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SubjectRe: [PATCH v2 05/10] drm/fourcc: Add DRM_FORMAT_C[124]
On Mon, 14 Mar 2022 14:30:18 +0100
Geert Uytterhoeven <geert@linux-m68k.org> wrote:

> On Mon, Mar 7, 2022 at 9:53 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > Introduce fourcc codes for color-indexed frame buffer formats with two,
> > four, and sixteen colors, and provide a mapping from bit per pixel and
> > depth to fourcc codes.
> >
> > As the number of bits per pixel is less than eight, these rely on proper
> > block handling for the calculation of bits per pixel and pitch.
> >
> > Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
>
> > --- a/include/uapi/drm/drm_fourcc.h
> > +++ b/include/uapi/drm/drm_fourcc.h
> > @@ -99,7 +99,10 @@ extern "C" {
> > #define DRM_FORMAT_INVALID 0
> >
> > /* color index */
> > -#define DRM_FORMAT_C8 fourcc_code('C', '8', ' ', ' ') /* [7:0] C */
> > +#define DRM_FORMAT_C1 fourcc_code('C', '1', ' ', ' ') /* [7:0] C0:C1:C2:C3:C4:C5:C6:C7 1:1:1:1:1:1:1:1 eight pixels/byte */
> > +#define DRM_FORMAT_C2 fourcc_code('C', '2', ' ', ' ') /* [7:0] C0:C1:C2:C3 2:2:2:2 four pixels/byte */
> > +#define DRM_FORMAT_C4 fourcc_code('C', '4', ' ', ' ') /* [7:0] C0:C1 4:4 two pixels/byte */
> > +#define DRM_FORMAT_C8 fourcc_code('C', '8', ' ', ' ') /* [7:0] C 8 one pixel/byte */
> >
> > /* 8 bpp Red */
> > #define DRM_FORMAT_R8 fourcc_code('R', '8', ' ', ' ') /* [7:0] R */
>
> After replying to Ilia's comment[1], I realized the CFB drawing
> operations use native byte and bit ordering, unless
> FBINFO_FOREIGN_ENDIAN is set.
> While Amiga, Atari, and Sun-3 use big-endian bit ordering,
> e.g. Acorn VIDC[2] uses little endian, and SH7760[3] is configurable
> (sh7760fb configures ordering to match host order).
> BTW, ssd130{7fb,x}_update_rect() both assume little-endian, so I
> guess they are broken on big-endian.
> Fbtest uses big-endian bit ordering, so < 8 bpp is probably broken
> on little-endian.
>
> Hence the above should become:
>
> #define DRM_FORMAT_C1 fourcc_code('C', '1', ' ', ' ') /*
> [7:0] C7:C6:C5:C4:C3:C2:C1:C0 1:1:1:1:1:1:1:1 eight pixels/byte */
> #define DRM_FORMAT_C2 fourcc_code('C', '2', ' ', ' ') /*
> [7:0] C3:C2:C1:C0 2:2:2:2 four pixels/byte */
> #define DRM_FORMAT_C4 fourcc_code('C', '4', ' ', ' ') /*
> [7:0] C1:C0 4:4 two pixels/byte */
>
> The same changes should be made for DRM_FORMAT_[RD][124].
>
> The fbdev emulation code should gain support for these with and without
> DRM_FORMAT_BIG_ENDIAN, the latter perhaps only on big-endian platforms?
>
> [1] https://lore.kernel.org/r/CAKb7UvgEdm9U=+RyRwL0TGRfA_Qc7NbhCWoZOft2DKdXggtKYw@mail.gmail.com/
> [2] See p.30 of the VIDC datasheet
> http://chrisacorns.computinghistory.org.uk/docs/Acorn/Misc/Acorn_VIDC_Datasheet.pdf
> [3] See p.1178 of the SH7660 datasheet
> https://datasheet.octopart.com/HD6417760BL200AV-Renesas-datasheet-14105759.pdf

Hi Geert,

why would CPU endianess affect the order of bits in a byte?

Do you mean that bit 0 one machine is (1 << 0), and on another machine
bit 0 is (1 << 7)?

In C, we have only one way to address bits of a byte and that is with
arithmetic. You cannot take the address of a bit any other way, can you?

Can we standardise on "bit n of a byte is addressed as (1 << n)"?

I don't mind in which order the pixels are inside a byte, as long as it
doesn't change by CPU endianess. If you need both directions, then use
two different drm_fourcc codes that do not change their meaning by CPU
endianess. Just like we have XRGB and BGRX formats.

I would not like to see DRM_FORMAT_BIG_ENDIAN used for this, it would
conflate a whole new concept into the mess that is little- vs.
big-endian.


Thanks,
pq
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