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drm/i915: Don't read 'HEAD' MMIO register in LRC mode

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The logical ring code was updating the software ring 'head' value
by reading the hardware 'HEAD' register. In LRC mode, this is not
valid as the hardware is not necessarily executing the same context
that is being processed by the software. Thus reading the h/w HEAD
could put an unrelated (undefined, effectively random) value into
the s/w 'head' -- A Bad Thing for the free space calculations.

Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
This commit is contained in:
Dave Gordon 2014-11-18 20:07:21 +00:00 committed by Daniel Vetter
parent 57e215135f
commit d65621c496
1 changed files with 0 additions and 1 deletions
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drivers/gpu/drm/i915/intel_lrc.c
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@ -986,7 +986,6 @@ static int logical_ring_wait_for_space(struct intel_ringbuffer *ringbuf,
end = jiffies + 60 * HZ;
do {
ringbuf->head = I915_READ_HEAD(ring);
ringbuf->space = intel_ring_space(ringbuf);
if (ringbuf->space >= bytes) {
ret = 0;