DRM_FORMAT_* byte order (was: Re: [PATCH] drm: virtio: fix virtio_gpu_cursor_formats)

Gerd Hoffmann kraxel at redhat.com
Thu Apr 6 08:29:43 UTC 2017


> >  static const uint32_t virtio_gpu_cursor_formats[] = {
> > +#ifdef __BIG_ENDIAN
> > +	DRM_FORMAT_BGRA8888,
> > +#else
> >  	DRM_FORMAT_ARGB8888,
> > +#endif
> DRM formats are supposed to be little endian, so this isn't really
> correct.

Well, maybe they where *intended* to be little endian at some point in
the past.  The actual code appears to interpret them as native endian

Lets take a simple example, the bochs driver (qemu sdvga).  It supports
32 bpp with depth 24 (DRM_FORMAT_XRGB8888) as the one and only
framebuffer format (see bochs_user_framebuffer_create).  We still had to
add a special register to the virtual hardware so the guest can signal
to the host whenever the framebuffer is big endian or little endian (see
bochs_hw_init), so both ppc64 and ppc64le guests work properly with the
qemu stdvga.

So, bigendian guests assume that DRM_FORMAT_XRGB8888 is big endian not
little endian.  And given that the fourcc codes are used in the
userspace/kernel API too (see DRM_IOCTL_MODE_ADDFB2) I think we can't
change that any more ...


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