[Mesa-dev] Question about implementing viewport transfer and const load in nir

[Qiang Yu]

Hi guys,

When working on lima gp compiler, I come across two problems about
inserting extra uniform
and instructions in nir for the driver and don't know where's the
right place to do it. So I'd like
to hear your opinion and if there's other driver already did so.

1. viewport transfer
lima gp needs embed viewport transfer into vertex shader, so need to
insert a uniform which
holds the scale and transfer and some instruction to apply the
calculation to the gl_Position
varying output. If do this in driver callback create_vs_state(), seems
won't affect the state
tracker to allocate uniform space for it, maybe add something in
st_create_vp_variant()?

2. const load
lima gp needs const be loaded just as uniform, so I have to allocate
uniform space for them.
Besides the same problem as 1 (where to do it), the const node may be
eliminated after driver
nir optimization and won't have a base filed as uniform.

Seems some of these (space allocation) need be done in non-driver
layer and some (instruction
insertion and uniform base assign to const) can be done in driver.

BTW. lima gp can only have one uniform buffer, so I can't just use a
dedicated uniform buffer
for viewport transfer and const uniform.

Thanks,
Qiang