.com>, <sakari.ailus@iki.fi>,
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From:   Sowjanya Komatineni <skomatineni@nvidia.com>
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Date:   Tue, 7 Jul 2020 13:29:13 -0700
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On 7/7/20 12:35 PM, Hans Verkuil wrote:
> On 07/07/2020 21:25, Sowjanya Komatineni wrote:
>> On 7/7/20 12:01 PM, Sowjanya Komatineni wrote:
>>>
>>> On 7/6/20 2:10 AM, Hans Verkuil wrote:
>>>>> +static void tegra_vi_graph_cleanup(struct tegra_vi *vi)
>>>>> +{
>>>>> +	struct tegra_vi_channel *chan;
>>>>> +
>>>>> +	list_for_each_entry(chan, &vi->vi_chans, list) {
>>>>> +		video_unregister_device(&chan->video);
>>>>> +		mutex_lock(&chan->video_lock);
>>>>> +		vb2_queue_release(&chan->queue);
>>>> No need for this since this is done in vb2_fop_release().
>>>>
>>>> In fact, vb2_queue_release should never be called by drivers. Just usi=
ng
>>>> vb2_fop_release or __vb2_fop_release is sufficient.
>>>>
>>>> The confusion is due to the fact that the name suggests that vb2_queue=
_release
>>>> has to be balanced with vb2_queue_init, but that's not the case. Perha=
ps
>>>> vb2_queue_stop or something like that might be a better name. I'll hav=
e to
>>>> think about this since I see that a lot of drivers do this wrong.
>>>>
>>>>> +		mutex_unlock(&chan->video_lock);
>>>>> +		v4l2_async_notifier_unregister(&chan->notifier);
>>>>> +		v4l2_async_notifier_cleanup(&chan->notifier);
>>>>> +	}
>>>>> +}
>>>>> +
>>> vb2_queue_release() here is called to stop streaming a head before medi=
a links are removed in case of when driver unbind happens while
>>> userspace application holds video device with active streaming in progr=
ess.
>>>
>>> Without vb2_queue_release() here streaming will be active during the dr=
iver unbind and by the time vb2_queue_release() happens from
>>> vb2_fop_release(), async notifiers gets unregistered and media links wi=
ll be removed which causes channel stop stream to crash as we can't
>>> retrieve sensor subdev=C2=A0 thru media entity pads to execute s_stream=
 on subdev.
>>>
>> I think we don't need async notifier unbind. Currently media links are r=
emoved during unbind so during notifier unregister all subdevs gets
>> unbind and links removed.
>>
>> media_device_unregister during video device release callback takes care =
of media entity unregister and removing links.
>>
>> So, will try by removing notifier unbind along with removing vb2_queue_r=
elease during cleanup.
>>
> I actually wonder if vb2_queue_release shouldn't be called from video_unr=
egister_device.
>
> I'll look into this tomorrow.
>
> Regards,
>
> 	Hans

Thanks Hans.

Tried without notifier unbind to remove media links and I still see=20
crash due to below diff reason now.

With userspace app holding video device node with active streaming in=20
progress when I do driver unbind, v4l2_device release callback=20
tegra_v4l2_dev_release() happens prior to vb2_fops_release() ->=20
vb2_queue_release().

All channels resources and channel memory is freed during v4l2_device=20
release callback.

Letting vb2_queue_release() to happen thru vb2_fops_release() causes=20
crash as stop streaming tries to retrieve subdev thru channel media pads=20
and channel memory is freed by that time.

So, doing vb2_queue_release() during driver unbind -> tegra_vi_exit() ->=20
tegra_vi_graph_cleanup(), stops subdev stream properly and then on=20
v4l2_device release channel memory gets freed and this works which is=20
the existing implementation in the patch.

I remember adding vb2_queue_release() during graph cleanup for TPG as=20
well for the same reason to allow driver unbind while holding video=20
device from user space so media pad can be accessible to stop stream=20
before channel cleanup.

Regards,

Sowjanya