[PATCH i-g-t 1/3] lib/xe/xe_spin: fix premature batch exit on BO resubmission

[Marcin Bernatowicz <marcin.bernatowicz@linux.intel.com>]

COND_BBE reads ticks_delta from memory. When the same BO is resubmitted,
COND_BBE can read the value left by the previous batch(elapsed >= ctx_ticks
at that batch's exit), satisfying the exit condition on the very first loop
iteration before any new elapsed time is stored.

Write sentinel 0 to ticks_delta at batch start and use
MI_SEMAPHORE_WAIT to poll until it is visible in memory before entering
the loop. This guarantees COND_BBE cannot read the stale value from the
previous batch.

Also switch from inverted ticks_delta logic (STOREINV + ~ctx_ticks) to
direct elapsed ticks (STORE + MAD_LT_IDD + ctx_ticks) for readability.
The pad loop between SRM and COND_BBE is removed as it does not help.

Suggested-by: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>
Signed-off-by: Marcin Bernatowicz <marcin.bernatowicz@linux.intel.com>
Cc: Adam Miszczak <adam.miszczak@linux.intel.com>
Cc: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Cc: Lukasz Laguna <lukasz.laguna@intel.com>
Cc: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>
---
 lib/xe/xe_spin.c | 31 +++++++++++++++++--------------
 1 file changed, 17 insertions(+), 14 deletions(-)

diff --git a/lib/xe/xe_spin.c b/lib/xe/xe_spin.c
index 874310789..2863d2163 100644
--- a/lib/xe/xe_spin.c
+++ b/lib/xe/xe_spin.c
@@ -52,7 +52,6 @@ void xe_spin_init(struct xe_spin *spin, struct xe_spin_opts *opts)
 	uint64_t start_addr = opts->addr + offsetof(struct xe_spin, start);
 	uint64_t end_addr = opts->addr + offsetof(struct xe_spin, end);
 	uint64_t ticks_delta_addr = opts->addr + offsetof(struct xe_spin, ticks_delta);
-	uint64_t pad_addr = opts->addr + offsetof(struct xe_spin, pad);
 	uint64_t timestamp_addr = opts->addr + offsetof(struct xe_spin, timestamp);
 	int b = 0;
 	uint32_t devid;
@@ -77,6 +76,18 @@ void xe_spin_init(struct xe_spin *spin, struct xe_spin_opts *opts)
 	spin->batch[b++] = start_addr >> 32;
 	spin->batch[b++] = 0xc0ffee;
 
+	if (opts->ctx_ticks) {
+		/* Write sentinel and wait until it is visible in memory before loop. */
+		spin->batch[b++] = MI_STORE_DWORD_IMM_GEN4;
+		spin->batch[b++] = ticks_delta_addr;
+		spin->batch[b++] = ticks_delta_addr >> 32;
+		spin->batch[b++] = 0;
+		spin->batch[b++] = MI_SEMAPHORE_WAIT | MI_SEMAPHORE_POLL | MI_SEMAPHORE_SAD_EQ_SDD;
+		spin->batch[b++] = 0;
+		spin->batch[b++] = ticks_delta_addr;
+		spin->batch[b++] = ticks_delta_addr >> 32;
+	}
+
 	loop_addr = opts->addr + b * sizeof(uint32_t);
 
 	if (opts->preempt)
@@ -101,12 +112,12 @@ void xe_spin_init(struct xe_spin *spin, struct xe_spin_opts *opts)
 		spin->batch[b++] = opts->use_queue_timestamp ? QUEUE_TIMESTAMP : CTX_TIMESTAMP;
 		spin->batch[b++] = CS_GPR(NOW_TS);
 
-		/* delta = now - start; inverted to match COND_BBE */
+		/* delta = now - start */
 		spin->batch[b++] = MI_MATH(4);
 		spin->batch[b++] = MI_MATH_LOAD(MI_MATH_REG_SRCA, MI_MATH_REG(NOW_TS));
 		spin->batch[b++] = MI_MATH_LOAD(MI_MATH_REG_SRCB, MI_MATH_REG(START_TS));
 		spin->batch[b++] = MI_MATH_SUB;
-		spin->batch[b++] = MI_MATH_STOREINV(MI_MATH_REG(NOW_TS), MI_MATH_REG_ACCU);
+		spin->batch[b++] = MI_MATH_STORE(MI_MATH_REG(NOW_TS), MI_MATH_REG_ACCU);
 
 		/* Save delta for reading by COND_BBE */
 		spin->batch[b++] = MI_STORE_REGISTER_MEM_GEN8 | MI_SRM_CS_MMIO;
@@ -114,17 +125,9 @@ void xe_spin_init(struct xe_spin *spin, struct xe_spin_opts *opts)
 		spin->batch[b++] = ticks_delta_addr;
 		spin->batch[b++] = ticks_delta_addr >> 32;
 
-		/* Delay between SRM and COND_BBE to post the writes */
-		for (int n = 0; n < 8; n++) {
-			spin->batch[b++] = MI_STORE_DWORD_IMM_GEN4;
-			spin->batch[b++] = pad_addr;
-			spin->batch[b++] = pad_addr >> 32;
-			spin->batch[b++] = 0xc0ffee;
-		}
-
-		/* Break if delta [time elapsed] > ns */
-		spin->batch[b++] = MI_COND_BATCH_BUFFER_END | MI_DO_COMPARE | 2;
-		spin->batch[b++] = ~(opts->ctx_ticks);
+		/* Break if delta [time elapsed] >= ns */
+		spin->batch[b++] = MI_COND_BATCH_BUFFER_END | MI_DO_COMPARE | MAD_LT_IDD | 2;
+		spin->batch[b++] = opts->ctx_ticks;
 		spin->batch[b++] = ticks_delta_addr;
 		spin->batch[b++] = ticks_delta_addr >> 32;
 	}
-- 
2.43.0