[PATCH 1/2] gpu: host1x: skip redundant syncpoint loads in host1x_syncpt_wait()

[Tanmay Patil]

In host1x_syncpt_wait(), the hardware syncpoint value was loaded
initially for expiry check, and then loaded a second time to
populate the caller's value pointer. Reuse a single load for
both purposes.

After dma_fence_wait_timeout(), the previous code reloaded the syncpoint
value for the expiry check, which is only required in the timeout case.
On success (i.e., return value > 0, or return value == 0 with zero
jiffies remaining), the ISR has already cached the value before
signaling the fence. The value pointer can therefore be populated using
the cached value using host1x_syncpt_read_min() without MMIO access.
Only the timeout path requires a fresh load, move host1x_syncpt_load()
under that path.

Measured Syncpoint wait latency (50000 samples):
  Average latency:   12.2 us  -> 10.6 us
  99.99 pct latency: 62.96 us -> 51.90 us

Signed-off-by: Tanmay Patil <tanmayp@xxxxxxxxxx>
---
 drivers/gpu/host1x/syncpt.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/host1x/syncpt.c b/drivers/gpu/host1x/syncpt.c
index acc7d82e0585..807c74fc6a0a 100644
--- a/drivers/gpu/host1x/syncpt.c
+++ b/drivers/gpu/host1x/syncpt.c
@@ -222,11 +222,12 @@ int host1x_syncpt_wait(struct host1x_syncpt *sp, u32 thresh, long timeout,
 {
 	struct dma_fence *fence;
 	long wait_err;
+	u32 curr;
 
-	host1x_hw_syncpt_load(sp->host, sp);
+	curr = host1x_syncpt_load(sp);
 
 	if (value)
-		*value = host1x_syncpt_load(sp);
+		*value = curr;
 
 	if (host1x_syncpt_is_expired(sp, thresh))
 		return 0;
@@ -245,21 +246,25 @@ int host1x_syncpt_wait(struct host1x_syncpt *sp, u32 thresh, long timeout,
 		host1x_fence_cancel(fence);
 	dma_fence_put(fence);
 
-	if (value)
-		*value = host1x_syncpt_load(sp);
-
 	/*
 	 * Don't rely on dma_fence_wait_timeout return value,
 	 * since it returns zero both on timeout and if the
 	 * wait completed with 0 jiffies left.
 	 */
-	host1x_hw_syncpt_load(sp->host, sp);
-	if (wait_err == 0 && !host1x_syncpt_is_expired(sp, thresh))
+	if (wait_err == 0 && !host1x_syncpt_is_expired(sp, thresh)) {
+		if (value)
+			*value = host1x_syncpt_load(sp);
+
 		return -EAGAIN;
-	else if (wait_err < 0)
+	} else if (wait_err < 0) {
 		return wait_err;
-	else
+	} else {
+		/* Success, read the value cached by ISR */
+		if (value)
+			*value = host1x_syncpt_read_min(sp);
+
 		return 0;
+	}
 }
 EXPORT_SYMBOL(host1x_syncpt_wait);
 
-- 
2.54.0