drm/i915: Make ring buffer size of a LRC context configurable

This patch introduces an option for configuring the ring buffer size
of a LRC context after the context creation.

v9:
- Fix an identation issue. (Chris)

v8:
- Rename the data member in i915_gem_context. (Chris)

Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1466078825-6662-6-git-send-email-zhi.a.wang@intel.com

drivers/gpu/drm/i915/i915_drv.h

@@ -880,6 +880,7 @@ struct i915_gem_context {
 		int pin_count;
 		bool initialised;
 	} engine[I915_NUM_ENGINES];
+	u32 ring_size;
 
 	struct list_head link;
 

drivers/gpu/drm/i915/i915_gem_context.c

@@ -295,6 +295,7 @@ __create_hw_context(struct drm_device *dev,
 	ctx->remap_slice = ALL_L3_SLICES(dev_priv);
 
 	ctx->hang_stats.ban_period_seconds = DRM_I915_CTX_BAN_PERIOD;
+	ctx->ring_size = 4 * PAGE_SIZE;
 
 	return ctx;
 

drivers/gpu/drm/i915/intel_lrc.c

@@ -2527,7 +2527,7 @@ static int execlists_context_deferred_alloc(struct i915_gem_context *ctx,
 		return PTR_ERR(ctx_obj);
 	}
 
-	ringbuf = intel_engine_create_ringbuffer(engine, 4 * PAGE_SIZE);
+	ringbuf = intel_engine_create_ringbuffer(engine, ctx->ring_size);
 	if (IS_ERR(ringbuf)) {
 		ret = PTR_ERR(ringbuf);
 		goto error_deref_obj;