The Allwinner backend has a commit bit in order to push the new configuration to the actual hardware. We've always been using that bit.
However, we also should poll for that bit to clear, which we don't. Accessing any register while a commit is pending is forbidden, and will for example show a symptom of reading another, random, register.
If you get this during a read/modify/write cycle, this will result in random register corruption, which are pretty bad.
This can be shown using the following program (while the backend is active): http://code.bulix.org/gdl44p-161437?raw
Fortunately for us, this is not really likely to happen. The window where it can happen is quite thin, and it only happens during a modeset, since it's the only time we commit some new configuration.
Unfortunately for us, QT does a ridiculous amount of modeset, and will just hit that window after a while, creating a distorded (since the register we read/modify/write also has scaling attributes) or with weird colors (since it also has a invertion of red and blue components). And might fix itself later on by reading another random register with proper values for these fields.
Let me know what you think, Maxime
Changes from v1: - Renamed the function drm_atomic_helper_commit_tail_rpm - Dropped the changes in the rcar-du driver - Enhanced the documentation based on Daniel's comments
Maxime Ripard (4): drm/atomic: implement drm_atomic_helper_commit_tail for runtime_pm users drm/sun4i: Use the runtime_pm commit_tail variant drm/sun4i: engine: Add commit_poll function drm/sun4i: make sure we don't have a commit pending
drivers/gpu/drm/drm_atomic_helper.c | 49 +++++++++++++++-------- drivers/gpu/drm/exynos/exynos_drm_fb.c | 27 +------------- drivers/gpu/drm/rockchip/rockchip_drm_fb.c | 21 +---------- drivers/gpu/drm/sun4i/sun4i_backend.c | 14 +++++++- drivers/gpu/drm/sun4i/sun4i_crtc.c | 3 +- drivers/gpu/drm/sun4i/sun4i_framebuffer.c | 6 +++- drivers/gpu/drm/sun4i/sunxi_engine.h | 14 +++++++- include/drm/drm_atomic_helper.h | 1 +- 8 files changed, 74 insertions(+), 61 deletions(-)
base-commit: 5771a8c08880cdca3bfb4a3fc6d309d6bba20877