[Intel-gfx] [RFC 10/39] drm/i915: Disable hardware semaphores when GPU scheduler is enabled

[John.C.Harrison at Intel.com]

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

Hardware sempahores require seqno values to be continuously incrementing.
However, the scheduler's reordering of batch buffers means that the seqno values
going through the hardware could be out of order. Thus semaphores can not be
used.

On the other hand, the scheduler superceeds the need for hardware semaphores
anyway. Having one ring stall waiting for something to complete on another ring
is inefficient if that ring could be working on some other, independent task.
This is what the scheduler is meant to do - keep the hardware as busy as
possible by reordering batch buffers to avoid dependency stalls.

Change-Id: I95d1fceacd370455a9720d7dca55cfd0a1f6beaa
For: VIZ-1587
Signed-off-by: John Harrison <John.C.Harrison at Intel.com>
---
 drivers/gpu/drm/i915/i915_drv.c         | 9 +++++++++
 drivers/gpu/drm/i915/i915_scheduler.c   | 7 +++++++
 drivers/gpu/drm/i915/i915_scheduler.h   | 1 +
 drivers/gpu/drm/i915/intel_ringbuffer.c | 4 ++++
 4 files changed, 21 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
index db48aee..abd7efc 100644
--- a/drivers/gpu/drm/i915/i915_drv.c
+++ b/drivers/gpu/drm/i915/i915_drv.c
@@ -34,6 +34,7 @@
 #include "i915_drv.h"
 #include "i915_trace.h"
 #include "intel_drv.h"
+#include "i915_scheduler.h"
 
 #include <linux/console.h>
 #include <linux/module.h>
@@ -516,6 +517,14 @@ void intel_detect_pch(struct drm_device *dev)
 
 bool i915_semaphore_is_enabled(struct drm_device *dev)
 {
+	/* Hardware semaphores are not compatible with the scheduler due to the
+	 * seqno values being potentially out of order. However, semaphores are
+	 * also not required as the scheduler will handle interring dependencies
+	 * and try do so in a way that does not cause dead time on the hardware.
+	 */
+	if (i915_scheduler_is_enabled(dev))
+		return false;
+
 	if (INTEL_INFO(dev)->gen < 6)
 		return false;
 
diff --git a/drivers/gpu/drm/i915/i915_scheduler.c b/drivers/gpu/drm/i915/i915_scheduler.c
index 0d1cbe3..f7fd9a4 100644
--- a/drivers/gpu/drm/i915/i915_scheduler.c
+++ b/drivers/gpu/drm/i915/i915_scheduler.c
@@ -38,6 +38,13 @@ static int         i915_scheduler_priority_bump(struct i915_scheduler *scheduler
 						struct i915_scheduler_queue_entry *target,
 						uint32_t bump);
 
+bool i915_scheduler_is_enabled(struct drm_device *dev)
+{
+	struct drm_i915_private *dev_priv = dev->dev_private;
+
+	return dev_priv->scheduler != NULL;
+}
+
 int i915_scheduler_init(struct drm_device *dev)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
diff --git a/drivers/gpu/drm/i915/i915_scheduler.h b/drivers/gpu/drm/i915/i915_scheduler.h
index 6b2585a..88cbfba 100644
--- a/drivers/gpu/drm/i915/i915_scheduler.h
+++ b/drivers/gpu/drm/i915/i915_scheduler.h
@@ -84,6 +84,7 @@ enum {
 	i915_sf_submitting          = (1 << 1),
 };
 
+bool        i915_scheduler_is_enabled(struct drm_device *dev);
 int         i915_scheduler_init(struct drm_device *dev);
 int         i915_scheduler_queue_execbuffer(struct i915_scheduler_queue_entry *qe);
 int         i915_scheduler_handle_irq(struct intel_engine_cs *ring);
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 83a5254..df0cd48 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -32,6 +32,7 @@
 #include <drm/i915_drm.h>
 #include "i915_trace.h"
 #include "intel_drv.h"
+#include "i915_scheduler.h"
 
 bool
 intel_ring_initialized(struct intel_engine_cs *ring)
@@ -1379,6 +1380,9 @@ gen6_ring_sync(struct drm_i915_gem_request *waiter_req,
 	u32 wait_mbox = signaller->semaphore.mbox.wait[waiter->id];
 	int ret;
 
+	/* Arithmetic on sequence numbers is unreliable with a scheduler. */
+	BUG_ON(i915_scheduler_is_enabled(signaller->dev));
+
 	/* Throughout all of the GEM code, seqno passed implies our current
 	 * seqno is >= the last seqno executed. However for hardware the
 	 * comparison is strictly greater than.
-- 
1.9.1