Hi Sylwester,
On Sat, Nov 26, 2011 at 01:21:01PM +0100, Sylwester Nawrocki wrote:
...
> Did you have a chance to look at this RFC [1] ? I have reviewed usage
> of VIDIOC_[S/G]_JPEGCOMP ioctls and there are also some links to previous
> discussion there.
I have to admit I had completely missed this RFC. I'll take a look at it.
>
> >
> > 2) Define a new control for jpeg quality. Its value range can be what the
> > hardware supports and the user space gets much better information on the
> > capabilities of the hardware and the granularity of the quality setting.
>
> This was my conclusion as well. That sounds like a most effective and
> flexible
> solution. IIRC, Hans also suggested those things might be a perfect candidate
> for a control class.
Sounds like a good idea. Would the class be intended for all compressed
formats or just jpeg? I have to say I don't know much about the
alternatives, except that their compression ratio tends to be much better on
equivalent quality.
>
> >
> > I might even favour the second one. I also wonder how many user space
> > applications use this IOCTL, so if we're breaking anything by not supporting
> > it.
>
> I imagine we could deprecate 'quality' and 'jpeg_markers' fields of
> v4l2_jpegcompression
> and during deprecation period add support for the controls for these
> parameters,
> so the application can adapt and fall back to a control based interface once
> the
> ioctl is removed in the kernel.
> This should be possible for almost all drivers as they virtually use
> G/S_JPEGCOMP
> ioctls for image quality only.
>
> >
> > Or we could decide to do option 1 right now and implement 2) later on. I can
> > write a patch to change the documentation.
>
> The documentation could always be improved. But I personally would like to
> see these
> ioctls die, as they're really hopeless as a part of public API. I though about
> something like
>
> /* The ioctls to configure/query selected data segment in a jpeg encoder */
> #define VIDIOC_G_JPEGCOMP _IOR('V', 61, struct v4l2_jpegcomp)
> #define VIDIOC_S_JPEGCOMP _IOW('V', 62, struct v4l2_jpegcomp)
>
> /**
> * struct v4l2_jpegcomp - JPEG segment data structure
> *
> * @id: field indicating what standard JPEG data segment @data contains
> * @data: points the segment data
> * @length: length of the segment
> */
> struct v4l2_jpegcomp {
> __u32 id;
>
> int length;
> void *data;
> };
>
>
> As a side note, our plan was to get merged the S5P JPEG codec in v3.3, with
> the old jpeg ioctls and then switch to the controls when they're ready,
> possibly
> in subsequent kernel release. That's why the driver is marked as experimental.
Sounds good to me.
--
Sakari Ailus
e-mail: sakari.ai...@iki.fi jabber/XMPP/Gmail: sai...@retiisi.org.uk
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