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

[Henri Verbeet]

On Thu, 9 Jul 2020 at 13:47, Axel Davy <davyaxel0 at gmail.com> wrote:
> I have another explanation for this behaviour:
>
> For the first DISCARD you try, the buffer backing is busy because it is
> doing to be used for rendering not yet finished.
> Thus the DISCARD does actually do something.
>
> Then when you map for reading the buffer it flushes everything and
> rendering has to finish. Thus for the next DISCARD, the driver finds
> that the driver backing is not busy, it decides to keep it.
>
> Definitely you should discard buffer data if the buffer is not bound. It
> will happen in a lot of games. A buffer not bound doesn't mean it is not
> used by the GPU. What you really want is to not discard when the buffer
> is not bound nor used in any pending rendering. That is of course much
> harder to detect. I would bet your application just needs the DISCARD to
> do nothing before the buffer is used for rendering for the first time.
>
Yes, what Axel said. I suspect the application is essentially
depending on an optimisation to not bother discarding the buffer if we
wouldn't have to wait to map it. I.e., similar to the redundant
discard filtering we do a bit further down in
buffer_resource_sub_resource_map(), and similar to the Vulkan discard
handling we do in adapter_vk_map_bo_address(). In its current form,
this patch will introduce stalls when mapping buffers that have been
unbound, but are still in use by the GPU.