[PATCH 1/3] drm/i915: Fix negative remaining time after retire requests

[Janusz Krzysztofik]

Commit b97060a99b01 ("drm/i915/guc: Update intel_gt_wait_for_idle to work
with GuC") extended the API of intel_gt_retire_requests_timeout with an
extra argument 'remaining_timeout', intended for passing back unconsumed
portion of requested timeout when 0 (success) is returned.  However, when
request retirement happens to succeed despite an error returned by
dma_fence_wait_timeout(), the error code (a negative value) is passed back
instead of remaining time.  If a user then passes that negative value
forward as requested timeout to another wait, a WARN or BUG can be
triggered.

Instead of copying the value of timeout variable to *remaining_timeout
before return, update the *remaining_timeout after each DMA fence wait.
Set it to 0 on -ETIME or -EINTR, and assume no time has been consumed on
other errors returned from the wait.

TODO: Reconsider how we should respond to -ERESTARTSYS.

Fixes: b97060a99b01 ("drm/i915/guc: Update intel_gt_wait_for_idle to work with GuC")
Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik at linux.intel.com>
Cc: stable at vger.kernel.org # v5.15+
---
 drivers/gpu/drm/i915/gt/intel_gt_requests.c | 25 ++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_gt_requests.c b/drivers/gpu/drm/i915/gt/intel_gt_requests.c
index edb881d756309..ae994c2b7a29a 100644
--- a/drivers/gpu/drm/i915/gt/intel_gt_requests.c
+++ b/drivers/gpu/drm/i915/gt/intel_gt_requests.c
@@ -138,6 +138,9 @@ long intel_gt_retire_requests_timeout(struct intel_gt *gt, long timeout,
 	unsigned long active_count = 0;
 	LIST_HEAD(free);
 
+	if (remaining_timeout)
+		*remaining_timeout = timeout;
+
 	flush_submission(gt, timeout); /* kick the ksoftirqd tasklets */
 	spin_lock(&timelines->lock);
 	list_for_each_entry_safe(tl, tn, &timelines->active_list, link) {
@@ -163,6 +166,25 @@ long intel_gt_retire_requests_timeout(struct intel_gt *gt, long timeout,
 								 timeout);
 				dma_fence_put(fence);
 
+				if (remaining_timeout) {
+					/*
+					 * If we get an error here but request
+					 * retirement succeeds anyway
+					 * (!active_count) and we return 0, the
+					 * caller may want to spend remaining
+					 * time on waiting for other events.
+					 * TODO: Review the list of error codes
+					 * on which we should reduce the
+					 * remaininig time to 0.
+					 */
+					if (timeout == -ETIME ||
+					    timeout == -EINTR)
+						*remaining_timeout = 0;
+					else if (timeout >= 0)
+						*remaining_timeout = timeout;
+					/* else assume no time consumed */
+				}
+
 				/* Retirement is best effort */
 				if (!mutex_trylock(&tl->mutex)) {
 					active_count++;
@@ -196,9 +218,6 @@ out_active:	spin_lock(&timelines->lock);
 	if (flush_submission(gt, timeout)) /* Wait, there's more! */
 		active_count++;
 
-	if (remaining_timeout)
-		*remaining_timeout = timeout;
-
 	return active_count ? timeout : 0;
 }
 
-- 
2.25.1