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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Tue, 12 Jul 2022 10:43:17 +0200
Message-ID: <CAMuHMdVhxE9aayG8qRMwUuBryiR_ng08m63_+GY8htFCSmUiWg@mail.gmail.com>
Subject: Re: [PATCH 1/3] drm/fourcc: Add missing big-endian XRGB1555 and
 RGB565 formats
To:     Gerd Hoffmann <kraxel@redhat.com>
Cc:     =?UTF-8?Q?Michel_D=C3=A4nzer?= <michel.daenzer@mailbox.org>,
        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>,
        =?UTF-8?Q?Noralf_Tr=C3=B8nnes?= <noralf@tronnes.org>,
        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>
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Hi Gerd,

On Tue, Jul 12, 2022 at 10:39 AM Gerd Hoffmann <kraxel@redhat.com> wrote:
> On Tue, Jul 12, 2022 at 10:01:15AM +0200, Geert Uytterhoeven wrote:
> > > 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.
>
> Fixed big endian should use 'DRM_FORMAT_RGB565 | DRM_FORMAT_BIG_ENDIAN'.

True.

> As described above DRM_FORMAT_HOST_RGB565 means bigendian on bigendian
> hosts and little endian on little endian hosts.  Which is not correct
> when your hardware does big endian no matter what.

But (a) drm_driver_legacy_fb_format() uses DRM_FORMAT_HOST_RGB565
if quirk_addfb_prefer_host_byte_order is set, and (b)
quirk_addfb_prefer_host_byte_order must be set on big-endian as
per commit eae06120f1974e1a ("drm: refuse ADDFB2 ioctl for broken
bigendian drivers").

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