Re: [PATCH v4 5/5] drm/ofdrm: Support big-endian scanout buffers

From: Thomas Zimmermann <tzimmermann@suse.de>
To: Arnd Bergmann <arnd@arndb.de>,
	Javier Martinez Canillas <javierm@redhat.com>,
	David Airlie <airlied@linux.ie>, Daniel Vetter <daniel@ffwll.ch>,
	Helge Deller <deller@gmx.de>, Maxime Ripard <maxime@cerno.tech>,
	sam@ravnborg.org, Michal Suchanek <msuchanek@suse.de>,
	Michael Ellerman <mpe@ellerman.id.au>,
	benh@kernel.crashing.org, Paul Mackerras <paulus@samba.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	mark.cave-ayland@ilande.co.uk
Cc: linux-fbdev@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	dri-devel@lists.freedesktop.org
Subject: Re: [PATCH v4 5/5] drm/ofdrm: Support big-endian scanout buffers
Date: Wed, 12 Oct 2022 16:31:14 +0200	[thread overview]
Message-ID: <a81e1acf-64dd-f69d-d97f-4a1af534e8e6@suse.de> (raw)
In-Reply-To: <76d8a408-fc3e-4bd1-91c5-8278f7469979@app.fastmail.com>

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Hi

Am 12.10.22 um 15:12 schrieb Arnd Bergmann:
> On Wed, Oct 12, 2022, at 2:00 PM, Thomas Zimmermann wrote:
>>
>> Could well be. But ofdrm intents to replace offb and this test has
>> worked well in offb for almost 15 yrs. If there are bug reports, I'm
>> happy to take patches, but until then I see no reason to change it.
> 
> I wouldn't change the code in offb unless a user reports a bug,
> but I don't see a point in adding the same mistake to ofdrm if we
> know it can't work on real hardware.

As I said, this has worked with offb and apparently on real hardware. 
For all I know, ATI hardware (before it became AMD) was used in PPC 
Macintoshs and assumed big-endian access on those machines.

> I tried to find out where this is configured in qemu, but it seems
> to depend on the framebuffer backend there: most are always little-endian,
> ati/bochs/vga-pci/virtio-vga are configurable from the guest through
> some register setting, but vga.c picks a default from the
> 'TARGET_WORDS_BIGENDIAN' macro, which I think is set differently
> between qemu-system-ppc64le and qemu-system-ppc64.
> 
> If you are using the framebuffer code from vga.c, I would guess that
> that you can run a big-endian kernel with qemu-system-ppc64,
> or a little-endian kernel with qemu-system-ppc64le and get the
> correct colors, while running a little-endian kernel with
> qemu-system-ppc64 and vga.c, or using a different framebuffer
> emulation on a big-endian kernel would give you the wrong colors.

If qemu doesn't give us the necessary DT property, it's a qemu bug. In 
in the absence of the property, picking the kernel's endianess is a 
sensible choice.

Best regards
Thomas

> 
> Which combinations did you actually test?
> 
>       Arnd

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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