[Intel-gfx] [RFC 22/44] drm/i915: Ensure OLS & PLR are always in sync

[John.C.Harrison at Intel.com]

From: John Harrison <John.C.Harrison at Intel.com>

The new seqno alloction code pre-allocates a 'lazy' request structure and then
tries to allocate the 'lazy' seqno. The seqno allocation can potential wrap
around zero and when doing so, tries to idle the ring by waiting for all
oustanding work to complete. With a scheduler in place, this can mean first
submitting extra work to the ring. However, at this point in time, the lazy
request is valid but the lazy seqno is not. Some existing code was getting
confused by this state and Bad Things would happen.

The safest solution is to still allocate the lazy request in advance (to avoid
having to roll back in an out of memory sitation) but to save the pointer in a
local variable rather than immediately updating the lazy pointer. Only after a
valid seqno has been acquired is the lazy request pointer actually updated.

This guarantees that both lazy values are either invalid or both valid. There
can no longer be an inconsistent state.
---
 drivers/gpu/drm/i915/intel_ringbuffer.c |   27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 737c41b..1ef0cbd 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -1665,20 +1665,31 @@ int intel_ring_idle(struct intel_engine_cs *ring)
 int
 intel_ring_alloc_seqno(struct intel_engine_cs *ring)
 {
-	if (ring->outstanding_lazy_seqno)
+	int ret;
+	struct drm_i915_gem_request *request;
+
+	/* NB: Some code seems to test the OLS and other code tests the PLR.
+	 * Therefore it is only safe if the two are kept in step. */
+
+	if (ring->outstanding_lazy_seqno) {
+		BUG_ON(ring->preallocated_lazy_request == NULL);
 		return 0;
+	}
 
-	if (ring->preallocated_lazy_request == NULL) {
-		struct drm_i915_gem_request *request;
+	BUG_ON(ring->preallocated_lazy_request != NULL);
 
-		request = kmalloc(sizeof(*request), GFP_KERNEL);
-		if (request == NULL)
-			return -ENOMEM;
+	request = kmalloc(sizeof(*request), GFP_KERNEL);
+	if (request == NULL)
+		return -ENOMEM;
 
-		ring->preallocated_lazy_request = request;
+	ret = i915_gem_get_seqno(ring->dev, &ring->outstanding_lazy_seqno);
+	if (ret) {
+		kfree(request);
+		return ret;
 	}
 
-	return i915_gem_get_seqno(ring->dev, &ring->outstanding_lazy_seqno);
+	ring->preallocated_lazy_request = request;
+	return 0;
 }
 
 static int __intel_ring_prepare(struct intel_engine_cs *ring,
-- 
1.7.9.5