[RFC 7/9] drm/i915: Interrupt driven fences

John.C.Harrison
2015-07-17T14:31:42Z

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

The intended usage model for struct fence is that the signalled status should be
set on demand rather than polled. That is, there should not be a need for a
'signaled' function to be called everytime the status is queried. Instead,
'something' should be done to enable a signal callback from the hardware which
will update the state directly. In the case of requests, this is the seqno
update interrupt. The idea is that this callback will only be enabled on demand
when something actually tries to wait on the fence.

This change removes the polling test and replaces it with the callback scheme.
Each fence is added to a 'please poke me' list at the start of
i915_add_request(). The interrupt handler then scans through the 'poke me' list
when a new seqno pops out and signals any matching fence/request. The fence is
then removed from the list so the entire request stack does not need to be
scanned every time. Note that the fence is added to the list before the commands
to generate the seqno interrupt are added to the ring. Thus the sequence is
guaranteed to be race free if the interrupt is already enabled.

Note that the interrupt is only enabled on demand (i.e. when __wait_request() is
called). Thus there is still a potential race when enabling the interrupt as the
request may already have completed. However, this is simply solved by calling
the interrupt processing code immediately after enabling the interrupt and
thereby checking for already completed requests.

Lastly, the ring clean up code has the possibility to cancel outstanding
requests (e.g. because TDR has reset the ring). These requests will never get
signalled and so must be removed from the signal list manually. This is done by
setting a 'cancelled' flag and then calling the regular notify/retire code path
rather than attempting to duplicate the list manipulatation and clean up code in
multiple places. This also avoid any race condition where the cancellation
request might occur after/during the completion interrupt actually arriving.

v2: Updated to take advantage of the request unreference no longer requiring the
mutex lock.

For: VIZ-5190
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
---
drivers/gpu/drm/i915/i915_drv.h | 8 ++
drivers/gpu/drm/i915/i915_gem.c | 132 +++++++++++++++++++++++++++++---
drivers/gpu/drm/i915/i915_irq.c | 2 +
drivers/gpu/drm/i915/intel_lrc.c | 1 +
drivers/gpu/drm/i915/intel_ringbuffer.c | 1 +
drivers/gpu/drm/i915/intel_ringbuffer.h | 1 +
6 files changed, 136 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 61c3db2..d7f1aa5 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -2163,7 +2163,11 @@ void i915_gem_track_fb(struct drm_i915_gem_object *old,
struct drm_i915_gem_request {
/** Underlying object for implementing the signal/wait stuff. */
struct fence fence;
+ struct list_head signal_list;
+ struct list_head unsignal_list;
struct list_head delay_free_list;
+ bool cancelled;
+ bool irq_enabled;

/** On Which ring this request was generated */
struct drm_i915_private *i915;
@@ -2241,6 +2245,10 @@ int i915_gem_request_alloc(struct intel_engine_cs *ring,
struct drm_i915_gem_request **req_out);
void i915_gem_request_cancel(struct drm_i915_gem_request *req);

+void i915_gem_request_submit(struct drm_i915_gem_request *req);
+void i915_gem_request_enable_interrupt(struct drm_i915_gem_request *req);
+void i915_gem_request_notify(struct intel_engine_cs *ring);
+
int i915_create_fence_timeline(struct drm_device *dev,
struct intel_context *ctx,
struct intel_engine_cs *ring);
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 482835a..7c589a9 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1222,6 +1222,11 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
if (list_empty(&req->list))
return 0;

+ /*
+ * Enable interrupt completion of the request.
+ */
+ i915_gem_request_enable_interrupt(req);
+
if (i915_gem_request_completed(req))
return 0;

@@ -1382,6 +1387,10 @@ static void i915_gem_request_retire(struct drm_i915_gem_request *request)
list_del_init(&request->list);
i915_gem_request_remove_from_client(request);

+ /* In case the request is still in the signal pending list */
+ if (!list_empty(&request->signal_list))
+ request->cancelled = true;
+
i915_gem_request_unreference(request);
}

@@ -2534,6 +2543,12 @@ void __i915_add_request(struct drm_i915_gem_request *request,
*/
request->postfix = intel_ring_get_tail(ringbuf);

+ /*
+ * Add the fence to the pending list before emitting the commands to
+ * generate a seqno notification interrupt.
+ */
+ i915_gem_request_submit(request);
+
if (i915.enable_execlists)
ret = ring->emit_request(request);
else {
@@ -2653,6 +2668,9 @@ static void i915_gem_request_free(struct drm_i915_gem_request *req)
i915_gem_context_unreference(ctx);
}

+ if (req->irq_enabled)
+ req->ring->irq_put(req->ring);
+
kmem_cache_free(req->i915->requests, req);
}

@@ -2668,24 +2686,105 @@ static const char *i915_gem_request_get_timeline_name(struct fence *req_fence)
return req->ring->name;
}

-static bool i915_gem_request_enable_signaling(struct fence *req_fence)
+/*
+ * The request has been submitted to the hardware so add the fence to the
+ * list of signalable fences.
+ *
+ * NB: This does not enable interrupts yet. That only occurs on demand when
+ * the request is actually waited on. However, adding it to the list early
+ * ensures that there is no race condition where the interrupt could pop
+ * out prematurely and thus be completely lost. The race is merely that the
+ * interrupt must be manually checked for after being enabled.
+ */
+void i915_gem_request_submit(struct drm_i915_gem_request *req)
{
- /* Interrupt driven fences are not implemented yet.*/
- WARN(true, "This should not be called!");
- return true;
+ fence_enable_sw_signaling(&req->fence);
}

-static bool i915_gem_request_is_completed(struct fence *req_fence)
+/*
+ * The request is being actively waited on, so enable interrupt based
+ * completion signalling.
+ */
+void i915_gem_request_enable_interrupt(struct drm_i915_gem_request *req)
+{
+ if (req->irq_enabled)
+ return;
+
+ WARN_ON(!req->ring->irq_get(req->ring));
+ req->irq_enabled = true;
+
+ /*
+ * Because the interrupt is only enabled on demand, there is a race
+ * where the interrupt can fire before anyone is looking for it. So
+ * do an explicit check for missed interrupts.
+ */
+ i915_gem_request_notify(req->ring);
+}
+
+static bool i915_gem_request_enable_signaling(struct fence *req_fence)
{
struct drm_i915_gem_request *req = container_of(req_fence,
typeof(*req), fence);
+
+ i915_gem_request_reference(req);
+ WARN_ON(!list_empty(&req->signal_list));
+ list_add_tail(&req->signal_list, &req->ring->fence_signal_list);
+
+ /*
+ * Note that signalling is always enabled for every request before
+ * that request is submitted to the hardware. Therefore there is
+ * no race condition whereby the signal could pop out before the
+ * request has been added to the list. Hence no need to check
+ * for completion, undo the list add and return false.
+ *
+ * NB: Interrupts are only enabled on demand. Thus there is still a
+ * race where the request could complete before the interrupt has
+ * been enabled. Thus care must be taken at that point.
+ */
+
+ return true;
+}
+
+void i915_gem_request_notify(struct intel_engine_cs *ring)
+{
+ struct drm_i915_gem_request *req, *req_next;
+ unsigned long flags;
u32 seqno;
+ LIST_HEAD(free_list);

- BUG_ON(req == NULL);
+ if (list_empty(&ring->fence_signal_list))
+ return;
+
+ seqno = ring->get_seqno(ring, false);
+
+ spin_lock_irqsave(&ring->fence_lock, flags);
+ list_for_each_entry_safe(req, req_next, &ring->fence_signal_list, signal_list) {
+ if (!req->cancelled) {
+ if (!i915_seqno_passed(seqno, req->seqno))
+ continue;

- seqno = req->ring->get_seqno(req->ring, false/*lazy_coherency*/);
+ fence_signal_locked(&req->fence);
+ }
+
+ list_del_init(&req->signal_list);
+ if (req->irq_enabled) {
+ req->ring->irq_put(req->ring);
+ req->irq_enabled = false;
+ }

- return i915_seqno_passed(seqno, req->seqno);
+ /* Can't unreference here because that might grab fence_lock */
+ list_add_tail(&req->unsignal_list, &free_list);
+ }
+ spin_unlock_irqrestore(&ring->fence_lock, flags);
+
+ /* It should now be safe to actually free the requests */
+ while (!list_empty(&free_list)) {
+ req = list_first_entry(&free_list,
+ struct drm_i915_gem_request, unsignal_list);
+ list_del(&req->unsignal_list);
+
+ i915_gem_request_unreference(req);
+ }
}

static void i915_fence_timeline_value_str(struct fence *fence, char *str, int size)
@@ -2711,7 +2810,6 @@ static const struct fence_ops i915_gem_request_fops = {
.get_driver_name = i915_gem_request_get_driver_name,
.get_timeline_name = i915_gem_request_get_timeline_name,
.enable_signaling = i915_gem_request_enable_signaling,
- .signaled = i915_gem_request_is_completed,
.wait = fence_default_wait,
.release = i915_gem_request_release,
.fence_value_str = i915_fence_value_str,
@@ -2791,6 +2889,7 @@ int i915_gem_request_alloc(struct intel_engine_cs *ring,
goto err;
}

+ INIT_LIST_HEAD(&req->signal_list);
fence_init(&req->fence, &i915_gem_request_fops, &ring->fence_lock,
ctx->engine[ring->id].fence_timeline.fence_context,
i915_fence_timeline_get_next_seqno(&ctx->engine[ring->id].fence_timeline));
@@ -2913,6 +3012,13 @@ static void i915_gem_reset_ring_cleanup(struct drm_i915_private *dev_priv,

i915_gem_request_retire(request);
}
+
+ /*
+ * Make sure any requests that were on the signal pending list get
+ * cleaned up.
+ */
+ i915_gem_request_notify(ring);
+ i915_gem_retire_requests_ring(ring);
}

void i915_gem_restore_fences(struct drm_device *dev)
@@ -2968,6 +3074,13 @@ i915_gem_retire_requests_ring(struct intel_engine_cs *ring)
{
WARN_ON(i915_verify_lists(ring->dev));

+ /*
+ * If no-one has waited on a request recently then interrupts will
+ * not have been enabled and thus no requests will ever be marked as
+ * completed. So do an interrupt check now.
+ */
+ i915_gem_request_notify(ring);
+
/* Retire requests first as we use it above for the early return.
* If we retire requests last, we may use a later seqno and so clear
* the requests lists without clearing the active list, leading to
@@ -5345,6 +5458,7 @@ init_ring_lists(struct intel_engine_cs *ring)
{
INIT_LIST_HEAD(&ring->active_list);
INIT_LIST_HEAD(&ring->request_list);
+ INIT_LIST_HEAD(&ring->fence_signal_list);
INIT_LIST_HEAD(&ring->delayed_free_list);
}

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index d87f173..e446509 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -853,6 +853,8 @@ static void notify_ring(struct intel_engine_cs *ring)

trace_i915_gem_request_notify(ring);

+ i915_gem_request_notify(ring);
+
wake_up_all(&ring->irq_queue);
}

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 9ee80f5..18dbd5c 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -1808,6 +1808,7 @@ static int logical_ring_init(struct drm_device *dev, struct intel_engine_cs *rin
ring->dev = dev;
INIT_LIST_HEAD(&ring->active_list);
INIT_LIST_HEAD(&ring->request_list);
+ INIT_LIST_HEAD(&ring->fence_signal_list);
INIT_LIST_HEAD(&ring->delayed_free_list);
spin_lock_init(&ring->fence_lock);
spin_lock_init(&ring->delayed_free_lock);
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 11494a3..83a5254 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -2040,6 +2040,7 @@ static int intel_init_ring_buffer(struct drm_device *dev,
INIT_LIST_HEAD(&ring->active_list);
INIT_LIST_HEAD(&ring->request_list);
INIT_LIST_HEAD(&ring->execlist_queue);
+ INIT_LIST_HEAD(&ring->fence_signal_list);
INIT_LIST_HEAD(&ring->delayed_free_list);
spin_lock_init(&ring->fence_lock);
spin_lock_init(&ring->delayed_free_lock);
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 68173a3..2e68b73 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -352,6 +352,7 @@ struct intel_engine_cs {
u32 (*get_cmd_length_mask)(u32 cmd_header);

spinlock_t fence_lock;
+ struct list_head fence_signal_list;
};

bool intel_ring_initialized(struct intel_engine_cs *ring);
--
1.9.1