drm/i915: Return a mask of the active rings in the high word of busy_ioctl

The intention is to help select which engine to use for copies with
interoperating clients - such as a GL client making a request to the X
server to perform a SwapBuffers, which may require copying from the
active GL back buffer to the X front buffer.

We choose to report a mask of the active rings to future proof the
interface against any changes which may allow for the object to reside
upon multiple rings.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: bikeshed away the write ring mask and add the explanation
Chris sent in a follow-up mail why we decided to use masks.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -3400,6 +3400,10 @@ i915_gem_busy_ioctl(struct drm_device *dev, void *data,
 	ret = i915_gem_object_flush_active(obj);
 
 	args->busy = obj->active;
+	if (obj->ring) {
+		BUILD_BUG_ON(I915_NUM_RINGS > 16);
+		args->busy |= intel_ring_flag(obj->ring) << 16;
+	}
 
 	drm_gem_object_unreference(&obj->base);
 unlock:

include/drm/i915_drm.h

@@ -704,7 +704,11 @@ struct drm_i915_gem_busy {
 	/** Handle of the buffer to check for busy */
 	__u32 handle;
 
-	/** Return busy status (1 if busy, 0 if idle) */
+	/** Return busy status (1 if busy, 0 if idle).
+	 * The high word is used to indicate on which rings the object
+	 * currently resides:
+	 *  16:31 - busy (r or r/w) rings (16 render, 17 bsd, 18 blt, etc)
+	 */
 	__u32 busy;
 };