[Intel-gfx] [PATCH] drm/i915: Use atomic for dev_priv->mm.bsd_engine_dispatch_index

[Joonas Lahtinen]

Use atomic type and operands for dev_priv->mm.bsd_engine_dispatch_index
to avoid one struct_mutex locking scenario.

Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Imre Deak <imre.deak at intel.com>
Cc: Zhao Yakui <yakui.zhao at intel.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
---
 drivers/gpu/drm/i915/i915_drv.h            |  2 +-
 drivers/gpu/drm/i915/i915_gem.c            |  2 ++
 drivers/gpu/drm/i915/i915_gem_execbuffer.c | 10 +++-------
 3 files changed, 6 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index c413587..43d5298 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1338,7 +1338,7 @@ struct i915_gem_mm {
 	bool interruptible;
 
 	/* the indicator for dispatch video commands on two BSD rings */
-	unsigned int bsd_engine_dispatch_index;
+	atomic_t bsd_engine_dispatch_index;
 
 	/** Bit 6 swizzling required for X tiling */
 	uint32_t bit_6_swizzle_x;
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index d37b441..2401818 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -4622,6 +4622,8 @@ i915_gem_load_init(struct drm_device *dev)
 
 	dev_priv->mm.interruptible = true;
 
+	atomic_set(&dev_priv->mm.bsd_engine_dispatch_index, 0);
+
 	spin_lock_init(&dev_priv->fb_tracking.lock);
 }
 
diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index 57d096a..9432d4c 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -1535,13 +1535,9 @@ gen8_dispatch_bsd_engine(struct drm_i915_private *dev_priv,
 	struct drm_i915_file_private *file_priv = file->driver_priv;
 
 	/* Check whether the file_priv has already selected one ring. */
-	if ((int)file_priv->bsd_engine < 0) {
-		/* If not, use the ping-pong mechanism to select one. */
-		mutex_lock(&dev_priv->drm.struct_mutex);
-		file_priv->bsd_engine = dev_priv->mm.bsd_engine_dispatch_index;
-		dev_priv->mm.bsd_engine_dispatch_index ^= 1;
-		mutex_unlock(&dev_priv->drm.struct_mutex);
-	}
+	if ((int)file_priv->bsd_engine < 0)
+		file_priv->bsd_engine = atomic_fetch_xor(1,
+			 &dev_priv->mm.bsd_engine_dispatch_index);
 
 	return file_priv->bsd_engine;
 }
-- 
2.5.5