[Intel-gfx] [PATCH] drm/i915: Prevent recursion by retiring requests when the ring is full

[Chris Wilson]

As the VM do not track activity of objects and instead use a large
hammer to forcibly idle and evict all of their associated objects when
one is released, it is possible for that to cause a recursion when we
need to wait for free space on a ring and call retire requests.
(intel_ring_begin -> intel_ring_wait_request ->
i915_gem_retire_requests_ring -> i915_gem_context_free ->
i915_gem_evict_vm -> i915_gpu_idle -> intel_ring_begin etc)

In order to remove the requirement for calling retire-requests from
intel_ring_wait_request, we have to inline a couple of steps from
retiring requests, notably we have to record the position of the request
we wait for and use that to update the available ring space.

Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/intel_ringbuffer.c | 25 +++++--------------------
 1 file changed, 5 insertions(+), 20 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c 
b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 10ff32d09c14..0da7c257159a 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -1431,28 +1431,16 @@ void intel_cleanup_ring_buffer(struct intel_ring_buffer 
*ring)
        cleanup_status_page(ring);
 }
 
-static int intel_ring_wait_seqno(struct intel_ring_buffer *ring, u32 seqno)
-{
-       int ret;
-
-       ret = i915_wait_seqno(ring, seqno);
-       if (!ret)
-               i915_gem_retire_requests_ring(ring);
-
-       return ret;
-}
-
 static int intel_ring_wait_request(struct intel_ring_buffer *ring, int n)
 {
        struct drm_i915_gem_request *request;
-       u32 seqno = 0;
+       u32 seqno = 0, tail;
        int ret;
 
-       i915_gem_retire_requests_ring(ring);
-
        if (ring->last_retired_head != -1) {
                ring->head = ring->last_retired_head;
                ring->last_retired_head = -1;
+
                ring->space = ring_space(ring);
                if (ring->space >= n)
                        return 0;
@@ -1469,6 +1457,7 @@ static int intel_ring_wait_request(struct 
intel_ring_buffer *ring, int n)
                        space += ring->size;
                if (space >= n) {
                        seqno = request->seqno;
+                       tail = request->tail;
                        break;
                }
 
@@ -1483,15 +1472,11 @@ static int intel_ring_wait_request(struct 
intel_ring_buffer *ring, int n)
        if (seqno == 0)
                return -ENOSPC;
 
-       ret = intel_ring_wait_seqno(ring, seqno);
+       ret = i915_wait_seqno(ring, seqno);
        if (ret)
                return ret;
 
-       if (WARN_ON(ring->last_retired_head == -1))
-               return -ENOSPC;
-
-       ring->head = ring->last_retired_head;
-       ring->last_retired_head = -1;
+       ring->head = tail;
        ring->space = ring_space(ring);
        if (WARN_ON(ring->space < n))
                return -ENOSPC;
-- 
1.8.5.3

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