Does gbm_bo_map() implicitly synchronise?

[Pierre Ossman]

On 17/06/2024 18:09, Michel Dänzer wrote:
>>
>> Can I know whether it is needed or not? Or should I be cautious and always do it?
> 
> Assuming GBM in the X server uses the GPU HW driver, I'd say it shouldn't be needed.
> 

It does not (except the driver libgbm loads). We're trying to use this 
in Xvnc, so it's all CPU. We're just trying to make sure the 
applications can use the full power of the GPU to render their stuff 
before handing it over to the X server. :)

> 
>> A recording of the issue is available here, in case the behaviour rings a bell for anyone:
>>
>> http://www.cendio.com/~ossman/dri3/Screencast%20from%202024-06-17%2017-06-50.webm
> 
> Interesting. Looks like the surroundings (drop shadow region?) of the window move along with it first, then the surroundings get fixed up in the next frame.
> 
> As far as I know, mutter doesn't move window contents like that on the client side; it always redraws the damaged output region from scratch. So I wonder if the initial move together with surroundings is actually a blit on the X server side (possibly triggered by mutter moving the X window in its function as window manager). And then the surroundings fixing themselves up is the correct output from mutter via DRI3/Present.
> 
> If so, the issue isn't synchronization, it's that the first blit happens at all.
> 

Hmm... The source of the blit is CopyWindow being called as a result of 
the window moving. But I would have expected that to be inhibited by the 
fact that a compositor is active. It's also surprising that this only 
happens if DRI3 is involved.

I would also have expected something similar with software rendering. 
Albeit with a PutImage instead of PresentPixmap for the correct data. 
But everything works there.

I will need to dig further.

Regards,
-- 
Pierre Ossman           Software Development
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