drm/i915/gt: Disable arbitration around Braswell's pdp updates

Braswell's pdp workaround is full of dragons, that may be being angered
when they are interrupted. Let's not take that risk and disable
arbitration during the update.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111105735.21515-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_execlists_submission.c

@@ -2539,6 +2539,14 @@ static int emit_pdps(struct i915_request *rq)
 	 * GPU hangs to forcewake errors and machine lockups!
 	 */
 
+	cs = intel_ring_begin(rq, 2);
+	if (IS_ERR(cs))
+		return PTR_ERR(cs);
+
+	*cs++ = MI_ARB_ON_OFF | MI_ARB_DISABLE;
+	*cs++ = MI_NOOP;
+	intel_ring_advance(rq, cs);
+
 	/* Flush any residual operations from the context load */
 	err = engine->emit_flush(rq, EMIT_FLUSH);
 	if (err)
@@ -2564,7 +2572,8 @@ static int emit_pdps(struct i915_request *rq)
 		*cs++ = i915_mmio_reg_offset(GEN8_RING_PDP_LDW(base, i));
 		*cs++ = lower_32_bits(pd_daddr);
 	}
-	*cs++ = MI_NOOP;
+	*cs++ = MI_ARB_ON_OFF | MI_ARB_ENABLE;
+	intel_ring_advance(rq, cs);
 
 	intel_ring_advance(rq, cs);