drm: document uAPI page-flip flags

Document flags accepted by the page-flip and atomic IOCTLs.

v2 (Pekka):
- Mention DRM_EVENT_FLIP_COMPLETE in DRM_MODE_PAGE_FLIP_EVENT docs.
- Expand DRM_MODE_ATOMIC_NONBLOCK and DRM_MODE_ATOMIC_ALLOW_MODESET
  description.
v3:
- Fix struct field ref syntax (Daniel)
- Clarify when artifacts are no longer displayed (Daniel)
- Add note about sinks deciding to show artifacts on their own (Pekka, Daniel)
v4:
- Fix typo (Pekka)

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/505107/

include/uapi/drm/drm_mode.h

@@ -935,12 +935,31 @@ struct hdr_output_metadata {
 	};
 };
 
+/**
+ * DRM_MODE_PAGE_FLIP_EVENT
+ *
+ * Request that the kernel sends back a vblank event (see
+ * struct drm_event_vblank) with the &DRM_EVENT_FLIP_COMPLETE type when the
+ * page-flip is done.
+ */
 #define DRM_MODE_PAGE_FLIP_EVENT 0x01
+/**
+ * DRM_MODE_PAGE_FLIP_ASYNC
+ *
+ * Request that the page-flip is performed as soon as possible, ie. with no
+ * delay due to waiting for vblank. This may cause tearing to be visible on
+ * the screen.
+ */
 #define DRM_MODE_PAGE_FLIP_ASYNC 0x02
 #define DRM_MODE_PAGE_FLIP_TARGET_ABSOLUTE 0x4
 #define DRM_MODE_PAGE_FLIP_TARGET_RELATIVE 0x8
 #define DRM_MODE_PAGE_FLIP_TARGET (DRM_MODE_PAGE_FLIP_TARGET_ABSOLUTE | \
 				   DRM_MODE_PAGE_FLIP_TARGET_RELATIVE)
+/**
+ * DRM_MODE_PAGE_FLIP_FLAGS
+ *
+ * Bitmask of flags suitable for &drm_mode_crtc_page_flip_target.flags.
+ */
 #define DRM_MODE_PAGE_FLIP_FLAGS (DRM_MODE_PAGE_FLIP_EVENT | \
 				  DRM_MODE_PAGE_FLIP_ASYNC | \
 				  DRM_MODE_PAGE_FLIP_TARGET)
@@ -1034,11 +1053,53 @@ struct drm_mode_destroy_dumb {
 	__u32 handle;
 };
 
-/* page-flip flags are valid, plus: */
+/**
+ * DRM_MODE_ATOMIC_TEST_ONLY
+ *
+ * Do not apply the atomic commit, instead check whether the hardware supports
+ * this configuration.
+ *
+ * See &drm_mode_config_funcs.atomic_check for more details on test-only
+ * commits.
+ */
 #define DRM_MODE_ATOMIC_TEST_ONLY 0x0100
+/**
+ * DRM_MODE_ATOMIC_NONBLOCK
+ *
+ * Do not block while applying the atomic commit. The &DRM_IOCTL_MODE_ATOMIC
+ * IOCTL returns immediately instead of waiting for the changes to be applied
+ * in hardware. Note, the driver will still check that the update can be
+ * applied before retuning.
+ */
 #define DRM_MODE_ATOMIC_NONBLOCK  0x0200
+/**
+ * DRM_MODE_ATOMIC_ALLOW_MODESET
+ *
+ * Allow the update to result in temporary or transient visible artifacts while
+ * the update is being applied. Applying the update may also take significantly
+ * more time than a page flip. All visual artifacts will disappear by the time
+ * the update is completed, as signalled through the vblank event's timestamp
+ * (see struct drm_event_vblank).
+ *
+ * This flag must be set when the KMS update might cause visible artifacts.
+ * Without this flag such KMS update will return a EINVAL error. What kind of
+ * update may cause visible artifacts depends on the driver and the hardware.
+ * User-space that needs to know beforehand if an update might cause visible
+ * artifacts can use &DRM_MODE_ATOMIC_TEST_ONLY without
+ * &DRM_MODE_ATOMIC_ALLOW_MODESET to see if it fails.
+ *
+ * To the best of the driver's knowledge, visual artifacts are guaranteed to
+ * not appear when this flag is not set. Some sinks might display visual
+ * artifacts outside of the driver's control.
+ */
 #define DRM_MODE_ATOMIC_ALLOW_MODESET 0x0400
 
+/**
+ * DRM_MODE_ATOMIC_FLAGS
+ *
+ * Bitfield of flags accepted by the &DRM_IOCTL_MODE_ATOMIC IOCTL in
+ * &drm_mode_atomic.flags.
+ */
 #define DRM_MODE_ATOMIC_FLAGS (\
 		DRM_MODE_PAGE_FLIP_EVENT |\
 		DRM_MODE_PAGE_FLIP_ASYNC |\