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    SubjectRe: [PATCH 3/5] drm: fix drm_mode_addfb() on big endian machines.
    On Mon, Sep 3, 2018 at 12:45 PM, Daniel Vetter <daniel@ffwll.ch> wrote:
    > On Mon, Sep 03, 2018 at 12:57:54PM +0200, Gerd Hoffmann wrote:
    >> Userspace on big endian machhines typically expects the ADDFB ioctl
    >> returns a big endian framebuffer. drm_mode_addfb() will call
    >> drm_mode_addfb2() unconditionally with little endian DRM_FORMAT_*
    >> values though, which is wrong. This patch fixes that.
    >>
    >> Drivers (both kernel and xorg) have quirks in place to deal with the
    >> broken drm_mode_addfb() behavior. Because of this we can't just change
    >> drm_mode_addfb() behavior for everybody without breaking things. So add
    >> a new driver feature flag DRIVER_PREFER_HOST_BYTE_ORDER, so drivers can
    >> opt-in.
    >>
    >> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
    >> ---
    >> include/drm/drm_drv.h | 1 +
    >> drivers/gpu/drm/drm_framebuffer.c | 11 +++++++++++
    >> 2 files changed, 12 insertions(+)
    >>
    >> diff --git a/include/drm/drm_drv.h b/include/drm/drm_drv.h
    >> index 46a8009784..9cf12596cd 100644
    >> --- a/include/drm/drm_drv.h
    >> +++ b/include/drm/drm_drv.h
    >> @@ -57,6 +57,7 @@ struct drm_printer;
    >> #define DRIVER_KMS_LEGACY_CONTEXT 0x20000
    >> #define DRIVER_SYNCOBJ 0x40000
    >> #define DRIVER_PREFER_XBGR_30BPP 0x80000
    >> +#define DRIVER_PREFER_HOST_BYTE_ORDER 0x100000
    >
    > Hm, not a huge fan of using driver_flags for random little quirks. I think
    > a boolean in sturct drm_mode_config would be much better. Bonus if you
    > also move the 30bpp hack over to that. Something like
    > mode_config.quirk_addfb_host_byte_order and
    > mode_config.quirk_addfb_prefer_xbgr_30bpp or whatever. That has the upside
    > of giving us a really nice place to put a huge comment about what this is
    > supposed to do.
    >
    > I think otherwise this looks overall rather reasonable. I think the only
    > other driver that ever cared about big endian was radeon/amdgpu. Would be
    > good to get at least an ack from amd folks, or a "meh, stopped caring".

    and nouveau.

    I do believe that ADDFB should be made to always prefer host byte
    order -- this is how all of the existing implementations work in
    practice. However e.g. nouveau wants DRM_FORMAT_XRGB8888. But it still
    treats it as host byte order. This will become more important in a
    world where ADDFB2 is more common.

    So, I think that this change should be applied, drivers (I suspect
    just nouveau and radeon) fixed up to consume the "new" formats, and
    then made to be the one-and-only way for ADDFB to function. That way
    we'll no longer be lying about the DRM_FORMAT being used.

    Cheers,

    -ilia

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