linux-kernel - Re: [PATCH 1/3] drm/fourcc: Add missing big-endian XRGB1555 and RGB565 formats

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Message-ID: <CAMuHMdVrf7fgzumcSnZJ3OMGqA34YExXcF3O15YXYpA1ykgKyQ@mail.gmail.com>
Date:   Tue, 12 Jul 2022 10:01:15 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     Michel Dänzer <michel.daenzer@...lbox.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Noralf Trønnes <noralf@...nnes.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        "Linux/m68k" <linux-m68k@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] drm/fourcc: Add missing big-endian XRGB1555 and
 RGB565 formats

Hi Gerd,

On Tue, Jul 12, 2022 at 9:47 AM Gerd Hoffmann <kraxel@...hat.com> wrote:
> On Mon, Jul 11, 2022 at 05:30:30PM +0200, Geert Uytterhoeven wrote:
> > > > Cirrus is the only driver setting quirk_addfb_prefer_host_byte_order
> > > > and supporting RGB565 or XRGB1555, but no one tried that on big-endian?
> > > > Cirrus does not support DRM_FORMAT_RGB565 | DRM_FORMAT_BIG_ENDIAN
> > > > in cirrus_fb_create, so you cannot get a graphical text console.
> > > >
> > > > Do we need these definitions on little-endian platforms, too?
> > > > Would it be better to use "DRM_FORMAT_{XRGB1555,RGB565} |
> > > > DRM_FORMAT_BIG_ENDIAN" instead of "DRM_FORMAT_HOST_{XRGB1555,RGB565}" in
> > > > formats[]?
> > >
> > > The intention of DRM_FORMAT_HOST_* is that they are macros in
> > > include/drm/drm_fourcc.h which just map to little endian formats
> > > defined in drivers/gpu/drm/drm_fourcc.c. Since this is not possible
> > > for big endian hosts for XRGB1555 or RGB565 (or any other format
> > > with non-8-bit components), this isn't applicable here.
>
> It IMHO is not applicable to any physical hardware.  It's used by
> virtio-gpu where the supported format depends on the byte order
> (it is argb8888 in native byte order).  Only virtual hardware can
> have that kind of behavior.
>
> And we can probably drop the DRM_FORMAT_HOST_* variants for 1555 and
> 565, they are not used anywhere.

Atari DRM supports (big-endian) RGB565, so it uses
DRM_FORMAT_HOST_RGB565.

The alternative is to drop the quirk_addfb_prefer_host_byte_order
requirement on big-endian, and always use a little-endian RGB565
shadow frame buffer, at the expense of never being able to get rid
of the copying and byteswapping.

[Cirrus discussion removed]

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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