Re: [PATCH] fbdev: fix color component field length documentation

[Geert Uytterhoeven <geert@linux-m68k.org>]

On Tue, Mar 31, 2009 at 00:00, Michal Januszewski <spock@gentoo.org> wrote:
> The documentation about the meaning of the color component bitfield lengths
> in pseudocolor modes is inconsistent.  Fix it, so that it indicates the
> correct interpretation everywhere, i.e. that the 1 << length is the number
> of palette entries.
>
> Signed-off-by: Michal Januszewski <spock@gentoo.org>
> Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
> Cc: Ville Syrjälä <syrjala@sci.fi>
> Cc: Geert Uytterhoeven <geert.uytterhoeven@gmail.com>

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>

Except for this:

> --- a/include/linux/fb.h
> +++ b/include/linux/fb.h
> @@ -172,8 +172,11 @@ struct fb_fix_screeninfo {
>  /* Interpretation of offset for color fields: All offsets are from the right,
>  * inside a "pixel" value, which is exactly 'bits_per_pixel' wide (means: you
>  * can use the offset as right argument to <<). A pixel afterwards is a bit
> - * stream and is written to video memory as that unmodified. This implies
> - * big-endian byte order if bits_per_pixel is greater than 8.
> + * stream and is written to video memory as that unmodified.
> + *
> + * For pseudocolor, offset is always 0 and length, which should be the same
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This is not correct. Offset can be non-zero, e.g. for a 8 bpp frame buffer with
64 palette entries, where the palette index is stored in the upper 6 bits of the
8-bit pixel value, offset would be 2, and length would be 6.
Not that I've seen that (so far)...

> + * for all color components, indicates the number of available palette entries
> + * (i.e. # of entries = 1 << length).
>  */
>  struct fb_bitfield {
>        __u32 offset;                   /* beginning of bitfield        */
>

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

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							    -- Linus Torvalds