drm/vkms: update todo

Drop issues already resolved in vkms:

- CRC API Improvements to [1] add igt test to check extreme alpha values
  and [2] alpha blending;
- [3] prime buffer sharing;
- [4] writeback support;

On the other hand, we also found or thought about other improvements since
the last update of this document:

- better support for IGT tests
- improvements to writeback support
- syzbot report

Finally, we reorder items by the assumed complexity.

[1] https://patchwork.freedesktop.org/series/55944/
[2] https://patchwork.freedesktop.org/series/80823/
[3] https://patchwork.freedesktop.org/series/63212/
[4] https://patchwork.freedesktop.org/series/81177/

v2:
- Link to syzbot dashboard

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201006223006.gu55pjtuaigkh6il@smtp.gmail.com

Documentation/gpu/vkms.rst

@@ -10,36 +10,24 @@
 TODO
 ====
 
-CRC API Improvements
---------------------
-
-- Optimize CRC computation ``compute_crc()`` and plane blending ``blend()``
-
-- Use the alpha value to blend vaddr_src with vaddr_dst instead of
-  overwriting it in ``blend()``.
-
-- Add igt test to check cleared alpha value for XRGB plane format.
-
-- Add igt test to check extreme alpha values i.e. fully opaque and fully
-  transparent (intermediate values are affected by hw-specific rounding modes).
-
-Runtime Configuration
----------------------
-
-We want to be able to reconfigure vkms instance without having to reload the
-module. Use/Test-cases:
-
-- Hotplug/hotremove connectors on the fly (to be able to test DP MST handling of
-  compositors).
+If you want to do any of the items listed below, please share your interest
+with VKMS maintainers.
 
-- Configure planes/crtcs/connectors (we'd need some code to have more than 1 of
-  them first).
+IGT better support
+------------------
 
-- Change output configuration: Plug/unplug screens, change EDID, allow changing
-  the refresh rate.
+- Investigate: (1) test cases on kms_plane that are failing due to timeout on
+  capturing CRC; (2) when running kms_flip test cases in sequence, some
+  successful individual test cases are failing randomly.
 
-The currently proposed solution is to expose vkms configuration through
-configfs.  All existing module options should be supported through configfs too.
+- VKMS already has support for vblanks simulated via hrtimers, which can be
+  tested with kms_flip test; in some way, we can say that VKMS already mimics
+  the real hardware vblank. However, we also have virtual hardware that does
+  not support vblank interrupt and completes page_flip events right away; in
+  this case, compositor developers may end up creating a busy loop on virtual
+  hardware. It would be useful to support Virtual Hardware behavior in VKMS
+  because this can help compositor developers to test their features in
+  multiple scenarios.
 
 Add Plane Features
 ------------------
@@ -55,34 +43,50 @@ There's lots of plane features we could add support for:
 - Additional buffer formats, especially YUV formats for video like NV12.
   Low/high bpp RGB formats would also be interesting.
 
-- Async updates (currently only possible on cursor plane using the legacy cursor
-  api).
+- Async updates (currently only possible on cursor plane using the legacy
+  cursor api).
+
+For all of these, we also want to review the igt test coverage and make sure
+all relevant igt testcases work on vkms.
+
+Prime Buffer Sharing
+--------------------
 
-For all of these, we also want to review the igt test coverage and make sure all
-relevant igt testcases work on vkms.
+- Syzbot report - WARNING in vkms_gem_free_object:
+  https://syzkaller.appspot.com/bug?extid=e7ad70d406e74d8fc9d0
+
+Runtime Configuration
+---------------------
+
+We want to be able to reconfigure vkms instance without having to reload the
+module. Use/Test-cases:
+
+- Hotplug/hotremove connectors on the fly (to be able to test DP MST handling
+  of compositors).
+
+- Configure planes/crtcs/connectors (we'd need some code to have more than 1 of
+  them first).
+
+- Change output configuration: Plug/unplug screens, change EDID, allow changing
+  the refresh rate.
+
+The currently proposed solution is to expose vkms configuration through
+configfs.  All existing module options should be supported through configfs
+too.
 
 Writeback support
 -----------------
 
-Currently vkms only computes a CRC for each frame. Once we have additional plane
-features, we could write back the entire composited frame, and expose it as:
+- The writeback and CRC capture operations share the use of composer_enabled
+  boolean to ensure vblanks. Probably, when these operations work together,
+  composer_enabled needs to refcounting the composer state to proper work.
 
-- Writeback connector. This is useful for testing compositors if you don't have
-  hardware with writeback support.
+- Add support for cloned writeback outputs and related test cases using a
+  cloned output in the IGT kms_writeback.
 
 - As a v4l device. This is useful for debugging compositors on special vkms
   configurations, so that developers see what's really going on.
 
-Prime Buffer Sharing
---------------------
-
-We already have vgem, which is a gem driver for testing rendering, similar to
-how vkms is for testing the modeset side. Adding buffer sharing support to vkms
-allows us to test them together, to test synchronization and lots of other
-features. Also, this allows compositors to test whether they work correctly on
-SoC chips, where the display and rendering is very often split between 2
-drivers.
-
 Output Features
 ---------------
 
@@ -93,7 +97,10 @@ Output Features
 - Add support for link status, so that compositors can validate their runtime
   fallbacks when e.g. a Display Port link goes bad.
 
-- All the hotplug handling describe under "Runtime Configuration".
+CRC API Improvements
+--------------------
+
+- Optimize CRC computation ``compute_crc()`` and plane blending ``blend()``
 
 Atomic Check using eBPF
 -----------------------