drm/i915/slpc: Balance the inc/dec for num_waiters

As seen in some recent failures, SLPC num_waiters value is < 0.
This happens because the inc/dec are not balanced. We should skip
decrement for the same conditions as the increment. Currently, we
do that for power saving profile mode. This patch also ensures that
num_waiters is incremented in the case min_softlimit is at boost
freq. It ensures that we don't reduce the frequency while this request
is in flight.

v2: Add Fixes tags

Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/13598
Fixes: f864a29 ("drm/i915/slpc: Optmize waitboost for SLPC")
Fixes: 4a82ceb ("drm/i915/slpc: Add sysfs for SLPC power profiles")
Cc: Sk Anirban <sk.anirban@intel.com>
Reviewed-by: Sk Anirban <sk.anirban@intel.com>
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://lore.kernel.org/r/20250428183555.3250021-1-vinay.belgaumkar@intel.com
(cherry picked from commit d26e550)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

Author: vsbelgaum

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

@@ -1001,6 +1001,10 @@ void intel_rps_dec_waiters(struct intel_rps *rps)
 	if (rps_uses_slpc(rps)) {
 		slpc = rps_to_slpc(rps);
 
+		/* Don't decrement num_waiters for req where increment was skipped */
+		if (slpc->power_profile == SLPC_POWER_PROFILES_POWER_SAVING)
+			return;
+
 		intel_guc_slpc_dec_waiters(slpc);
 	} else {
 		atomic_dec(&rps->num_waiters);
@@ -1029,11 +1033,15 @@ void intel_rps_boost(struct i915_request *rq)
 			if (slpc->power_profile == SLPC_POWER_PROFILES_POWER_SAVING)
 				return;
 
-			if (slpc->min_freq_softlimit >= slpc->boost_freq)
-				return;
-
 			/* Return if old value is non zero */
 			if (!atomic_fetch_inc(&slpc->num_waiters)) {
+				/*
+				 * Skip queuing boost work if frequency is already boosted,
+				 * but still increment num_waiters.
+				 */
+				if (slpc->min_freq_softlimit >= slpc->boost_freq)
+					return;
+
 				GT_TRACE(rps_to_gt(rps), "boost fence:%llx:%llx\n",
 					 rq->fence.context, rq->fence.seqno);
 				queue_work(rps_to_gt(rps)->i915->unordered_wq,