[Mesa-dev] Re: [Nouveau] [RFC] mesa/st: Avoid passing a NULL buffer to the drivers

[Tobias Klausmann]

On 12.01.2015 01:57, Ilia Mirkin wrote:
> On Sun, Jan 11, 2015 at 7:43 PM, Tobias Klausmann
> <tobias.johannes.klausmann at mni.thm.de> wrote:
>>
>> On 11.01.2015 06:05, Ilia Mirkin wrote:
>>> Can you elaborate a bit as to why that's the right thing to do?
>>>
>>> On Wed, Jan 7, 2015 at 1:52 PM, Tobias Klausmann
>>> <tobias.johannes.klausmann at mni.thm.de> wrote:
>>>> If we capture transform feedback from n stream in (n-1) buffers we face a
>>>> NULL buffer, use the buffer (n-1) to capture the output of stream n.
>>>>
>>>> This fixes one piglit test with nvc0:
>>>>      arb_gpu_shader5-xfb-streams-without-invocations
>>>>
>>>> Signed-off-by: Tobias Klausmann <tobias.johannes.klausmann at mni.thm.de>
>>>> ---
>>>>    src/mesa/state_tracker/st_cb_xformfb.c | 5 +++++
>>>>    1 file changed, 5 insertions(+)
>>>>
>>>> diff --git a/src/mesa/state_tracker/st_cb_xformfb.c
>>>> b/src/mesa/state_tracker/st_cb_xformfb.c
>>>> index 8f75eda..5a12da4 100644
>>>> --- a/src/mesa/state_tracker/st_cb_xformfb.c
>>>> +++ b/src/mesa/state_tracker/st_cb_xformfb.c
>>>> @@ -123,6 +123,11 @@ st_begin_transform_feedback(struct gl_context *ctx,
>>>> GLenum mode,
>>>>          struct st_buffer_object *bo =
>>>> st_buffer_object(sobj->base.Buffers[i]);
>>>>
>>>>          if (bo) {
>>>> +         if (!bo->buffer)
>>>> +            /* If we capture transform feedback from n streams into
>>>> (n-1)
>>>> +             * buffers we have to write to buffer (n-1) for stream n.
>>>> +             */
>>>> +            bo = st_buffer_object(sobj->base.Buffers[i-1]);
>>>>             /* Check whether we need to recreate the target. */
>>>>             if (!sobj->targets[i] ||
>>>>                 sobj->targets[i] == sobj->draw_count ||
>>>> --
>>>> 2.2.1
>> Quoted from Ilia Mirkin, to specify what shall be elaborated:
>> "Can you explain (on-list) why using buffer n - 1 is the right thing to
>> do to capture output of stream n? I would have thought that the output
>> for that stream should be discarded or something.
>>
>> Like with a spec quotation or some other justification. i.e. why is
>> the code you wrote correct? Why is it better than, say, bo =
>> buffers[0], or some other thing entirely?"
>>
>> Yeah thats the most concerning point i see as well. The problem is that
>> there is a interaction between arb_gpu_shader5 and arb_transform_feedback3,
>> but after a bit of reading i think the patch is actually what we should do:
>>
>>  From the arb_transfrom_feedback3 spec:
>> "
>> (3) How might you use transform feedback with geometry shaders and
>>          multiple vertex streams?
>>
>>        RESOLVED:  As a simple example, let's say you are processing triangles
>>        and capture both processed triangle vertices and some values that are
>>        computed per-primitive (e.g., facet normal).  The geometry shader
>>        might declare its outputs like the following:
>>
>>          layout(stream = 0) out vec4 position;
>>          layout(stream = 0) out vec4 texcoord;
>>          layout(stream = 1) out vec4 normal;
>>
>>        "position" and "texcoord" would be per-vertex attributes written to
>>        vertex stream 0; "normal" would be a per-triangle facet normal.  The
>>        geometry shader would emit three vertices to stream zero (the
>> processed
>>        input vertices) and a single vertex to stream one (the per-triangle
>>        data).  The transform feedback API usage for this case would be
>>        something like:
>>
>>          // Set up buffer objects 21 and 22 to capture data for per-vertex
>> and
>>          // per primitive values.
>>          glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, 21);
>>          glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 1, 22);
>>
>>          // Set up XFB to capture position and texcoord to buffer binding
>>          // point 0 (buffer 21 bound), and normal to binding point 1 (buffer
>>          // 22 bound).
>>          char *strings[] = { "position", "texcoord", "gl_NextBuffer",
>>                              "normal" };
>> "
>>
>> -> Especially the comments are enlightening as to where the outputs should
>> go. Thats what happens with the
>> "arb_gpu_shader5-xfb-streams-without-invocations" test, where two
>> stream(outputs) are captured into one buffer.
>>
>> One might argue now if we have to count .Buffers[i-1] for all buffers after
>> this...
>>
>> Comments and additional feedback is always appreciated!
> The thing you're quoting is talking about the case where everything's
> supposed to work. I haven't investigated, but I'm guessing that the
> test has a layout(stream=1) but no buffer is bound at index 1.

Actually no, the layout reads like this:
     layout(stream = 0) out float stream0_0_out;
     layout(stream = 1) out vec2 stream1_0_out;
     layout(stream = 2) out float stream2_0_out;
     layout(stream = 2) out vec4 stream2_1_out;
where no buffer is bound to stream2_1_out.

>   Is that right? In that case, I would imagine that the TF output should
> actually just get dropped on the floor. I would assume that this is in
> the ARB_tf3 spec, but I don't have time to go digging right now.

As the layout read more like the one from the specs, i'd still go with 
[n-1] and honestly (not proven correct) i assume that a stream without 
an output is just wrong and should not even get through the linker...

Tobias