[Intel-gfx] [PATCH] drm/i915: Use MLC (l3$) for context objects

[Chris Wilson]

Enabling context support increases SwapBuffers latency by about 20%
(measured on an i7-3720qm). We can offset that loss slightly by enabling
faster caching for the contexts. As they are not backed by any
particular cache (such as the sampler or render caches) our only option
is to select the generic mid-level cache. This reduces the latency of
the swap by about 5%.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Ben Widawsky <ben at bwidawsk.net>
Cc: Kenneth Graunke <kenneth at whitecape.org>
---
 drivers/gpu/drm/i915/i915_gem_context.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index 338cdfc..129b63d 100644
--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -154,6 +154,13 @@ create_hw_context(struct drm_device *dev,
 		return ERR_PTR(-ENOMEM);
 	}
 
+	if (INTEL_INFO(dev)->gen >= 7) {
+		ret = i915_gem_object_set_cache_level(ctx->obj,
+						      I915_CACHE_LLC_MLC);
+		if (ret)
+			goto err_out;
+	}
+
 	/* The ring associated with the context object is handled by the normal
 	 * object tracking code. We give an initial ring value simple to pass an
 	 * assertion in the context switch code.
-- 
1.7.10.4