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Date: Wed, 12 Oct 2022 18:01:52 +0200
From: Michal =?iso-8859-1?Q?Such=E1nek?= <msuchanek@suse.de>
To: Ville =?iso-8859-1?Q?Syrj=E4l=E4?= <ville.syrjala@linux.intel.com>
Subject: Re: [PATCH v4 5/5] drm/ofdrm: Support big-endian scanout buffers
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Cc: linux-fbdev@vger.kernel.org, Thomas Zimmermann <tzimmermann@suse.de>, Arnd Bergmann <arnd@arndb.de>, David Airlie <airlied@linux.ie>, Helge Deller <deller@gmx.de>, linuxppc-dev@lists.ozlabs.org, mark.cave-ayland@ilande.co.uk, Javier Martinez Canillas <javierm@redhat.com>, dri-devel@lists.freedesktop.org, Paul Mackerras <paulus@samba.org>, Maxime Ripard <maxime@cerno.tech>, Daniel Vetter <daniel@ffwll.ch>, Geert Uytterhoeven <geert@linux-m68k.org>, sam@ravnborg.org
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On Wed, Oct 12, 2022 at 05:59:45PM +0300, Ville Syrjälä wrote:
> On Wed, Oct 12, 2022 at 04:31:14PM +0200, Thomas Zimmermann wrote:
> > 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.
> 
> At least mach64 class hardware has two frame buffer apertures, and
> byte swapping can be configured separately for each. But that means
> you only get correct byte swapping for at most two bpps at the same
> time (and that only if you know which aperture to access each time).
> IIRC Rage 128 already has the surface register stuff where you
> could byte swap a limited set of ranges independently. And old
> mga hardware has just one byte swap setting for the whole frame
> buffer aperture, so only one bpp at a time.
> 
> That kind of horrible limitations of the byte swappers is the
> main reason why I wanted to make drm fourcc endianness explicit.
> Simply assuming host endianness would end in tears on big endian
> as soon as you need to access stuff with two bpps at the same time.
> Much better to just switch off those useless byte swappers and
> swap by hand when necessary.

If you have hardware-specific driver, sure.

This is firmware-provided framebuffer, though. You get one framebuffer
address, and one endian - whatever the firmware set up and described in
the DT.

Thanks

Michal