drm/i915: Add I915_GEM_PARAM_HAS_SEMAPHORES

Userspace tries to estimate the cost of ring switching based on whether
the GPU and GEM supports semaphores. (If we have multiple rings and no
semaphores, userspace assumes that the cost of switching rings between
batches is exorbitant and will endeavour to keep the next batch on the
active ring - as a coarse approximation to tracking both destination and
source surfaces.) Currently userspace has to guess whether semaphores
exist based on the chipset generation and the module parameter,
i915.semaphores. This is a crude and inaccurate guess as the defaults
internally depend upon other chipset features being enabled or disabled,
nor does it extend well into the future. By exporting a HAS_SEMAPHORES
parameter, we can easily query the driver and obtain an accurate answer.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_dma.c

@@ -1009,6 +1009,9 @@ static int i915_getparam(struct drm_device *dev, void *data,
 	case I915_PARAM_HAS_WAIT_TIMEOUT:
 		value = 1;
 		break;
+	case I915_PARAM_HAS_SEMAPHORES:
+		value = i915_semaphore_is_enabled(dev);
+		break;
 	default:
 		DRM_DEBUG_DRIVER("Unknown parameter %d\n",
 				 param->param);

include/drm/i915_drm.h

@@ -311,6 +311,7 @@ typedef struct drm_i915_irq_wait {
 #define I915_PARAM_HAS_LLC     	 	 17
 #define I915_PARAM_HAS_ALIASING_PPGTT	 18
 #define I915_PARAM_HAS_WAIT_TIMEOUT	 19
+#define I915_PARAM_HAS_SEMAPHORES	 20
 
 typedef struct drm_i915_getparam {
 	int param;