[RFC] Standardize YUV support in the fbdev API

[Hans Verkuil]

On Monday, May 23, 2011 13:55:21 Marek Szyprowski wrote:
> Hello,
> 
> On Wednesday, May 18, 2011 8:54 AM Hans Verkuil wrote:
> 
> > On Wednesday, May 18, 2011 00:44:26 Felipe Contreras wrote:
> > > On Wed, May 18, 2011 at 1:07 AM, Laurent Pinchart
> > > <laurent.pinchart at ideasonboard.com> wrote:
> > > > I need to implement support for a YUV frame buffer in an fbdev driver.
> > As the
> > > > fbdev API doesn't support this out of the box, I've spent a couple of
> > days
> > > > reading fbdev (and KMS) code and thinking about how we could cleanly
> > add YUV
> > > > support to the API. I'd like to share my findings and thoughts, and
> > hopefully
> > > > receive some comments back.
> > > >
> > > > The terms 'format', 'pixel format', 'frame buffer format' and 'data
> > format'
> > > > will be used interchangeably in this e-mail. They all refer to the way
> > pixels
> > > > are stored in memory, including both the representation of a pixel as
> > integer
> > > > values and the layout of those integer values in memory.
> > >
> > > This is a great proposal. It was about time!
> > >
> > > > The third solution has my preference. Comments and feedback will be
> > > > appreciated. I will then work on a proof of concept and submit patches.
> > >
> > > I also would prefer the third solution. I don't think there's much
> > > difference from the user-space point of view, and a new ioctl would be
> > > cleaner. Also the v4l2 fourcc's should do.
> > 
> > I agree with this.
> > 
> > We might want to take the opportunity to fix this section of the V4L2 Spec:
> > 
> > http://www.xs4all.nl/~hverkuil/spec/media.html#pixfmt-rgb
> > 
> > There are two tables, 2.6 and 2.7. But 2.6 is almost certainly wrong and
> > should be removed.
> 
> That's definitely true. I was confused at the beginning when I saw 2
> different tables describing the same pixel formats.
> 
>  I suspect many if not all V4L2 drivers are badly broken for
> > big-endian systems and report the wrong pixel formats.
> > 
> > Officially the pixel formats reflect the contents of the memory. But
> > everything is swapped on a big endian system, so you are supposed to 
> > report a different pix format.
> 
> I always thought that pix_format describes the layout of video data in
> memory on byte level, which is exactly the same on both little- and big-
> endian systems.

Correct.

> You can notice swapped data only if you access memory
> by units larger than byte, like 16bit or 32bit integers. BTW - I would
> really like to avoid little- and big- endian flame, but your statement
> about 'everything is swapped on a big endian system' is completely
> wrong. It is rather the characteristic of little-endian system not big
> endian one if you display the content of the same memory first using
> byte access and then using word/long access.

You are correct, I wasn't thinking it through.
 
> > I can't remember seeing any driver do that. Some have built-in swapping,
> > though, and turn that on if needed.
> 
> The drivers shouldn't do ANY byte swapping at all. Only tools that
> extract pixel data with some 'accelerated' methods (like 32bit integer
> casting and bit-level shifting) should be aware of endianess.
> 
> > I really need to run some tests, but I've been telling myself this for
> > years now :-(
> 
> I've checked the BTTV board in my PowerMac/G4 and the display was
> correct with xawtv. It is just a matter of selecting correct pix format
> basing on the information returned by xsever. 
> 
> Best regards
> 

Just forget my post (except for the part of cleaning up the tables :-) ).

Regards,

	Hans