DRI3 damage tracking?

[Pierre Ossman]

Hi everyone,

I'm playing around with adding DRI3 support to Xvnc, so that OpenGL and 
Vulkan can be accelerated for headless VNC sessions. Everything seems to 
be working, but there is one inefficiency that I'm trying to resolve, 
and that's minimising the shuffling of data between the GPU and CPU.

The X server can follow what it modifies, so knowing what to push back 
to the GPU is fairly straightforward. But I cannot find any details 
about the other direction. I.e. what the DRI3 clients modify via direct 
rendering.

Presently, I have to assume that everything that the X server wants to 
read is modified, and always read that back from the GPU.

So, is there some information the X server can see about what has been 
modified? Or perhaps at least if the buffer has been touched at all 
since the last synchronisation?

Ideally, the copying could be completely avoided by accessing the same 
memory. But apparently¹, gbm_bo_map() isn't guaranteed to actually map 
memory.

¹ 
https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/945#note_1483370

Regards,
-- 
Pierre Ossman           Software Development
Cendio AB               https://cendio.com
Teknikringen 8          https://twitter.com/ThinLinc
583 30 Linköping        https://facebook.com/ThinLinc
Phone: +46-13-214600

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