drm/i915: Replace global bsd_dispatch_index with random seed

We keep a global seed for the legacy BSD round-robin selector, but in
our testing of multiple simultaneous client workloads, a random seed
spreads the load more evenly. (As even as an initial round-robin selector
can be!) Removing the global is one less variable we have to find a home
for!

We can simulate multi-client (both same and mixed workloads) using
igt/gem_wsim to work out optimal strategies and then compare our
simulation with the actual transcoder on multi-engine machines. This
fixed round-robin turns out to be one of the worst methods.

No user is advised to use this method; the current suggestion is to use
a virtual engine for agnostic batches, randomised submission or using
the busyness tracking to select the most idle engine at the time of
dispatch. At the present time, intel-media is explicit, but libva still
seems to use it, with the exception of batches that must execute on vcs0.
Oh well.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190809091010.23281-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c

@@ -2120,8 +2120,7 @@ gen8_dispatch_bsd_engine(struct drm_i915_private *dev_priv,
 
 	/* Check whether the file_priv has already selected one ring. */
 	if ((int)file_priv->bsd_engine < 0)
-		file_priv->bsd_engine = atomic_fetch_xor(1,
-			 &dev_priv->mm.bsd_engine_dispatch_index);
+		file_priv->bsd_engine = get_random_int() & 1;
 
 	return file_priv->bsd_engine;
 }

drivers/gpu/drm/i915/i915_drv.h

@@ -749,9 +749,6 @@ struct i915_gem_mm {
 
 	u64 unordered_timeline;
 
-	/* the indicator for dispatch video commands on two BSD rings */
-	atomic_t bsd_engine_dispatch_index;
-
 	/** Bit 6 swizzling required for X tiling */
 	u32 bit_6_swizzle_x;
 	/** Bit 6 swizzling required for Y tiling */

drivers/gpu/drm/i915/i915_gem.c

@@ -1653,8 +1653,6 @@ int i915_gem_init_early(struct drm_i915_private *dev_priv)
 	i915_gem_init__mm(dev_priv);
 	i915_gem_init__pm(dev_priv);
 
-	atomic_set(&dev_priv->mm.bsd_engine_dispatch_index, 0);
-
 	spin_lock_init(&dev_priv->fb_tracking.lock);
 
 	err = i915_gemfs_init(dev_priv);