• Chris Wilson's avatar
    drm/i915: Serialise updates to GGTT with access through GGTT on Braswell · 5bab6f60
    Chris Wilson authored
    When accessing through the GTT from one CPU whilst concurrently updating
    the GGTT PTEs in another thread, the hardware likes to return random
    data. As we have strong serialisation prevent us from modifying the PTE
    of an active GTT mmapping, we have to conclude that it whilst modifying
    other PTE's that error occurs. (I have not looked for any pattern such
    as modifying PTE within the same page or cacheline as active PTE -
    though checking whether revoking neighbouring objects should be enough
    to test that theory.) The corruption also seems restricted to Braswell
    and disappears with maxcpus=0. This patch stops all access through the
    GTT by other CPUs when we update any PTE by stopping the machine around
    the GGTT update.
    
    Note that splitting up the 64 bit write into two 32 bit writes was
    tried and found to fail too.
    
    Testcase: igt/gem_concurrent_blit
    Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89079Signed-off-by: default avatarChris Wilson <chris@chris-wilson.co.uk>
    [danvet: Add note about 2x 32bits failing too.]
    Signed-off-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
    5bab6f60
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