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 5823cca39d585e4b4a32b1292eed0015da9c3276)
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

diff --git a/Documentation/gpu/todo.rst b/Documentation/gpu/todo.rst
index 009d8e6..77fbfe9 100644 (file)
--- a/Documentation/gpu/todo.rst
+++ b/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.
 
 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
 ===========================
 
 Subsystem-wide refactorings
 ===========================
 


@@ -168,6 +171,22 @@ Contact: Daniel Vetter, respective driver maintainers
 
 Level: Advanced
 
 
 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
 ------------------------------------------------------------
 
 Convert logging to drm_* functions with drm_device paramater
 ------------------------------------------------------------