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Date:   Mon, 14 Mar 2022 17:05:39 +0200
From:   Pekka Paalanen <ppaalanen@gmail.com>
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     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>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
        Linux/m68k <linux-m68k@vger.kernel.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 05/10] drm/fourcc: Add DRM_FORMAT_C[124]
Message-ID: <20220314170539.17400f93@eldfell>
In-Reply-To: <CAMuHMdVq19wpA_7nKKTm-G2EmK3cMxxP6nbR_u=vkazqCZ=KhQ@mail.gmail.com>
References: <cover.1646683502.git.geert@linux-m68k.org>
        <8d3c0cc370b0214244b01a64c588e5e506531716.1646683502.git.geert@linux-m68k.org>
        <CAMuHMdVq19wpA_7nKKTm-G2EmK3cMxxP6nbR_u=vkazqCZ=KhQ@mail.gmail.com>
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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> =20
>=20
> > --- 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 */ =20
>=20
> 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.
>=20
> Hence the above should become:
>=20
>     #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 */
>=20
> The same changes should be made for DRM_FORMAT_[RD][124].
>=20
> The fbdev emulation code should gain support for these with and without
> DRM_FORMAT_BIG_ENDIAN, the latter perhaps only on big-endian platforms?
>=20
> [1] https://lore.kernel.org/r/CAKb7UvgEdm9U=3D+RyRwL0TGRfA_Qc7NbhCWoZOft2=
DKdXggtKYw@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-141=
05759.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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