[Intel-gfx] [RFC] drm/i915: check that rpm ref is held when writing to ringbuf in stolen mem

[daniele.ceraolospurio at intel.com]

From: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>

While running some tests on the scheduler patches with rpm enabled I
came across a corruption in the ringbuffer, which was root-caused to
the GPU being suspended while commands were being emitted to the
ringbuffer. The access to memory was failing because the GPU needs to
be awake when accessing stolen memory (where my ringbuffer was located).
Since we have this constraint it looks like a sensible idea to check that
we hold a refcount when we emit commands.

Cc: John Harrison <John.C.Harrison at Intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
---
 drivers/gpu/drm/i915/intel_lrc.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 3761eaf..f9e8d74 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -1105,6 +1105,11 @@ int intel_logical_ring_begin(struct drm_i915_gem_request *req, int num_dwords)
 	if (ret)
 		return ret;
 
+	// If the ringbuffer is in stolen memory we need to be sure that the
+	// gpu is awake before writing to it
+	if (req->ringbuf->obj->stolen && num_dwords > 0)
+		assert_rpm_wakelock_held(dev_priv);
+
 	req->ringbuf->space -= num_dwords * sizeof(uint32_t);
 	return 0;
 }
-- 
1.9.1