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

[Paul Gofman]

On 7/9/20 13:11, Henri Verbeet wrote:
> 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.

Yes, dropping discard flag for any unbound buffer maps was a bad idea. I
don't see a good way to check if buffer is actually used in GL during
mapping, so I think I should leave it for now until persistent buffer
mapping changes are there.