[libcamera-devel] [PATCH] libcamera: v4l2_videodevice: Explain multiplanar bytesused reasoning
Kieran Bingham
kieran.bingham at ideasonboard.com
Fri Oct 15 12:57:14 CEST 2021
Quoting Laurent Pinchart (2021-10-14 21:12:18)
> Hi Kieran,
>
> Thank you for the patch.
>
> On Thu, Oct 14, 2021 at 09:54:05AM +0100, Kieran Bingham wrote:
> > The ternary operation used to get the total bytesused of a V4L2 single
> > planar format which is stored in a multiplanar buffer can easily be
> > mis-read to think it's a bug, and appears to be reading the value of the
> > first of N planes as the total.
> >
> > Directly explain the reasoning for why it looks like the condition is
> > inverted, as it is correct that the total bytes used is stored in only
> > the first plane of the multiplanar buffer.
> >
> > Signed-off-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> > ---
> > src/libcamera/v4l2_videodevice.cpp | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > diff --git a/src/libcamera/v4l2_videodevice.cpp b/src/libcamera/v4l2_videodevice.cpp
> > index ba5f88cd41ed..f4752f8bb23f 100644
> > --- a/src/libcamera/v4l2_videodevice.cpp
> > +++ b/src/libcamera/v4l2_videodevice.cpp
> > @@ -1711,6 +1711,7 @@ FrameBuffer *V4L2VideoDevice::dequeueBuffer()
> > return buffer;
> > }
> >
> > + /* Single planar format stored in a multiplanar buffer */
>
> Now I find this a bit confusing :-) How about the following ?
>
> /*
> * With a V4L2 single-planar format, all the data is stored in a
> * single memory plane. The number of bytes used is conveyed
> * through that plane when using the V4L2 multi-planar API, or
> * set directly in the buffer when using the V4L2 single-planar
> * API.
> */
>
> It's a bit verbose perhaps ?
Perhaps, but I don't mind verbose there.
The line on it's own looks like there is a bug because of the inversion.
So it warrants an explanation. And if you've written one - we can add
it.
--
Kieran
>
> > unsigned int bytesused = multiPlanar ? planes[0].bytesused
> > : buf.bytesused;
> >
>
> --
> Regards,
>
> Laurent Pinchart
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