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

[Guennadi Liakhovetski <g.liakhovetski@gmx.de>]

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.

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/