Hi Gerd,
On Thu, Nov 24, 2022 at 8:29 AM Gerd Hoffmann <kraxel@xxxxxxxxxx> wrote:
+#ifdef __BIG_ENDIAN
Why do we need the #ifdef here? Iirc some hw has big endian flags in the
scanout registers, so could supprt this unconditionally if there's no
#ifdef around the format defines. Some drivers might then also want a
DRM_FORMAT_FOO_BE define to simplify tables and stuff, but that's more a
bikeshed.
"Limit this to big-endian platforms, as there is currently no need
to support these formats on little-endian platforms."
Will anyone make use of this? In theory, all of the 16-bpp formats
can have a big-endian counterpart.
Highly unlikely. Dealing with 16-bpp formats in non-native byte order
is a PITA because it isn't enough to simply adjust the masks and shifts
to pick the correct bits and be done with it.
The qemu stdvga happens to have a register to switch framebuffer
byteorder (so both x64 and ppc are happy), and the bochs drm driver
actually supports that no matter what the cpu byte order is, but it
supports only DRM_FORMAT_XRGB8888 + DRM_FORMAT_BGRX8888.
Supporting 16 bpp in the driver wouldn't be that much of a problem, but
processing the framebuffer on the host side when emulating a big endian
guest on a little endian host is painful. I think I can't ask pixman to
do a conversation from DRM_FORMAT_RGB565 | DRM_FORMAT_BIG_ENDIAN to
DRM_FORMAT_XRGB8888 on a little endian machine.
Indeed. But you can do a quick 16-bit byteswap, and convert from
DRM_FORMAT_RGB565 to DRM_FORMAT_XRGB8888?
There's a similar issue with Cairo, cfr. '[PATCH libdrm v2 08/10]
util: Fix pwetty on big-endian"[1].
BTW, does pixman support converting DRM_FORMAT_RGB565 to
DRM_FORMAT_XRGB8888 on a big-endian machine?
[1] https://lore.kernel.org/all/e8597038478f12e9eda5e86b309b52988f69f2eb.1657302103.git.geert@xxxxxxxxxxxxxx
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
