[PATCH v3 5/5] drm/xe: Sample gpu timestamp closer to exec queues

[Lucas De Marchi]

Move the force_wake_get to the beginning of the function so the gpu
timestamp can be closer to the sampled value for exec queues. This
avoids additional delays waiting for force wake ack which can make the
proportion between cycles/total_cycles fluctuate either to more or to
less than the real value, depending if the delay happened in the first
or second sample of a top-like application:

	sample 0:
		read_exec_queue_timestamp
					<<<< (A)
		read_gpu_timestamp
	sample 1:
		read_exec_queue_timestamp
					<<<<< (B)
		read_gpu_timestamp

Additional delay in (A) will cause the utilization to be bigger
than it should. Additional delay in (B) will cause the utilization to be
smaller than it should.

With this a LNL system that was failing after ~60 iterations, get to run
to 100 without a failure. This is still not perfect, and it's easy to
introduce errors by just loading the CPU with `stress --cpu $(nproc)` -
the same igt test in this case fails after 2 or 3 iterations.

Although not perfect, this could be viewed as just extracting that ugly
loop to "find the forcewake for any available engine" into a separate
function, and at the same time improving the results a little bit.

Signed-off-by: Lucas De Marchi <lucas.demarchi at intel.com>
---
 drivers/gpu/drm/xe/xe_drm_client.c | 63 ++++++++++++++++++------------
 1 file changed, 39 insertions(+), 24 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_drm_client.c b/drivers/gpu/drm/xe/xe_drm_client.c
index 24a0a7377abf2..874c2157baf49 100644
--- a/drivers/gpu/drm/xe/xe_drm_client.c
+++ b/drivers/gpu/drm/xe/xe_drm_client.c
@@ -269,6 +269,40 @@ static void show_meminfo(struct drm_printer *p, struct drm_file *file)
 	}
 }
 
+static bool force_wake_get_for_gpu_timestamp(struct xe_device *xe,
+					     struct xe_hw_engine **phwe,
+					     unsigned int *pfw_ref)
+{
+	struct xe_gt *gt;
+	unsigned long gt_id;
+
+	for_each_gt(gt, xe, gt_id) {
+		struct xe_hw_engine *hwe;
+		enum xe_force_wake_domains domain;
+		unsigned int fw_ref;
+		struct xe_force_wake *fw;
+
+		hwe = xe_gt_any_hw_engine(gt);
+		if (!hwe)
+			continue;
+
+		domain = xe_hw_engine_to_fw_domain(hwe);
+		fw = gt_to_fw(gt);
+
+		fw_ref = xe_force_wake_get(fw, domain);
+		if (!xe_force_wake_ref_has_domain(fw_ref, domain)) {
+			xe_force_wake_put(fw, fw_ref);
+			return false;
+		}
+
+		*phwe = hwe;
+		*pfw_ref = fw_ref;
+		return true;
+	}
+
+	return false;
+}
+
 static void show_run_ticks(struct drm_printer *p, struct drm_file *file)
 {
 	unsigned long class, i, gt_id, capacity[XE_ENGINE_CLASS_MAX] = { };
@@ -282,6 +316,9 @@ static void show_run_ticks(struct drm_printer *p, struct drm_file *file)
 
 	xe_pm_runtime_get(xe);
 
+	if (!force_wake_get_for_gpu_timestamp(xe, &hwe, &fw_ref))
+		return;
+
 	/* Accumulate all the exec queues from this client */
 	mutex_lock(&xef->exec_queue.lock);
 	xa_for_each(&xef->exec_queue.xa, i, q) {
@@ -295,27 +332,7 @@ static void show_run_ticks(struct drm_printer *p, struct drm_file *file)
 	}
 	mutex_unlock(&xef->exec_queue.lock);
 
-	/* Get the total GPU cycles */
-	for_each_gt(gt, xe, gt_id) {
-		enum xe_force_wake_domains fw;
-
-		hwe = xe_gt_any_hw_engine(gt);
-		if (!hwe)
-			continue;
-
-		fw = xe_hw_engine_to_fw_domain(hwe);
-
-		fw_ref = xe_force_wake_get(gt_to_fw(gt), fw);
-		if (!xe_force_wake_ref_has_domain(fw_ref, fw)) {
-			hwe = NULL;
-			xe_force_wake_put(gt_to_fw(gt), fw_ref);
-			break;
-		}
-
-		gpu_timestamp = xe_hw_engine_read_timestamp(hwe);
-		xe_force_wake_put(gt_to_fw(gt), fw_ref);
-		break;
-	}
+	gpu_timestamp = xe_hw_engine_read_timestamp(hwe);
 
 	/*
 	 * Wait for any exec queue going away: their cycles will get updated on
@@ -324,11 +341,9 @@ static void show_run_ticks(struct drm_printer *p, struct drm_file *file)
 	wait_var_event(&xef->exec_queue.pending_removal,
 		       !atomic_read(&xef->exec_queue.pending_removal));
 
+	xe_force_wake_put(gt_to_fw(hwe->gt), fw_ref);
 	xe_pm_runtime_put(xe);
 
-	if (unlikely(!hwe))
-		return;
-
 	for (class = 0; class < XE_ENGINE_CLASS_MAX; class++) {
 		const char *class_name;
 
-- 
2.47.0