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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Mon, 14 Mar 2022 14:30:18 +0100
Message-ID: <CAMuHMdVq19wpA_7nKKTm-G2EmK3cMxxP6nbR_u=vkazqCZ=KhQ@mail.gmail.com>
Subject: Re: [PATCH v2 05/10] drm/fourcc: Add DRM_FORMAT_C[124]
To:     Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Javier Martinez Canillas <javierm@redhat.com>,
        Sam Ravnborg <sam@ravnborg.org>, Helge Deller <deller@gmx.de>
Cc:     DRI Development <dri-devel@lists.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
        "Linux/m68k" <linux-m68k@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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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