drm/i915: Reserve ring buffer space for i915_add_request() commands
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.
v3: Incremented the reserved size to accommodate Ironlake (after finally
managing to run on an ILK system). Also fixed missing wrap code in LRC mode.
v4: Added extra comment and removed duplicate WARN (feedback from Tomas).
For: VIZ-5115
CC: Tomas Elf <tomas.elf@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 80b509b..b9e0989 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2485,6 +2485,13 @@
} 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 @@
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;
}
@@ -2665,6 +2675,26 @@
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;
@@ -2673,6 +2703,13 @@
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)
{