[Intel-gfx] [PATCH 01/14] drm/i915/guc: Do not use 0 for GuC doorbell cookie

[Michał Winiarski]

Apparently, this value is reserved and may be interpreted as changing
doorbell ownership. Even though we're not observing any side effects
now, let's skip over it to be consistent with the spec.

Suggested-by: Sagar Arun Kamble <sagar.a.kamble at intel.com>
Signed-off-by: Michał Winiarski <michal.winiarski at intel.com>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Sagar Arun Kamble <sagar.a.kamble at intel.com>
---
 drivers/gpu/drm/i915/i915_guc_submission.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_guc_submission.c b/drivers/gpu/drm/i915/i915_guc_submission.c
index a2e8114b739d..02ca2a412c62 100644
--- a/drivers/gpu/drm/i915/i915_guc_submission.c
+++ b/drivers/gpu/drm/i915/i915_guc_submission.c
@@ -475,9 +475,10 @@ static void guc_ring_doorbell(struct i915_guc_client *client)
 	/* pointer of current doorbell cacheline */
 	db = __get_doorbell(client);
 
-	/* we're not expecting the doorbell cookie to change behind our back */
+	/* we're not expecting the doorbell cookie to change behind our back,
+	 * we also need to treat 0 as a reserved value */
 	cookie = READ_ONCE(db->cookie);
-	WARN_ON_ONCE(xchg(&db->cookie, cookie + 1) != cookie);
+	WARN_ON_ONCE(xchg(&db->cookie, cookie + 1 ?: cookie + 2) != cookie);
 
 	/* XXX: doorbell was lost and need to acquire it again */
 	GEM_BUG_ON(db->db_status != GUC_DOORBELL_ENABLED);
-- 
2.13.6