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Subject: Re: [RFC PATCH v11 6/9] media: tegra: Add Tegra210 Video input driver
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From:   Sowjanya Komatineni <skomatineni@nvidia.com>
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On 5/2/20 1:48 PM, Dmitry Osipenko wrote:
> 02.05.2020 19:55, Sowjanya Komatineni =D0=BF=D0=B8=D1=88=D0=B5=D1=82:
>> Also stop stream ioctl request happens during suspend where both threads
>> will be stopped properly. done thread stop happens only after finishing
>> all outstanding buffers.
> Do you mean that V4L core takes care of stopping the streami on suspend
> and re-starting it on resume from suspend?
>
>> Stop stream request happens from streaming applications so even without
>> driver suspend/resume implementation currently, streaming will be
>> stopped prior to system=C2=A0 suspend where both threads will be stopped
>> properly (after finishing out standing buffers) and will be resumed by
>> application on system resume
> All userspace is frozen on suspend. System suspension is transparent for
> userspace applications. I'm not sure what you're meaning here.
>
>> Also tested suspending while streaming with this unconditional freeze, I
>> don't see any issue as application stops stream where v4l_streamoff gets
>> executed during suspend and on resume streaming starts where
>> v4l_streamon happens.
>>
>> So, I don't see any issue with existing implementation of unconditional
>> freeze.
> I don't understand why freezing is needed at all if V4L core takes care
> of stopping the stream on suspend, what is the point? If there is no
> real point, then let's make threads non-freezable and done with that.

video device fops unlocked_ioctl is set to video_ioctl2() in vi driver.

video device fops unlocked_ioctl gets executed with stream off cmd=20
during suspend and stream on cmd during resume which eventually calls=20
v4l_streamoff and v4l_streamon during system suspend/resume.

My understanding to have freezable threads is during system suspend user=20
space applications are frozen prior to kernel freeze and during suspend=20
when opened video character device node gets closed these ioctl gets=20
invoked and stream off during suspend and stream on during resume=20
happens. So probably we still need to use freezable threads to sync with=20
user space application when frozen before really entering suspend.

Will wait for Thierry/Hans comment to correct if my above understanding=20
is wrong and help clarify if we need freezable threads at all in this=20
case...

Note: I see other drivers using freezable threads for capture drivers.


Assuming we use freezable threads, I was saying we don't need=20
conditional try_to_freeze() like you pointed because even if finish=20
thread freeze happens prior to frame capture initiated by start thread,=20
vi hardware will still continue to update this single ongoing buffer and=20
will finish max within 200ms and actually there is no direct processing=20
of this done by finish thread itself except that it returns buffers back=20
when done and in this case it returns back when unfreeze/wake up happens.

So, I don't see any issue of unconditional try_to_freeze() even with=20
freezable threads.

Thanks

Sowjanya