Re: [PATCH 1/6 v4] V4L: add two new ioctl()s for multi-size videobuffer management

[Hans Verkuil <hverkuil@xs4all.nl>]

On Monday, August 22, 2011 19:21:18 Laurent Pinchart wrote:
> Hi Hans,
> 
> On Monday 22 August 2011 17:52:12 Hans Verkuil wrote:
> > On Monday, August 22, 2011 17:42:36 Laurent Pinchart wrote:
> > > On Monday 22 August 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. Pawel,
> > > > Sakari, Laurent: please, reply, whether you're ok with this.
> > > 
> > > Sakari is on holidays this week.
> > > 
> > > > 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.
> > > 
> > > How much common code would be pushed down to drivers ? I don't think this
> > > is a real issue. I like Pawel's proposal of removing .queue_setup()
> > > better.
> > 
> > I still don't see what removing queue_setup will solve or improve.
> 
> It will remove handling of the format in vb2 (even if it's a pass-through 
> operation). I think it would be cleaner that way. It will also avoid going 
> back and forth between drivers and vb2, which would improve code readability.

I very much doubt it will be more readable or cleaner. For one thing it is
inconsistent with the other ioctl ops where you just call a vb2 function to
handle it. Suddenly here you have to do lots of things to make it work.

> > I'd say leave it as it is to keep the diff as small as possible and someone
> > can always attempt to remove it later. Removing queue_setup is independent
> > from multi-size videobuffer management and we should not mix the two.
> 
> Guennadi's patch will (at least in my opinion) be cleaner if built on top of 
> queue_setup() removal.

Really? Just adding a single v4l2_format pointer is less clean than removing
queue_setup? That would really surprise me.

Anyway, let Guennadi choose what is easiest. It's an implementation detail in
the end and I just want to get this functionality in.

Regards,

	Hans