Re: [RFC PATCH V4 1/4] media: v4l2-mem2mem: add v4l2_m2m_suspend, v4l2_m2m_resume

[Tomasz Figa <tfiga@chromium.org>]

On Wed, Jun 10, 2020 at 03:52:39PM -0300, Ezequiel Garcia wrote:
> Hi everyone,
> 
> Thanks for the patch.
> 
> On Wed, 10 Jun 2020 at 07:33, Tomasz Figa <tfiga@chromium.org> wrote:
> >
> > On Wed, Jun 10, 2020 at 12:29 PM Hans Verkuil <hverkuil@xs4all.nl> wrote:
> > >
> > > On 21/05/2020 19:11, Tomasz Figa wrote:
> > > > Hi Jerry,
> > > >
> > > > On Wed, Dec 04, 2019 at 08:47:29PM +0800, Jerry-ch Chen wrote:
> > > >> From: Pi-Hsun Shih <pihsun@chromium.org>
> > > >>
> > > >> Add two functions that can be used to stop new jobs from being queued /
> > > >> continue running queued job. This can be used while a driver using m2m
> > > >> helper is going to suspend / wake up from resume, and can ensure that
> > > >> there's no job running in suspend process.
[snip]
> > >
> > > I assume this will be part of a future patch series that calls these new functions?
> >
> > The mtk-jpeg encoder series depends on this patch as well, so I guess
> > it would go together with whichever is ready first.
> >
> > I would also envision someone changing the other existing drivers to
> > use the helpers, as I'm pretty much sure some of them don't handle
> > suspend/resume correctly.
> >
> 
> This indeed looks very good. If I understood the issue properly,
> the change would be useful for both stateless (e.g. hantro, et al)
> and stateful (e.g. coda) codecs.
> 
> Hantro uses pm_runtime_force_suspend, and I believe that
> could is enough for proper suspend/resume operation.

Unfortunately, no. :(

If the decoder is already decoding a frame, that would forcefully power
off the hardware and possibly even cause a system lockup if we are
unlucky to gate a clock in the middle of a bus transaction.

I just inspected the code now and actually found one more bug in its
power management handling. device_run() calls clk_bulk_enable() before
pm_runtime_get_sync(), but only the latter is guaranteed to actually
power on the relevant power domains, so we end up clocking unpowered
hardware.

> 
> I'm not seeing any code in CODA to handle this, so not sure
> how it's handling suspend/resume.
> 
> Maybe we can have CODA as the first user, given it's a well-maintained
> driver and should be fairly easy to test.

I remember checking a number of drivers using the m2m helpers randomly
and none of them implemented suspend/resume correctly. I suppose that
was not discovered because normally the userspace itself would stop the
operation before the system is suspended, although it's not an API
guarantee.

Best regards,
Tomasz

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