[Intel-gfx] [PATCH 02/55] drm/i915: Reserve ring buffer space for i915_add_request() commands

[John.C.Harrison at Intel.com]

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

It is a bad idea for i915_add_request() to fail. The work will already have been
send to the ring and will be processed, but there will not be any tracking or
management of that work.

The only way the add request call can fail is if it can't write its epilogue
commands to the ring (cache flushing, seqno updates, interrupt signalling). The
reasons for that are mostly down to running out of ring buffer space and the
problems associated with trying to get some more. This patch prevents that
situation from happening in the first place.

When a request is created, it marks sufficient space as reserved for the
epilogue commands. Thus guaranteeing that by the time the epilogue is written,
there will be plenty of space for it. Note that a ring_begin() call is required
to actually reserve the space (and do any potential waiting). However, that is
not currently done at request creation time. This is because the ring_begin()
code can allocate a request. Hence calling begin() from the request allocation
code would lead to infinite recursion! Later patches in this series remove the
need for begin() to do the allocate. At that point, it becomes safe for the
allocate to call begin() and really reserve the space.

Until then, there is a potential for insufficient space to be available at the
point of calling i915_add_request(). However, that would only be in the case
where the request was created and immediately submitted without ever calling
ring_begin() and adding any work to that request. Which should never happen. And
even if it does, and if that request happens to fall down the tiny window of
opportunity for failing due to being out of ring space then does it really
matter because the request wasn't doing anything in the first place?

v2: Updated the 'reserved space too small' warning to include the offending
sizes. Added a 'cancel' operation to clean up when a request is abandoned. Added
re-initialisation of tracking state after a buffer wrap to keep the sanity
checks accurate.

For: VIZ-5115
Signed-off-by: John Harrison <John.C.Harrison at Intel.com>
---
 drivers/gpu/drm/i915/i915_drv.h         |    1 +
 drivers/gpu/drm/i915/i915_gem.c         |   37 +++++++++++++++++
 drivers/gpu/drm/i915/intel_lrc.c        |    9 ++++
 drivers/gpu/drm/i915/intel_ringbuffer.c |   68 ++++++++++++++++++++++++++++++-
 drivers/gpu/drm/i915/intel_ringbuffer.h |   10 +++++
 5 files changed, 123 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 0347eb9..eba1857 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -2187,6 +2187,7 @@ struct drm_i915_gem_request {
 
 int i915_gem_request_alloc(struct intel_engine_cs *ring,
 			   struct intel_context *ctx);
+void i915_gem_request_cancel(struct drm_i915_gem_request *req);
 void i915_gem_request_free(struct kref *req_ref);
 
 static inline uint32_t
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 68f1d1e..6f51416 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2485,6 +2485,13 @@ int __i915_add_request(struct intel_engine_cs *ring,
 	} else
 		ringbuf = ring->buffer;
 
+	/*
+	 * To ensure that this call will not fail, space for its emissions
+	 * should already have been reserved in the ring buffer. Let the ring
+	 * know that it is time to use that space up.
+	 */
+	intel_ring_reserved_space_use(ringbuf);
+
 	request_start = intel_ring_get_tail(ringbuf);
 	/*
 	 * Emit any outstanding flushes - execbuf can fail to emit the flush
@@ -2567,6 +2574,9 @@ int __i915_add_request(struct intel_engine_cs *ring,
 			   round_jiffies_up_relative(HZ));
 	intel_mark_busy(dev_priv->dev);
 
+	/* Sanity check that the reserved size was large enough. */
+	intel_ring_reserved_space_end(ringbuf);
+
 	return 0;
 }
 
@@ -2666,6 +2676,26 @@ int i915_gem_request_alloc(struct intel_engine_cs *ring,
 	if (ret)
 		goto err;
 
+	/*
+	 * Reserve space in the ring buffer for all the commands required to
+	 * eventually emit this request. This is to guarantee that the
+	 * i915_add_request() call can't fail. Note that the reserve may need
+	 * to be redone if the request is not actually submitted straight
+	 * away, e.g. because a GPU scheduler has deferred it.
+	 *
+	 * Note further that this call merely notes the reserve request. A
+	 * subsequent call to *_ring_begin() is required to actually ensure
+	 * that the reservation is available. Without the begin, if the
+	 * request creator immediately submitted the request without adding
+	 * any commands to it then there might not actually be sufficient
+	 * room for the submission commands. Unfortunately, the current
+	 * *_ring_begin() implementations potentially call back here to
+	 * i915_gem_request_alloc(). Thus calling _begin() here would lead to
+	 * infinite recursion! Until that back call path is removed, it is
+	 * necessary to do a manual _begin() outside.
+	 */
+	intel_ring_reserved_space_reserve(req->ringbuf, MIN_SPACE_FOR_ADD_REQUEST);
+
 	ring->outstanding_lazy_request = req;
 	return 0;
 
@@ -2674,6 +2704,13 @@ err:
 	return ret;
 }
 
+void i915_gem_request_cancel(struct drm_i915_gem_request *req)
+{
+	intel_ring_reserved_space_cancel(req->ringbuf);
+
+	i915_gem_request_unreference(req);
+}
+
 struct drm_i915_gem_request *
 i915_gem_find_active_request(struct intel_engine_cs *ring)
 {
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 6a5ed07..e62d396 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -687,6 +687,9 @@ static int logical_ring_wait_for_space(struct intel_ringbuffer *ringbuf,
 	unsigned space;
 	int ret;
 
+	/* The whole point of reserving space is to not wait! */
+	WARN_ON(ringbuf->reserved_in_use);
+
 	if (intel_ring_space(ringbuf) >= bytes)
 		return 0;
 
@@ -747,6 +750,9 @@ static int logical_ring_wrap_buffer(struct intel_ringbuffer *ringbuf,
 	uint32_t __iomem *virt;
 	int rem = ringbuf->size - ringbuf->tail;
 
+	/* Can't wrap if space has already been reserved! */
+	WARN_ON(ringbuf->reserved_in_use);
+
 	if (ringbuf->space < rem) {
 		int ret = logical_ring_wait_for_space(ringbuf, ctx, rem);
 
@@ -770,6 +776,9 @@ static int logical_ring_prepare(struct intel_ringbuffer *ringbuf,
 {
 	int ret;
 
+	if (!ringbuf->reserved_in_use)
+		bytes += ringbuf->reserved_size;
+
 	if (unlikely(ringbuf->tail + bytes > ringbuf->effective_size)) {
 		ret = logical_ring_wrap_buffer(ringbuf, ctx);
 		if (unlikely(ret))
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index d934f85..74c2222 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -2103,6 +2103,9 @@ static int ring_wait_for_space(struct intel_engine_cs *ring, int n)
 	unsigned space;
 	int ret;
 
+	/* The whole point of reserving space is to not wait! */
+	WARN_ON(ringbuf->reserved_in_use);
+
 	if (intel_ring_space(ringbuf) >= n)
 		return 0;
 
@@ -2130,6 +2133,9 @@ static int intel_wrap_ring_buffer(struct intel_engine_cs *ring)
 	struct intel_ringbuffer *ringbuf = ring->buffer;
 	int rem = ringbuf->size - ringbuf->tail;
 
+	/* Can't wrap if space has already been reserved! */
+	WARN_ON(ringbuf->reserved_in_use);
+
 	if (ringbuf->space < rem) {
 		int ret = ring_wait_for_space(ring, rem);
 		if (ret)
@@ -2180,16 +2186,74 @@ int intel_ring_alloc_request_extras(struct drm_i915_gem_request *request)
 	return 0;
 }
 
-static int __intel_ring_prepare(struct intel_engine_cs *ring,
-				int bytes)
+void intel_ring_reserved_space_reserve(struct intel_ringbuffer *ringbuf, int size)
+{
+	/* NB: Until request management is fully tidied up and the OLR is
+	 * removed, there are too many ways for get false hits on this
+	 * anti-recursion check! */
+	/*WARN_ON(ringbuf->reserved_size);*/
+	WARN_ON(ringbuf->reserved_in_use);
+
+	ringbuf->reserved_size = size;
+
+	/*
+	 * Really need to call _begin() here but that currently leads to
+	 * recursion problems! This will be fixed later but for now just
+	 * return and hope for the best. Note that there is only a real
+	 * problem if the create of the request never actually calls _begin()
+	 * but if they are not submitting any work then why did they create
+	 * the request in the first place?
+	 */
+}
+
+void intel_ring_reserved_space_cancel(struct intel_ringbuffer *ringbuf)
+{
+	WARN_ON(ringbuf->reserved_in_use);
+
+	ringbuf->reserved_size   = 0;
+	ringbuf->reserved_in_use = false;
+}
+
+void intel_ring_reserved_space_use(struct intel_ringbuffer *ringbuf)
+{
+	WARN_ON(ringbuf->reserved_in_use);
+
+	ringbuf->reserved_in_use = true;
+	ringbuf->reserved_tail   = ringbuf->tail;
+}
+
+void intel_ring_reserved_space_end(struct intel_ringbuffer *ringbuf)
+{
+	WARN_ON(!ringbuf->reserved_in_use);
+	WARN(ringbuf->tail > ringbuf->reserved_tail + ringbuf->reserved_size,
+	     "request reserved size too small: %d vs %d!\n",
+	     ringbuf->tail - ringbuf->reserved_tail, ringbuf->reserved_size);
+
+	ringbuf->reserved_size   = 0;
+	ringbuf->reserved_in_use = false;
+}
+
+static int __intel_ring_prepare(struct intel_engine_cs *ring, int bytes)
 {
 	struct intel_ringbuffer *ringbuf = ring->buffer;
 	int ret;
 
+	if (!ringbuf->reserved_in_use)
+		bytes += ringbuf->reserved_size;
+
 	if (unlikely(ringbuf->tail + bytes > ringbuf->effective_size)) {
+		WARN_ON(ringbuf->reserved_in_use);
+
 		ret = intel_wrap_ring_buffer(ring);
 		if (unlikely(ret))
 			return ret;
+
+		if(ringbuf->reserved_size) {
+			uint32_t size = ringbuf->reserved_size;
+
+			intel_ring_reserved_space_cancel(ringbuf);
+			intel_ring_reserved_space_reserve(ringbuf, size);
+		}
 	}
 
 	if (unlikely(ringbuf->space < bytes)) {
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 39f6dfc..39f795c 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -105,6 +105,9 @@ struct intel_ringbuffer {
 	int space;
 	int size;
 	int effective_size;
+	int reserved_size;
+	int reserved_tail;
+	bool reserved_in_use;
 
 	/** We track the position of the requests in the ring buffer, and
 	 * when each is retired we increment last_retired_head as the GPU
@@ -450,4 +453,11 @@ intel_ring_get_request(struct intel_engine_cs *ring)
 	return ring->outstanding_lazy_request;
 }
 
+#define MIN_SPACE_FOR_ADD_REQUEST	128
+
+void intel_ring_reserved_space_reserve(struct intel_ringbuffer *ringbuf, int size);
+void intel_ring_reserved_space_cancel(struct intel_ringbuffer *ringbuf);
+void intel_ring_reserved_space_use(struct intel_ringbuffer *ringbuf);
+void intel_ring_reserved_space_end(struct intel_ringbuffer *ringbuf);
+
 #endif /* _INTEL_RINGBUFFER_H_ */
-- 
1.7.9.5