[Intel-gfx] [PATCH v4 09/38] 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.

v4: Downgraded a BUG_ON to WARN_ON as the latter is preferred.

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 858d58c..e6be8f5 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>
@@ -581,6 +582,14 @@ 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 8cb9063..fc48955 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 00dc7f3..2d50d83 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);
 bool        i915_scheduler_notify_request(struct drm_i915_gem_request *req);
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 1dec252..a93bbce 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)
@@ -1411,6 +1412,9 @@ 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. */
+	WARN_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