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

[Ilia Mirkin]

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. 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.

  -ilia