drm/i915: avoid wait_for_atomic() in non-atomic host2guc_action()

Rather than using wait_for_atomic() when chacking for a response from
the GuC, we can get the effect of a hybrid spin/sleep wait by breaking
it into two stages. First, spin-wait for up to 10us to minimise latency
for "quick" commands; then, if that times out, sleep-wait for up 10ms
(the maximum allowed for a "slow" command).

Being able to do this depends on the recent patch
18f4b843 drm/i915: Use atomic waits for short non-atomic ones
and is similar to the hybrid approach in
1758b90e drm/i915: Use a hybrid scheme for fast register waits
(although we can't use that as-is, because that interface doesn't quite
match what we need here).
Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467815411-21756-1-git-send-email-david.s.gordon@intel.com

drivers/gpu/drm/i915/i915_guc_submission.c

@@ -97,8 +97,14 @@ static int host2guc_action(struct intel_guc *guc, u32 *data, u32 len)
 
 	I915_WRITE(HOST2GUC_INTERRUPT, HOST2GUC_TRIGGER);
 
-	/* No HOST2GUC command should take longer than 10ms */
-	ret = wait_for_atomic(host2guc_action_response(dev_priv, &status), 10);
+	/*
+	 * Fast commands should complete in less than 10us, so sample quickly
+	 * up to that length of time, then switch to a slower sleep-wait loop.
+	 * No HOST2GUC command should ever take longer than 10ms.
+	 */
+	ret = wait_for_us(host2guc_action_response(dev_priv, &status), 10);
+	if (ret)
+		ret = wait_for(host2guc_action_response(dev_priv, &status), 10);
 	if (status != GUC2HOST_STATUS_SUCCESS) {
 		/*
 		 * Either the GuC explicitly returned an error (which