[Bug 106175] amdgpu.dc=1 shows performance issues with Xorg compositors when moving windows

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=106175

--- Comment #52 from Brandon Wright <bearoso at gmail.com> ---
Ok, I think I understand what's going on. Forgive me if this sounds stupid, I'm
looking at the DRM code for the first time.

The old KMS interface uses what's flagged as "legacy" cursor updates. These are
"asynchronous" in that they're handled and passed to the hardware as they come
in. On the vertical retrace interrupt, it uses whatever the last data passed in
was. 

My theory is the DC interface isn't passing these on to the hardware
immediately. It's aggregating them until the next sync, when they're all
handled at once. And that is what's causing the disturbance at page-flip time.
High-report-rate mice might exacerbate it.

Intel's driver hasn't merged that async code yet. It's still using legacy
cursor updates and working around this.

The DC code seems to have a TODO comment in amdgpu_dm.c that suggests something
about the legacy_cursor_update flag, but it doesn't do anything with it.

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