drm/i915: Add comments to explain the BSD tail write workaround

Having had to dive into the bspec to understand what each stage of the
workaround meant, and how that the ring broadcasting IDLE corresponded
with the GT powering down the ring (i.e. rc6) add comments to aide
the next reader.

And since the register "is used to control all aspects of PSMI and power
saving functions" that makes it quite interesting to inspect with
regards to RC6 hangs, so add it to the error-state.

v2: Rediscover the piece of magic, set the RNCID to 0 before waiting for
the ring to wake up.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

1 files changed, 19 insertions, 8 deletions

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index d42d821c64d6..ddc48590ea60 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -1270,20 +1270,31 @@ static void gen6_bsd_ring_write_tail(struct intel_ring_buffer *ring,
         drm_i915_private_t *dev_priv = ring->dev->dev_private;
 
        /* Every tail move must follow the sequence below */
+
+        /* Disable notification that the ring is IDLE. The GT
+         * will then assume that it is busy and bring it out of rc6.
+         */
         I915_WRITE(GEN6_BSD_SLEEP_PSMI_CONTROL,
-                GEN6_BSD_SLEEP_PSMI_CONTROL_RC_ILDL_MESSAGE_MODIFY_MASK |
-                GEN6_BSD_SLEEP_PSMI_CONTROL_RC_ILDL_MESSAGE_DISABLE);
-        I915_WRITE(GEN6_BSD_RNCID, 0x0);
+                   _MASKED_BIT_ENABLE(GEN6_BSD_SLEEP_MSG_DISABLE));
+
+        /* Clear the context id. Here be magic! */
+        I915_WRITE64(GEN6_BSD_RNCID, 0x0);
 
+        /* Wait for the ring not to be idle, i.e. for it to wake up. */
         if (wait_for((I915_READ(GEN6_BSD_SLEEP_PSMI_CONTROL) &
-                GEN6_BSD_SLEEP_PSMI_CONTROL_IDLE_INDICATOR) == 0,
-                50))
-        DRM_ERROR("timed out waiting for IDLE Indicator\n");
+                      GEN6_BSD_SLEEP_INDICATOR) == 0,
+                     50))
+                DRM_ERROR("timed out waiting for the BSD ring to wake up\n");
 
+        /* Now that the ring is fully powered up, update the tail */
         I915_WRITE_TAIL(ring, value);
+        POSTING_READ(RING_TAIL(ring->mmio_base));
+
+        /* Let the ring send IDLE messages to the GT again,
+         * and so let it sleep to conserve power when idle.
+         */
         I915_WRITE(GEN6_BSD_SLEEP_PSMI_CONTROL,
-                GEN6_BSD_SLEEP_PSMI_CONTROL_RC_ILDL_MESSAGE_MODIFY_MASK |
-                GEN6_BSD_SLEEP_PSMI_CONTROL_RC_ILDL_MESSAGE_ENABLE);
+                   _MASKED_BIT_DISABLE(GEN6_BSD_SLEEP_MSG_DISABLE));
 }
 
 static int gen6_ring_flush(struct intel_ring_buffer *ring,