[Mesa-dev] Fwd: GPU-CPU sync during radeonQueryGetResult

[Vyacheslav Gonakhchyan]

Ilia, thanks a lot for the info.

So basically if I submit to GPU's command stream:
perform occlusion query,
write X to Y.
I know that query is completed when after reading Y address I get X.

Regards,
Vyacheslav

On Mon, Jul 6, 2015 at 9:13 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:

> I'm only really familiar with nouveau, but I think all GPU hardware
> works in roughly the same way. Basically you have some way of
> inserting "write X to address Y" into the command stream (aka a
> "fence"), after which you insert "write X+1 to address Y" and so on.
> If you want the CPU to wait on a given fence, you just do "while
> (*address < x);". If you have multiple GPU processing queues, you can
> usually also insert a "stall this queue until the value at address Y
> is at least X" command into the command stream.
>
> DRM uses implicit fences, so it knows which BOs are used for
> particular commands. So the flow goes something like "submit bunch of
> commands; submit fence write and attach that fence id to the BOs in
> the previous bunch of comands". Then to wait for a bo to become ready,
> you just wait until the GPU writes the appropriate number to memory
> address Y (from above).
>
> The mesa drivers can sometimes use clever tricks that avoid this
> sync'ing because it knows exactly how it emits the commands and
> perhaps it waits on something related earlier whereby it knows the
> other thing will be ready. No idea if that's the case here.
>
> Hope this helps,
>
>   -ilia
>
>
> On Mon, Jul 6, 2015 at 1:05 PM, Vyacheslav Gonakhchyan
> <ytriffy at gmail.com> wrote:
> > Ilia, thanks for the gallium link.
> > Do you know any links to high level info with broad strokes about how
> this
> > sync works? Frankly I do not know driver terminology and wanted to know
> more
> > about how this sync is performed for my research. I'm using mesa as a
> > reference because it has open implementation code. Occlusion query
> > functionality probably waits for z-buffer to become ready. Problem is
> that
> > usual synchronization techniques do not apply here. I'm thinking that
> driver
> > code gets notifications about state change. I want to know what kind of
> > notifications are available? Can query be performed in parallel with
> another
> > frame being processed or does it need complete GPU pipeline flush?
> >
> > Thanks,
> > Vyacheslav
> >
> > On Mon, Jul 6, 2015 at 8:32 PM, Ilia Mirkin <imirkin at alum.mit.edu>
> wrote:
> >>
> >> On Mon, Jul 6, 2015 at 11:29 AM, Vyacheslav Gonakhchyan
> >> <ytriffy at gmail.com> wrote:
> >> > Hi, everyone.
> >> >
> >> > Trying to understand method radeonQueryGetResult (more broadly GPU-CPU
> >> > sync).
> >> >
> >> > static void radeonQueryGetResult(struct gl_context *ctx, struct
> >> > gl_query_object *q)
> >> > {
> >> >     struct radeon_query_object *query = (struct radeon_query_object
> *)q;
> >> >         uint32_t *result;
> >> >     int i;
> >> >
> >> >     radeon_print(RADEON_STATE, RADEON_VERBOSE,
> >> >             "%s: query id %d, result %d\n",
> >> >             __func__, query->Base.Id, (int) query->Base.Result);
> >> >
> >> >     radeon_bo_map(query->bo, GL_FALSE);
> >> >         result = query->bo->ptr;
> >> >
> >> >     query->Base.Result = 0;
> >> >     for (i = 0; i < query->curr_offset/sizeof(uint32_t); ++i) {
> >> >         query->Base.Result += LE32_TO_CPU(result[i]);
> >> >         radeon_print(RADEON_STATE, RADEON_TRACE, "result[%d] = %d\n",
> i,
> >> > LE32_TO_CPU(result[i]));
> >> >     }
> >> >
> >> >     radeon_bo_unmap(query->bo);
> >> > }
> >> >
> >> > I don't know which part is responsible for blocking behavior (waiting
> >> > for
> >> > response from GPU). I suspect that radeon_bo_map does this magic.
> >> > Can someone point in the right direction?
> >>
> >> The radeon_bo_map defined in
> >> src/gallium/winsys/radeon/drm/radeon_drm_bo.c indeed has this magic.
> >> However the code in src/mesa/drivers/dri/radeon/radeon_queryobj.c
> >> references the radeon_bo_map in libdrm, which does not appear to wait.
> >>
> >> FWIW for nouveau, nouveau_bo_map will also implicitly do a
> >> nouveau_bo_wait, but that does not appear to be the case for radeon.
> >>
> >> Cheers,
> >>
> >>   -ilia
> >
> >
>
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