[libcamera-devel] [PATCH v1] v4l2_videodevice: Disable the watchdog timer when no buffers are queued

[Naushir Patuck]

Hi Kieran,

Thank you for the feedback!

On Thu, 5 May 2022 at 10:36, Kieran Bingham <kieran.bingham at ideasonboard.com>
wrote:

> Hi Naush,
>
> Quoting Naushir Patuck via libcamera-devel (2022-05-05 09:48:24)
> > Only enable/reset the watchdog timer when there are buffers queued in
> the V4L2
> > device. Otherwise, we may trigger spurious warnings when the watchdog
> times out
> > even if there are no buffers queued in the device.
>
> aha yes - I can see how that was tripping up on python interactive
> sessions or otherwise underflowing when it's not at all a fault of the
> V4L2VideoDevice.
>
>
> >
> > Signed-off-by: Naushir Patuck <naush at raspberrypi.com>
> > ---
> >  src/libcamera/v4l2_videodevice.cpp | 22 +++++++++++++++-------
> >  1 file changed, 15 insertions(+), 7 deletions(-)
> >
> > diff --git a/src/libcamera/v4l2_videodevice.cpp
> b/src/libcamera/v4l2_videodevice.cpp
> > index 5b4637b1a39e..430715afd554 100644
> > --- a/src/libcamera/v4l2_videodevice.cpp
> > +++ b/src/libcamera/v4l2_videodevice.cpp
> > @@ -1662,8 +1662,11 @@ int V4L2VideoDevice::queueBuffer(FrameBuffer
> *buffer)
> >                 return ret;
> >         }
> >
> > -       if (queuedBuffers_.empty())
> > +       if (queuedBuffers_.empty()) {
> >                 fdBufferNotifier_->setEnabled(true);
> > +               if (watchdogDuration_)
> > +
>  watchdog_.start(std::chrono::duration_cast<std::chrono::milliseconds>(watchdogDuration_));
> > +       }
>
> I guess this could also be set outside of the if (queuedBuffer_.empty())
> - but it would artificially delay the watchdog every time a buffer was
> queued. In the event that more than one buffer is required to be queued
> (to satisfy internal requirements perhaps?) that might actually be
> beneficial ... But I think either way is fine.
>

It could... I thought it might be ever so slightly more logically correct
if this
was inside the if() block.  I'll keep it in there unless there are further
objections.


>
> >
> >         queuedBuffers_[buf.index] = buffer;
> >
> > @@ -1742,16 +1745,21 @@ FrameBuffer *V4L2VideoDevice::dequeueBuffer()
> >                 return nullptr;
> >         }
> >
> > -       if (watchdogDuration_.count())
> > -
>  watchdog_.start(std::chrono::duration_cast<std::chrono::milliseconds>(watchdogDuration_));
> > -
> >         cache_->put(buf.index);
> >
> >         FrameBuffer *buffer = it->second;
> >         queuedBuffers_.erase(it);
> >
> > -       if (queuedBuffers_.empty())
> > +       if (queuedBuffers_.empty()) {
> >                 fdBufferNotifier_->setEnabled(false);
> > +               watchdog_.stop();
> > +       } else if (watchdogDuration_) {
> > +               /*
> > +                * Restart the watchdog timer if there are buffers still
> queued
> > +                * in the device.
> > +                */
> > +
>  watchdog_.start(std::chrono::duration_cast<std::chrono::milliseconds>(watchdogDuration_));
>
> Why do we need/have all these casts? Is either watchdogDuration_ not a
> duration? or is watchdog_.start() not accepting a duration? Either of
> those would remove a lot of casting surely?
>

This is a bit of a minor annoyance because of how I defined the underlying
type
of utils::Duration to a double. The std::chrono library forbids implicit
casts
from integer to float types.


> watchdogDuration_ is a utils::Duration which stores std::nano, and I see
> Timer start takes a std::chrono::milliseconds. I think it would make
> sense to add a 'void start(std::chrono::duration)' to the timer class
> and simplify these casts. But that doesn't have to be fixed in this
> patch.
>
> Would you like to do that as a patch on top? If you don't want to let me
> know and I'll handle it after this patch is merged.
>

What we've talked about doing is to switch utils::Duration to use an integer
underlying type, which would allow implicit conversions and remove the ugly
casts like the above. I believe Laurent was still thinking this through as
there
were also plans to make utils::Duration part of the public API...

Regards,
Naush


>
>
> Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
>
>
> > +       }
> >
> >         buffer->metadata_.status = buf.flags & V4L2_BUF_FLAG_ERROR
> >                                  ? FrameMetadata::FrameError
> > @@ -1847,7 +1855,7 @@ int V4L2VideoDevice::streamOn()
> >         }
> >
> >         state_ = State::Streaming;
> > -       if (watchdogDuration_.count())
> > +       if (watchdogDuration_ && !queuedBuffers_.empty())
> >
>  watchdog_.start(std::chrono::duration_cast<std::chrono::milliseconds>(watchdogDuration_));
> >
> >         return 0;
> > @@ -1924,7 +1932,7 @@ void
> V4L2VideoDevice::setDequeueTimeout(utils::Duration timeout)
> >         watchdogDuration_ = timeout;
> >
> >         watchdog_.stop();
> > -       if (watchdogDuration_.count() && state_ == State::Streaming)
> > +       if (watchdogDuration_ && state_ == State::Streaming &&
> !queuedBuffers_.empty())
> >
>  watchdog_.start(std::chrono::duration_cast<std::chrono::milliseconds>(timeout));
> >  }
> >
> > --
> > 2.25.1
> >
>
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