linux-kernel - Re: [RFC PATCH v11 6/9] media: tegra: Add Tegra210 Video input driver

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Date:   Sat, 2 May 2020 15:46:36 -0700
From:   Sowjanya Komatineni <skomatineni@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>, <thierry.reding@...il.com>,
        <jonathanh@...dia.com>, <frankc@...dia.com>, <hverkuil@...all.nl>,
        <sakari.ailus@....fi>, <helen.koike@...labora.com>
CC:     <sboyd@...nel.org>, <linux-media@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <linux-clk@...r.kernel.org>,
        <linux-tegra@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v11 6/9] media: tegra: Add Tegra210 Video input driver

On 5/2/20 1:48 PM, Dmitry Osipenko wrote:
> 02.05.2020 19:55, Sowjanya Komatineni пишет:
>> 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  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 
during suspend and stream on cmd during resume which eventually calls 
v4l_streamoff and v4l_streamon during system suspend/resume.

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

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

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

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

Thanks

Sowjanya