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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Thu, 24 Nov 2022 09:02:53 +0100
Message-ID: <CAMuHMdUJUUPPxS6VCHV1X9XMqzfACvu8qivUVO2pMbvD7rcQKg@mail.gmail.com>
Subject: Re: [PATCH resend v2] drm/fourcc: Add missing big-endian XRGB1555 and
 RGB565 formats
To:     Gerd Hoffmann <kraxel@redhat.com>
Cc:     Daniel Vetter <daniel@ffwll.ch>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@gmail.com>,
        dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org,
        linux-m68k@lists.linux-m68k.org, linux-kernel@vger.kernel.org
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Hi Gerd,

On Thu, Nov 24, 2022 at 8:29 AM Gerd Hoffmann <kraxel@redhat.com> wrote:
> > > > +#ifdef __BIG_ENDIAN
> > >
> > > Why do we need the #ifdef here? Iirc some hw has big endian flags in the
> > > scanout registers, so could supprt this unconditionally if there's no
> > > #ifdef around the format defines. Some drivers might then also want a
> > > DRM_FORMAT_FOO_BE define to simplify tables and stuff, but that's more a
> > > bikeshed.
> >
> >  "Limit this to big-endian platforms, as there is currently no need
> >  to support these formats on little-endian platforms."
> >
> > Will anyone make use of this? In theory, all of the 16-bpp formats
> > can have a big-endian counterpart.
>
> Highly unlikely.  Dealing with 16-bpp formats in non-native byte order
> is a PITA because it isn't enough to simply adjust the masks and shifts
> to pick the correct bits and be done with it.
>
> The qemu stdvga happens to have a register to switch framebuffer
> byteorder (so both x64 and ppc are happy), and the bochs drm driver
> actually supports that no matter what the cpu byte order is, but it
> supports only DRM_FORMAT_XRGB8888 + DRM_FORMAT_BGRX8888.
>
> Supporting 16 bpp in the driver wouldn't be that much of a problem, but
> processing the framebuffer on the host side when emulating a big endian
> guest on a little endian host is painful.  I think I can't ask pixman to
> do a conversation from DRM_FORMAT_RGB565 | DRM_FORMAT_BIG_ENDIAN to
> DRM_FORMAT_XRGB8888 on a little endian machine.

Indeed. But you can do a quick 16-bit byteswap, and convert from
DRM_FORMAT_RGB565 to DRM_FORMAT_XRGB8888?

There's a similar issue with Cairo, cfr. '[PATCH libdrm v2 08/10]
util: Fix pwetty on big-endian"[1].

BTW, does pixman support converting DRM_FORMAT_RGB565 to
DRM_FORMAT_XRGB8888 on a big-endian machine?

[1] https://lore.kernel.org/all/e8597038478f12e9eda5e86b309b52988f69f2eb.1657302103.git.geert@linux-m68k.org

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