On Mon, 4 Apr 2011, Hans Verkuil wrote:
[snip]
> >> > +/* VIDIOC_CREATE_BUFS */
> >> > +struct v4l2_create_buffers {
> >> > + __u32 index; /* output: buffers
> >> > index...index + count - 1 have
> >> been created */
> >> > + __u32 count;
> >> > + __u32 flags; /* V4L2_BUFFER_FLAG_* */
> >> > + enum v4l2_memory memory;
> >> > + __u32 size; /* Explicit size, e.g.,
> >> > for compressed streams */
> >>
> >> Hmm, shouldn't this be an array of size VIDEO_MAX_PLANES?
> >
> > Not sure. As the comment says, this is mainly for bitstream formats. For
> > any pixel-based format you really should just fill in the format below. If
> > you allow the user to specify planes, then you also need to know
> > alignment, contiguity, padding, etc.
> >
> >> > + struct v4l2_format format; /* "type" is used
> >> > always, the rest if
> >> size == 0 */
> >>
> >> Needs some reserved fields as well.
> >
> > v4l2_format is a union with 200 bytes, is this not enough?
>
> You can't add new fields to the v4l2_pix_format struct in this union,
> because that would mess up the G/S_FBUF ioctls. Really a V4L2 design flaw.
>
> So it's better to add some extra reserveds at the top level.
Can do that, sure, but just for understanding: as long as we're sure the
other union members don't exceed ("a bit fewer than") 200 bytes, why
cannot we change 200 to 196 in the union and add a reserver u32 to all
affected ioctl()s - if / when needed?
> >> > +};
> >> > +
> >> > /*
> >> > * I O C T L C O D E S F O R V I D E O D E V I C E S
> >> > *
> >> > @@ -1937,6 +1957,10 @@ struct v4l2_dbg_chip_ident {
> >> > #define VIDIOC_SUBSCRIBE_EVENT _IOW('V', 90, struct
> >> v4l2_event_subscription)
> >> > #define VIDIOC_UNSUBSCRIBE_EVENT _IOW('V', 91, struct
> >> v4l2_event_subscription)
> >> >
> >> > +#define VIDIOC_CREATE_BUFS _IOWR('V', 92, struct
> >> > v4l2_create_buffers)
> >> > +#define VIDIOC_DESTROY_BUFS _IOWR('V', 93, struct v4l2_buffer_span)
> >> > +#define VIDIOC_SUBMIT_BUF _IOW('V', 94, int)
> >>
> >> I don't really like the name. I think I'd go for _PRE_QBUF or _PREP_BUF
> >> or
> >> something like that.
> >
> > I didn't want to use any form of prepare, because we already have a
> > .buf_prepare() method, and IMHO it would be confusing. Pre-queue I didn't
> > like very much either, for the same reasons, why we rejected it at the
> > meeting, which was, I think, that it's not really doing anything with the
> > queue, right? What this ioctl() does is it passes buffer ownership from
> > the user-space to the kernel, right? Any good name for that? In fact
> > "submit" doesn't sound too bad to me:-)
>
> Well, naming isn't my strongest point. But isn't buf_prepare exactly the
> op that SUBMIT is supposed to call (besides pinning the buffers in
> memory)? That's the whole point of this ioctl. Anyway, I'd have to hear
> what others think.
I've been thinking about that too. Currently .buf_prepare() is called on
each QBUF, so, it performs a different task - a dynamic preparation,
necessary during a running capture / playback. Whereas the new ioctl()
should only perform a one-off operation - passing the ownership on the
buffer. Seems pretty different to me.
> >> BTW, I agree with other reviewers that DESTROY_BUFS shouldn't leave any
> >> holes.
> >
> > That was our decision at the meeting - I can well remember that, I was
> > surprised about that too, so, I think, I even asked to clarify this point.
> > But we can change it of course. Shall we return -EBUSY if the user is
> > trying to free buffers in the middle?
>
> I'm having second thoughts about this. If you have lots of buffers of
> different sizes, then it might make sense to destroy buffers in the
> middle.
>
> But we should perhaps just disallow it for now. We can always be more
> lenient later if necessary.
That would be the easiest for now. Do we all agree?
> >> > +
> >> > /* Reminder: when adding new ioctls please add support for them to
> >> > drivers/media/video/v4l2-compat-ioctl32.c as well! */
> >>
> >> Don't forget this! VIDIOC_CREATE_BUFS will need to be handled in
> >> compat-ioctl32.
> >
> > I didn't:
> >
> >> > drivers/media/video/v4l2-compat-ioctl32.c | 3 ++
> >
> > Or is that not enough? Is any special handling required? AFAICS, REQBUFS
> > is not treated specially either.
>
> No, you need special handling for the 'enum memory' and the struct
> v4l2_format. These have different sizes depending on the architecture.
Hm, I'll have a look at them then...
> BTW, REQBUFS and CREATE/DESTROY_BUFS should definitely co-exist. REQBUFS
> is compulsory, while CREATE/DESTROY are optional.
Sure, drivers _must_ implement REQBUFS and _may_ implement CREATE/DESTROY.
The question is - should we allow mixing of them? E.g., an app first
calling REQBUFS, then CREATE to add more buffers, and eventually
REQBUFS(0) to destroy them all?...
Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/
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