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

["Arnd Bergmann" <arnd@arndb.de>]

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.

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.

Which combinations did you actually test?

     Arnd