[PATCH] libcamera: v4l2_videodevice: Clarify V4L2M2MDevice

[Jacopo Mondi]

Hi Kieran

On Thu, Nov 28, 2024 at 10:53:58AM +0000, Kieran Bingham wrote:
> Quoting Jacopo Mondi (2024-11-27 08:12:11)
> > The documentation seems to suggest that to create a new M2M
> > execution context it is expected users to call V4L2M2MDevice::open()
> > multiple times on the same video device path.
> >
> > It is instead expected that multiple instances of the class are
> > created, one for each required execution context.
> >
> > Clarify it in the documentation of the V4L2M2MDevice class.
> >
> > Signed-off-by: Jacopo Mondi <jacopo.mondi at ideasonboard.com>
> > ---
> >  src/libcamera/v4l2_videodevice.cpp | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/src/libcamera/v4l2_videodevice.cpp b/src/libcamera/v4l2_videodevice.cpp
> > index 14eba0561d6a..156601d08d0c 100644
> > --- a/src/libcamera/v4l2_videodevice.cpp
> > +++ b/src/libcamera/v4l2_videodevice.cpp
> > @@ -2128,11 +2128,16 @@ V4L2PixelFormat V4L2VideoDevice::toV4L2PixelFormat(const PixelFormat &pixelForma
> >   * deviceNode which operate together using two queues to implement the V4L2
> >   * Memory to Memory API.
> >   *
> > - * The two devices should be opened by calling open() on the V4L2M2MDevice, and
> > - * can be closed by calling close on the V4L2M2MDevice.
>
> I would open this paragraph with a statement that it's possible to use
> multiple contexts. I don't think that itself is clear...
>
> """
> Memory to Memory devices in the kernel using the V4L2 M2M API can
> operate with multiple contexts for parallel operations on a single
> device. Each instance of a V4L2M2MDevice represents a single context.
> """

Indeed, that's helpful!

>
> Then continue with your text with one small typo:
>
> > + * User of this class should create a new instance of the V4L2M2MDevice for each
>
> s/User/Users/
>
> > + * desired execution context and then open it by calling open() on the
> > + * V4L2M2MDevice and close it by calling close() on the V4L2M2MDevice.
> >   *
> >   * Calling V4L2VideoDevice::open() and V4L2VideoDevice::close() on the capture
> >   * or output V4L2VideoDevice is not permitted.
> > + *
> > + * Once the M2M device is open users can operate on the output and capture queue

and s/queue/queues/

> > + * represented by the V4L2VideoDevice returned by the output() and capture()
> > + * functions.
> >   */
>
>
> Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>

Thanks
  j

>
> >
> >  /**
> > --
> > 2.47.0
> >