[PATCH] drm/uapi: Add a warning that mode flags must match the xrandr definitions

[ville.syrjala at linux.intel.com]

From: Ville Syrjälä <ville.syrjala at linux.intel.com>

Existing userspace expected the mode flags to match the xrandr
definitions 1:1, and even adding new flags in he previously unused
bits is likely to break existing userspace. Add a comment warning
people about this potential trap.

Cc: Shashank Sharma <shashank.sharma at intel.com>
Cc: "Lin, Jia" <lin.a.jia at intel.com>
Cc: Akashdeep Sharma <akashdeep.sharma at intel.com>
Cc: Jim Bride <jim.bride at linux.intel.com>
Cc: Jose Abreu <Jose.Abreu at synopsys.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: Emil Velikov <emil.l.velikov at gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
---
 include/uapi/drm/drm_mode.h | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
index 084b50a02dc5..01000c9f7c2c 100644
--- a/include/uapi/drm/drm_mode.h
+++ b/include/uapi/drm/drm_mode.h
@@ -47,7 +47,15 @@ extern "C" {
 #define DRM_MODE_TYPE_DRIVER	(1<<6)
 
 /* Video mode flags */
-/* bit compatible with the xorg definitions. */
+/* bit compatible with the xrandr RR_ definitions (bits 0-13)
+ *
+ * ABI warning: Existing userspace really expects
+ * the mode flags to match the xrandr definitions. Any
+ * changes that don't match the xrandr definitions will
+ * likely need a new client cap or some other mechanism
+ * to avoid breaking existing userspace. This includes
+ * allocating new flags in the previously unused bits!
+ */
 #define DRM_MODE_FLAG_PHSYNC			(1<<0)
 #define DRM_MODE_FLAG_NHSYNC			(1<<1)
 #define DRM_MODE_FLAG_PVSYNC			(1<<2)
-- 
2.7.4