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    SubjectRe: [PATCH v2 05/10] drm/fourcc: Add DRM_FORMAT_C[124]
    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

    Gr{oetje,eeting}s,

    Geert

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

    In personal conversations with technical people, I call myself a hacker. But
    when I'm talking to journalists I just say "programmer" or something like that.
    -- Linus Torvalds

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