drm/todo: Add entry for moving to dma_resv_lock

Requested by Thomas. I think it justifies a new level, since I tried
to make some forward progress on this last summer, and gave up (for
now). This is very tricky.
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Link: https://patchwork.freedesktop.org/patch/msgid/20210122133624.1751802-1-daniel.vetter@ffwll.ch
(cherry picked from commit 5823cca3)
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

Documentation/gpu/todo.rst

@@ -23,6 +23,9 @@ Advanced: Tricky tasks that need fairly good understanding of the DRM subsystem
 and graphics topics. Generally need the relevant hardware for development and
 testing.
 
+Expert: Only attempt these if you've successfully completed some tricky
+refactorings already and are an expert in the specific area
+
 Subsystem-wide refactorings
 ===========================
 
@@ -168,6 +171,22 @@ Contact: Daniel Vetter, respective driver maintainers
 
 Level: Advanced
 
+Move Buffer Object Locking to dma_resv_lock()
+---------------------------------------------
+
+Many drivers have their own per-object locking scheme, usually using
+mutex_lock(). This causes all kinds of trouble for buffer sharing, since
+depending which driver is the exporter and importer, the locking hierarchy is
+reversed.
+
+To solve this we need one standard per-object locking mechanism, which is
+dma_resv_lock(). This lock needs to be called as the outermost lock, with all
+other driver specific per-object locks removed. The problem is tha rolling out
+the actual change to the locking contract is a flag day, due to struct dma_buf
+buffer sharing.
+
+Level: Expert
+
 Convert logging to drm_* functions with drm_device paramater
 ------------------------------------------------------------