On Monday, August 22, 2011 15:54:03 Guennadi Liakhovetski wrote:
> We discussed a bit more with Hans on IRC, and below is my attempt of a
> summary. Hans, please, correct me, if I misunderstood anything.
Looks good, that's exactly what I meant.
Regards,
Hans
> Pawel,
> Sakari, Laurent: please, reply, whether you're ok with this.
>
> On Mon, 22 Aug 2011, Hans Verkuil wrote:
>
> > On Monday, August 22, 2011 12:40:25 Guennadi Liakhovetski wrote:
>
> [snip]
>
> > > It would be good if you also could have a look at my reply to this
Pawel's
> > > mail:
> > >
> > > http://article.gmane.org/gmane.linux.drivers.video-input-
> > infrastructure/36905
> > >
> > > and, specifically, at the vb2_parse_planes() function in it. That's my
> > > understanding of what would be needed, if we preserve .queue_setup() and
> > > use your last suggestion to include struct v4l2_format in struct
> > > v4l2_create_buffers.
> >
> > vb2_parse_planes can be useful as a utility function that 'normal' drivers
can
> > call from the queue_setup. But vb2 should not parse the format directly,
it
> > should just pass it on to the driver through the queue_setup function.
> >
> > You also mention: "All frame-format fields like fourcc code, width,
height,
> > colorspace are only input from the user. If the user didn't fill them in,
they
> > should not be used."
> >
> > I disagree with that. The user should fill in a full format description,
just
> > as with S/TRY_FMT. That's the information that the driver will use to set
up
> > the buffers. It could have weird rules like: if the fourcc is this, and
the
> > size is less than that, then we can allocate in this memory bank.
> >
> > It is also consistent with REQBUFS: there too the driver uses a full
format
> > (i.e. the last set format).
> >
> > I would modify queue_setup to something like this:
> >
> > int (*queue_setup)(struct vb2_queue *q, struct v4l2_format *fmt,
> > unsigned int *num_buffers,
> > unsigned int *num_planes, unsigned int sizes[],
> > void *alloc_ctxs[]);
> >
> > Whether fmt is left to NULL in the reqbufs case, or whether the driver has
to
> > call g_fmt first before calling vb2 is something that could be decided by
what
> > is easiest to implement.
>
> 1. VIDIOC_CREATE_BUFS passes struct v4l2_create_buffers from the user to
> the kernel, in which struct v4l2_format is embedded. The user _must_
> fill in .type member of struct v4l2_format. For .type ==
> V4L2_BUF_TYPE_VIDEO_CAPTURE or V4L2_BUF_TYPE_VIDEO_OUTPUT .fmt.pix is
> used, for .type == V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE or
> V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE .fmt.pix_mp is used. In both these
> cases the user _must_ fill in .width, .height, .pixelformat, .field,
> .colorspace by possibly calling VIDIOC_G_FMT or VIDIOC_TRY_FMT. The
> user also _may_ optionally fill in any further buffer-size related
> fields, if it believes to have any special requirements to them. On
> a successful return from the ioctl() .count and .index fields are
> filled in by the kernel, .format stays unchanged. The user has to call
> VIDIOC_QUERYBUF to retrieve specific buffer information.
>
> 2. Videobuf2 drivers, that implement .vidioc_create_bufs() operation, call
> vb2_create_bufs() with a pointer to struct v4l2_create_buffers as a
> second argument. vb2_create_bufs() in turn calls the .queue_setup()
> driver callback, whose prototype is modified as follows:
>
> int (*queue_setup)(struct vb2_queue *q, const struct v4l2_format *fmt,
> unsigned int *num_buffers,
> unsigned int *num_planes, unsigned int sizes[],
> void *alloc_ctxs[]);
>
> with &create->format as a second argument. As pointed out above, this
> struct is not modified by V4L, instead, the usual arguments 3-6 are
> filled in by the driver, which are then used by vb2_create_bufs() to
> call __vb2_queue_alloc().
>
> 3. vb2_reqbufs() shall call .queue_setup() with fmt == NULL, which will be
> a signal to the driver to use the current format.
>
> 4. We keep .queue_setup(), because its removal would inevitably push a
> part of the common code from vb2_reqbufs() and vb2_create_bufs() down
> into drivers, thus creating code redundancy and increasing its
> complexity.
>
> You have 24 hours to object, before I proceed with the next version;-)
>
> Thanks
> Guennadi
> ---
> Guennadi Liakhovetski, Ph.D.
> Freelance Open-Source Software Developer
> http://www.open-technology.de/
>
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