drm/TODO: Add drm_display_mode.hsync/vrefresh removal

Drivers shouldn't be using these values, add a TODO so someone removes
them.

Changes in v2:
- Add drm_display_mode.vrefresh removal (Ville)
- Add Sam's R-b and bonus points
Changes in v3:
- Add hsync removal todo item (Daniel)
- Change vrefresh wording to make removal less optional

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Bonus-points-awarded-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190129192637.73296-1-sean@poorly.run

Documentation/gpu/todo.rst

@@ -272,6 +272,34 @@ no one knows what that means) since underneath they just use dma_alloc_coherent.
 
 Contact: Laurent Pinchart, Daniel Vetter
 
+Convert direct mode.vrefresh accesses to use drm_mode_vrefresh()
+----------------------------------------------------------------
+
+drm_display_mode.vrefresh isn't guaranteed to be populated. As such, using it
+is risky and has been known to cause div-by-zero bugs. Fortunately, drm core
+has helper which will use mode.vrefresh if it's !0 and will calculate it from
+the timings when it's 0.
+
+Use simple search/replace, or (more fun) cocci to replace instances of direct
+vrefresh access with a call to the helper. Check out
+https://lists.freedesktop.org/archives/dri-devel/2019-January/205186.html for
+inspiration.
+
+Once all instances of vrefresh have been converted, remove vrefresh from
+drm_display_mode to avoid future use.
+
+Contact: Sean Paul
+
+Remove drm_display_mode.hsync
+-----------------------------
+
+We have drm_mode_hsync() to calculate this from hsync_start/end, since drivers
+shouldn't/don't use this, remove this member to avoid any temptations to use it
+in the future. If there is any debug code using drm_display_mode.hsync, convert
+it to use drm_mode_hsync() instead.
+
+Contact: Sean Paul
+
 Core refactorings
 =================