drm/i915: Document and reject invalid tiling modes

Through the GTT interface to the fence registers, we can only handle
linear, X and Y tiling. The more esoteric tiling patterns are ignored.
Document that the tiling ABI only supports upto Y tiling, and reject any
attempts to set a tiling mode other than NONE, X or Y.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470388464-28458-17-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_tiling.c

@@ -68,6 +68,9 @@ i915_tiling_ok(struct drm_device *dev, int stride, int size, int tiling_mode)
 	if (tiling_mode == I915_TILING_NONE)
 		return true;
 
+	if (tiling_mode > I915_TILING_LAST)
+		return false;
+
 	if (IS_GEN2(dev) ||
 	    (tiling_mode == I915_TILING_Y && HAS_128_BYTE_Y_TILING(dev)))
 		tile_width = 128;

include/uapi/drm/i915_drm.h

@@ -926,6 +926,7 @@ struct drm_i915_gem_caching {
 #define I915_TILING_NONE	0
 #define I915_TILING_X		1
 #define I915_TILING_Y		2
+#define I915_TILING_LAST	I915_TILING_Y
 
 #define I915_BIT_6_SWIZZLE_NONE		0
 #define I915_BIT_6_SWIZZLE_9		1