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

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Message-ID: <96a87833-d878-dde9-e335-9ea51a4ba406@mailbox.org>
Date:   Mon, 11 Jul 2022 17:22:54 +0200
From:   Michel Dänzer <michel.daenzer@...lbox.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.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>,
        Gerd Hoffmann <kraxel@...hat.com>
Cc:     linux-fbdev@...r.kernel.org, linux-m68k@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] drm/fourcc: Add missing big-endian XRGB1555 and
 RGB565 formats

On 2022-07-08 20:21, Geert Uytterhoeven wrote:
> As of commit eae06120f1974e1a ("drm: refuse ADDFB2 ioctl for broken
> bigendian drivers"), drivers must set the
> quirk_addfb_prefer_host_byte_order quirk to make the drm_mode_addfb()
> compat code work correctly on big-endian machines.
> 
> While that works fine for big-endian XRGB8888 and ARGB8888, which are
> mapped to the existing little-endian BGRX8888 and BGRA8888 formats, it
> does not work for big-endian XRGB1555 and RGB565, as the latter are not
> listed in the format database.
> 
> Fix this by adding the missing formats.  Limit this to big-endian
> platforms, as there is currently no need to support these formats on
> little-endian platforms.
> 
> Fixes: 6960e6da9cec3f66 ("drm: fix drm_mode_addfb() on big endian machines.")
> Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> ---
> 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's also doubtful that Cirrus hardware would access these formats as big endian (drivers/gpu/drm/tiny/cirrus.c has no endianness references at all, and the hardware was surely designed for x86 first and foremost).

Instead, fbcon (and user space) needs to convert to little endian when using DRM_FORMAT_HOST_{XRGB1555,RGB565} with the cirrus driver on big endian hosts.


-- 
Earthling Michel Dänzer            |                  https://redhat.com
Libre software enthusiast          |         Mesa and Xwayland developer