[PATCH] wined3d: Don't discard buffer data if buffer is not bound.

[Axel Davy]

On 10/07/2020 13:08, Paul Gofman wrote:
> Yes, sure, there should be some ways to introduce a workaround to make
> AION happy in the current buffer management logic, but probably a better
> solution is switching to persistent buffer mapping and overall buffer
> mapping logic redesign which AFAIK is being worked on and which I guess
> will obsolete such solutions. Also, the problem should probably not be
> present with the Vulkan backend.

I'm not sure how persistent buffer mapping does help with this, but let 
me add another piece of information:

The d3d9 vertex/index buffer memory management logic changed a lot 
between windows xp and vista+. It is also affected whether you have a 
single core or a multicore cpu (windows's d3d9 csmt equivalent seems to 
only activate if the app has access to at least 2 cores). I'm saying 
this to say that if the game relies on this behaviour, it is likely to 
be something the runtime does willingly since xp.

In the case of buffer locks, if you check the driver ddi you will notice 
efforts to be able to process locks without needing to block or flush 
the csmt thread.

In other words, the DISCARD can be processed before the internal gpu 
driver begins to process the calls needing the previous buffer data.

Thus if you observe a behaviour where DISCARD doesn't change the 
internal buffer, it is quite likely this behaviour is handled in the 
d3d9 runtime, not the internal gpu driver. If so, the logic behind this 
optimization must be quite simple, and probably doesn't check the buffer 
is busy or not.

In any case, I would suggest to check the behaviour on all vendors 
before going further.

Yours,

Axel Davy