Commit ecff665f5e3f (drm/ttm: make ttm reservation calls...) causes system hang on Radeon RS780

[Markus Trippelsdorf]

By simply copy/pasting a big document under LibreOffice my system hangs
itself up. Only a hard reset gets it working again.
see also: https://bugs.freedesktop.org/show_bug.cgi?id=66551

I've bisected the issue to:

commit ecff665f5e3f1c6909353e00b9420e45ae23d995
Author: Maarten Lankhorst <m.b.lankhorst at gmail.com>
Date:   Thu Jun 27 13:48:17 2013 +0200

    drm/ttm: make ttm reservation calls behave like reservation calls
    
    This commit converts the source of the val_seq counter to
    the ww_mutex api. The reservation objects are converted later,
    because there is still a lockdep splat in nouveau that has to
    resolved first.
    
    Signed-off-by: Maarten Lankhorst <maarten.lankhorst at canonical.com>
    Reviewed-by: Jerome Glisse <jglisse at redhat.com>
    Signed-off-by: Dave Airlie <airlied at redhat.com>

-- 
Markus