drm/i915: expose _SUBSLICE_MASK GETPARM

Assuming a uniform mask across all slices, this enables userspace to
determine the specific sub slices can be enabled. This information is
required, for example, to be able to analyse some OA counter reports
where the counter configuration depends on the HW sub slice
configuration.
Signed-off-by: Robert Bragg <robert@sixbynine.org>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>

drivers/gpu/drm/i915/i915_drv.c

@@ -379,6 +379,11 @@ static int i915_getparam(struct drm_device *dev, void *data,
 		if (!value)
 			return -ENODEV;
 		break;
+	case I915_PARAM_SUBSLICE_MASK:
+		value = INTEL_INFO(dev_priv)->sseu.subslice_mask;
+		if (!value)
+			return -ENODEV;
+		break;
 	default:
 		DRM_DEBUG("Unknown parameter %d\n", param->param);
 		return -EINVAL;

include/uapi/drm/i915_drm.h

@@ -421,6 +421,11 @@ typedef struct drm_i915_irq_wait {
 /* Query the mask of slices available for this system */
 #define I915_PARAM_SLICE_MASK		 46
 
+/* Assuming it's uniform for each slice, this queries the mask of subslices
+ * per-slice for this system.
+ */
+#define I915_PARAM_SUBSLICE_MASK	 47
+
 typedef struct drm_i915_getparam {
 	__s32 param;
 	/*