uapi/drm/drm_fourcc.h: Note on platform specificity for format modifiers

Make an additional note on DRM format modifiers for x and y tiling. These
format modifiers are defined for BDW+ platforms and therefore definition
is not valid for older gens. This is due to address swizzling for tiled
surfaces is no longer used. For newer platforms main memory controller has
a more effective address swizzling algorithm.

v2: Rephrase comment (Daniel)

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200506120827.12250-1-mika.kahola@intel.com

include/uapi/drm/drm_fourcc.h

@@ -354,9 +354,12 @@ extern "C" {
  * a platform-dependent stride. On top of that the memory can apply
  * platform-depending swizzling of some higher address bits into bit6.
  *
- * This format is highly platforms specific and not useful for cross-driver
- * sharing. It exists since on a given platform it does uniquely identify the
- * layout in a simple way for i915-specific userspace.
+ * Note that this layout is only accurate on intel gen 8+ or valleyview chipsets.
+ * On earlier platforms the is highly platforms specific and not useful for
+ * cross-driver sharing. It exists since on a given platform it does uniquely
+ * identify the layout in a simple way for i915-specific userspace, which
+ * facilitated conversion of userspace to modifiers. Additionally the exact
+ * format on some really old platforms is not known.
  */
 #define I915_FORMAT_MOD_X_TILED	fourcc_mod_code(INTEL, 1)
 
@@ -369,9 +372,12 @@ extern "C" {
  * memory can apply platform-depending swizzling of some higher address bits
  * into bit6.
  *
- * This format is highly platforms specific and not useful for cross-driver
- * sharing. It exists since on a given platform it does uniquely identify the
- * layout in a simple way for i915-specific userspace.
+ * Note that this layout is only accurate on intel gen 8+ or valleyview chipsets.
+ * On earlier platforms the is highly platforms specific and not useful for
+ * cross-driver sharing. It exists since on a given platform it does uniquely
+ * identify the layout in a simple way for i915-specific userspace, which
+ * facilitated conversion of userspace to modifiers. Additionally the exact
+ * format on some really old platforms is not known.
  */
 #define I915_FORMAT_MOD_Y_TILED	fourcc_mod_code(INTEL, 2)