Re: [PATCH 2/3] uapi: drm: New fourcc codes needed by Xilinx Video IP

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Ville,

On Tuesday, 28 November 2017 20:11:42 EET Ville Syrjälä wrote:
> On Tue, Nov 28, 2017 at 05:25:46PM +0000, Hyun Kwon wrote:
> > On Tuesday, November 28, 2017 6:44 AM Ville Syrjälä wrote:
> >> On Mon, Nov 27, 2017 at 06:27:32PM -0800, Hyun Kwon wrote:
> >>> From: Jeffrey Mouroux <jmouroux@xilinx.com>
> >>> 
> >>> The Xilinx Video Mixer and Xilinx Video Framebuffer DMA IP
> >>> support video memory formats that are not represented in the
> >>> current DRM fourcc library.  This patch adds those missing
> >>> fourcc codes.
> >>> 
> >>> Signed-off-by: Jeffrey Mouroux <jmouroux@xilinx.com>
> >>> Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
> >>> ---
> >>> 
> >>>  include/uapi/drm/drm_fourcc.h | 9 +++++++++
> >>>  1 file changed, 9 insertions(+)
> >>> 
> >>> diff --git a/include/uapi/drm/drm_fourcc.h
> >>> b/include/uapi/drm/drm_fourcc.h
> >>> index 3ad838d..83806d5 100644
> >>> --- a/include/uapi/drm/drm_fourcc.h
> >>> +++ b/include/uapi/drm/drm_fourcc.h
> >>> @@ -112,6 +112,13 @@ extern "C" {
> >>> 
> >>>  #define DRM_FORMAT_VYUY		fourcc_code('V', 'Y', 'U', 'Y')
> >>> /* [31:0] Y1:Cb0:Y0:Cr0 8:8:8:8 little endian */
> >>>  #define DRM_FORMAT_AYUV		fourcc_code('A', 'Y', 'U', 'V')
> >>> /* [31:0] A:Y:Cb:Cr 8:8:8:8 little endian */
> >>> +#define DRM_FORMAT_VUY888	fourcc_code('V', 'U', '2', '4') /* [23:0]
> >>> Cr:Cb:Y little endian */
> >>> +#define DRM_FORMAT_XVUY8888	fourcc_code('X', 'V', '2', '4') /* [31:0]
> >>> x:Cr:Cb:Y 8:8:8:8 little endian */
> >>> +#define DRM_FORMAT_XVUY2101010	fourcc_code('X', 'V', '3', '0')
> >>> /* [31:0] x:Cr:Cb:Y 2:10:10:10 little endian */
> >>> +
> >>> +/* Grey scale */
> >>> +#define DRM_FORMAT_Y8		fourcc_code('G', 'R', 'E', 'Y') /* 8-bit
> >>> packed greyscale Y:Y:Y:Y 8:8:8:8 */
> >>> +#define DRM_FORMAT_Y10		fourcc_code('Y', '1', '0', ' ') /*
> >>> 10-bit packed greyscale X:Y:Y:Y 2:10:10:10 */
> >> 
> >> These don't make sense to me. What does it even mean to have Y
> >> replicated three or four times for each pixel?
> > 
> > Each pixel has one Y component. The comment is to describe how components
> > are stored in 32bit, but I agree it's confusing. Will describe better.
> For the 8 bit Y format you should then define it as 8 bits. Unless of
> course it really is defined as a 32bit word containing 4 pixels. The
> 10 bit case would be even funky since there would have to be two
> bits of padding after every 3 pixels.
> 
> So are these really defined as 32 bit wits 3 or 4 pixels and potentially
> a few bits of extra padding packed into each word? That seems rather
> nuts to me because you can't even byte address each pixel.
> 
> I think such crazyness has no business living right next to the
> sane formats, hence we should put all these into their own little
> section of the header, and the names should somehow reflect that
> they are in fact "special".

Thee Y8 format isn't special as it's just a plain 8-bit format. The Y10 format 
is indeed not byte-addressable, but don't be too fast to dismiss it as crazy, 
this kind of format is pretty common outside of the desktop graphics world. 
There are even 10-bit formats with 3 components where the 8 LSBs of each 
component are stored in three bytes, and the 2 MSBs are grouped in a fourth 
byte with 2 bits of padding (or possibly with LSB and MSB switched, I can't 
remember right now).

-- 
Regards,

Laurent Pinchart

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