linux-kernel - Re: [PATCH resend v2] drm/fourcc: Add missing big-endian XRGB1555 and RGB565 formats

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Message-ID: <20221124111754.yrfewus6mqismc3b@sirius.home.kraxel.org>
Date:   Thu, 24 Nov 2022 12:17:54 +0100
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH resend v2] drm/fourcc: Add missing big-endian XRGB1555
 and RGB565 formats

  Hi,

> > 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?

Sure doable, but it's an extra step in a rarely tested code path ...

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

I don't think so.  When you can get the color bits with shifting and
masking pixman is happy.  For rgb565 (and xrgb1555) that works only on
native byte order.

take care,
  Gerd