[Mesa-dev] [RFC] gallium: add interface for writable shader images

[Ilia Mirkin]

On Thu, Jul 9, 2015 at 5:05 PM, Marek Olšák <maraeo at gmail.com> wrote:
> I'd like to discuss one more thing that will affect whether image
> slots will be global (shared by all shaders) or not.
>
> Which image unit an image uniform uses is not a compile-time thing,
> but it's specified later using glUniform1i. That means we need a
> per-shader table that maps image uniforms to global image units. One
> possible solution is to add this pipe_context function:
>
> void (*set_shader_image_mapping)(
>    struct pipe_context *, unsigned shader,
>    unsigned start_decl_index, unsigned count,
>    unsigned *decl_to_slot_mapping);
>
> This is only required if the shader image slots are global and not
> per-shader. (if they are per-shader, st/mesa can reorder the slots for
> each shader independently just like it already does for textures and
> UBOs) Shader storage buffer objects suffer from the same issue. Atomic
> counters don't.
>
> Therefore, image slots must be per-shader (like sampler slots) to
> avoid this craziness and keep things simple.

I think that's reasonable. The image slots can be fixed at
compile-time (like sampler slots), and we can just stick the right
images into the right places. But in order to do that, we need the
per-stage mapping, so making it like sampler views seems reasonable.

  -ilia