[PATCH v1 4/4] drm/xe: Raise hw_tlb_timeout to cover observed GuC ack latency

["Tales A. Mendonça" <talesam@gmail.com>]

On ARL-H (Arrow Lake-H) under sustained load, TLB invalidation acks can
arrive slightly later than the current fence timeout budget:

  tlb_inval_timeout_delay() = hw_tlb_timeout (HZ/4)
                            + 2 * xe_guc_ct_queue_proc_time_jiffies()

which with the 4K H2G buffer works out to 2.25s. When that happens the
fence is force-signalled with -ETIME even though the invalidation
completes an instant later:

  xe 0000:00:02.0: [drm] *ERROR* TLB invalidation fence timeout,
      seqno=15519 recv=15518

Local instrumentation that records the request and timeout times of a
timed-out invalidation and logs the deltas when its ack finally lands
measured, over ~24h of heavy use (kernel builds, 3 VMs, browser; ~311k
invalidations at ~7.4/s):

  - 34 fence timeouts, every one followed by its ack
  - request-to-ack: 2254-2355 ms (mean 2306 ms)
  - timeout-to-ack: 2-59 ms (mean 25 ms)

i.e. the ack always arrives, and always within tens of milliseconds of
the deadline - there is no firmware stall (nothing arrives seconds
late). A prior change guaranteeing a G2H CTB drain in the timeout path
confirmed the acks are genuinely not yet delivered when the timeout
fires, ruling out lost interrupts or an unprocessed CTB entry.

Signalling -ETIME on an invalidation that completes ~25 ms later is
worse than waiting a little longer: waiters see a spurious error for an
operation that succeeds. Raise hw_tlb_timeout to HZ/2, extending the
budget to 2.5s, which covers the observed worst case (2.355s) with
~150 ms of margin and eliminated all spurious timeouts in testing on
this workload.

Signed-off-by: Tales A. Mendonça <talesam@gmail.com>
---
 drivers/gpu/drm/xe/xe_guc_tlb_inval.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/xe/xe_guc_tlb_inval.c b/drivers/gpu/drm/xe/xe_guc_tlb_inval.c
index 91960dc4ba9..c5ceac5ab86 100644
--- a/drivers/gpu/drm/xe/xe_guc_tlb_inval.c
+++ b/drivers/gpu/drm/xe/xe_guc_tlb_inval.c
@@ -335,7 +335,7 @@ static long tlb_inval_timeout_delay(struct xe_tlb_inval *tlb_inval)
 	struct xe_guc *guc = tlb_inval->private;
 
 	/* this reflects what HW/GuC needs to process TLB inv request */
-	const long hw_tlb_timeout = HZ / 4;
+	const long hw_tlb_timeout = HZ / 2;
 
 	/* this estimates actual delay caused by the CTB transport */
 	long delay = xe_guc_ct_queue_proc_time_jiffies(&guc->ct);
-- 
2.55.0