[PATCH v3 2/3] drm/xe: Avoid unconditional VRAM reads in H2G path

[Matthew Brost <matthew.brost@intel.com>]

desc_read() issues an VRAM read which serializes the CPU and drains
posted writes on dGPU platforms. The H2G tracepoint evaluated its
arguments unconditionally, so even with tracing disabled the submission
path paid the full VRAM readf latency. Guard the tracepoint with
trace_xe_guc_ctb_h2g_enabled().

Adso move the descriptor status verification under CONFIG_DRM_XE_DEBUG.
This removes another unnecessary VRAM read in non-debug builfds.

This results in ~10× faster H2G submission and significantly reduces
lock contention across the driver.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
---
 drivers/gpu/drm/xe/xe_guc_ct.c | 25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_guc_ct.c b/drivers/gpu/drm/xe/xe_guc_ct.c
index ea07a27757d5..37842c93e0ee 100644
--- a/drivers/gpu/drm/xe/xe_guc_ct.c
+++ b/drivers/gpu/drm/xe/xe_guc_ct.c
@@ -939,22 +939,22 @@ static int h2g_write(struct xe_guc_ct *ct, const u32 *action, u32 len,
 	u32 full_len;
 	struct iosys_map map = IOSYS_MAP_INIT_OFFSET(&h2g->cmds,
 							 tail * sizeof(u32));
-	u32 desc_status;
 
 	full_len = len + GUC_CTB_HDR_LEN;
 
 	lockdep_assert_held(&ct->lock);
 	xe_gt_assert(gt, full_len <= GUC_CTB_MSG_MAX_LEN);
 
-	desc_status = desc_read(xe, h2g, status);
-	if (desc_status) {
-		xe_gt_err(gt, "CT write: non-zero status: %u\n", desc_status);
-		goto corrupted;
-	}
-
 	if (IS_ENABLED(CONFIG_DRM_XE_DEBUG)) {
 		u32 desc_tail = desc_read(xe, h2g, tail);
 		u32 desc_head = desc_read(xe, h2g, head);
+		u32 desc_status;
+
+		desc_status = desc_read(xe, h2g, status);
+		if (desc_status) {
+			xe_gt_err(gt, "CT write: non-zero status: %u\n", desc_status);
+			goto corrupted;
+		}
 
 		if (tail != desc_tail) {
 			desc_write(xe, h2g, status, desc_status | GUC_CTB_STATUS_MISMATCH);
@@ -1023,8 +1023,15 @@ static int h2g_write(struct xe_guc_ct *ct, const u32 *action, u32 len,
 	/* Update descriptor */
 	desc_write(xe, h2g, tail, h2g->info.tail);
 
-	trace_xe_guc_ctb_h2g(xe, gt->info.id, *(action - 1), full_len,
-			     desc_read(xe, h2g, head), h2g->info.tail);
+	/*
+	 * desc_read() performs an VRAM read which serializes the CPU and drains
+	 * posted writes on dGPU platforms. Tracepoints evaluate arguments even
+	 * when disabled, so guard the event to avoid adding µs-scale latency to
+	 * the fast H2G submission path when tracing is not active.
+	 */
+	if (trace_xe_guc_ctb_h2g_enabled())
+		trace_xe_guc_ctb_h2g(xe, gt->info.id, *(action - 1), full_len,
+				     desc_read(xe, h2g, head), h2g->info.tail);
 
 	return 0;
 
-- 
2.34.1