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

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Message-ID: <20220712083907.3ic7bltstaskz72n@sirius.home.kraxel.org>
Date:   Tue, 12 Jul 2022 10:39:07 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
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

On Tue, Jul 12, 2022 at 10:01:15AM +0200, Geert Uytterhoeven wrote:
> Hi Gerd,
> 
> > 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'.

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.

take care,
  Gerd