drm/i915/perf: avoid read back of head register
There's no need for the driver to keep reading back the head pointer from hardware since the hardware doesn't update it automatically. This way we can treat any invalid head pointer value as a software/driver bug instead of spurious hardware behaviour. This change is also a small stepping stone towards re-working how the head and tail state is managed as part of an improved workaround for the tail register race condition. Signed-off-by: NRobert Bragg <robert@sixbynine.org> Reviewed-by: NMatthew Auld <matthew.auld@intel.com> Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk> Link: http://patchwork.freedesktop.org/patch/msgid/20170511154345.962-4-lionel.g.landwerlin@intel.com
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