[Mesa-dev] Lowering viewport transformation in NIR

[Rob Clark]

On Thu, May 3, 2018 at 3:12 PM, Alyssa Rosenzweig <alyssa at rosenzweig.io> wrote:
> Hi all,
>
> Certain embedded GPUs do not implement coordinate transformation in
> hardware. Instead, section 12.5 "Coordinate Transformation" of the ES
> 3.2 specification is implemented in the vertex shader itself. Relevant
> examples include Midgard and vc4.
>
> To handle this, a lowering pass is needed to convert gl_Position writes
> to screen space writes. The vc4 driver lowers this in the backend IR;
> however, I don't think the pass needs to be specialised to the backend.
> For Midgard, I have written a NIR lowering pass to implement the same,
> which enables the lowered instructions themselves to be optimised.
>
> At the moment, this pass lives inside the (downstream) Midgard compiler.
> In the future, it will be necessary for the Bifrost compiler as well,
> should that use NIR. That said, Bifrost will share the same Gallium
> driver, so the pass could still live in the driver
> (src/gallium/drivers/panfrost).
>
> Should this pass be moved into common code (src/compiler/nir)? If so,
> what would the driver agnostic way of passing viewport parameters be?
> Both vc4 and Midgard currently use/will use special uniforms for this
> purpose. Similarly, is there a driver agnostic way of representing the
> transformed write? The Midgard pass emits `nir_instrinic_store_output`
> for the final value, but I'm not sure if this is generalisable.
>

maybe look at the user_clip_plane lowering as an example, ie.
nir_intrinsic_load_user_clip_plane

there are a few other examples of sysval's that lower to driver_params
(ie. things driver stuffs in special uniforms, basically)

BR,
-R


> I can send the Midgard code as an RFC if there's interest.
>
> Thanks,
>
> -Alyssa
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