asahi: plumb tri fan flatshading through common

not yet used in the GL driver but we should probably fix that.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29179>

src/asahi/compiler/agx_compile.c

@@ -3335,9 +3335,6 @@ agx_compile_shader_nir(nir_shader *nir, struct agx_shader_key *key,
 
    bool needs_libagx = true /* TODO: Optimize */;
 
-   if (nir->info.stage == MESA_SHADER_FRAGMENT)
-      NIR_PASS(_, nir, agx_nir_lower_interpolation);
-
    NIR_PASS(_, nir, nir_lower_vars_to_ssa);
 
    if (needs_libagx) {

src/asahi/compiler/agx_compile.h

@@ -234,6 +234,7 @@ void agx_link_libagx(nir_shader *nir, const nir_shader *libagx);
 void agx_preprocess_nir(nir_shader *nir, const nir_shader *libagx);
 bool agx_nir_lower_discard_zs_emit(nir_shader *s);
 bool agx_nir_lower_sample_mask(nir_shader *s);
+bool agx_nir_lower_interpolation(nir_shader *s);
 
 bool agx_nir_lower_cull_distance_fs(struct nir_shader *s,
                                     unsigned nr_distances);

src/asahi/compiler/agx_nir.h

@@ -9,7 +9,6 @@
 
 struct nir_shader;
 
-bool agx_nir_lower_interpolation(struct nir_shader *s);
 bool agx_nir_lower_algebraic_late(struct nir_shader *shader);
 bool agx_nir_fuse_selects(struct nir_shader *shader);
 bool agx_nir_fuse_algebraic_late(struct nir_shader *shader);

src/asahi/compiler/agx_nir_lower_interpolation.c

@@ -4,7 +4,7 @@
  */
 
 #include "compiler/shader_enums.h"
-#include "agx_nir.h"
+#include "agx_compile.h"
 #include "nir.h"
 #include "nir_builder.h"
 #include "nir_builder_opcodes.h"
@@ -82,8 +82,14 @@ interpolate_at_offset(nir_builder *b, nir_def *cf, nir_def *offset,
 static nir_def *
 interpolate_flat(nir_builder *b, nir_def *coefficients)
 {
-   /* Same value anywhere, so just take the constant (affine) component */
-   return nir_channel(b, coefficients, 2);
+   /* Same value anywhere, so just take the constant (affine) component. For
+    * triangle fans with the first provoking vertex, the CF layout is slightly
+    * different. I am unsure why, but Apple does the same and the bcsel is
+    * required for corrctness.
+    */
+   return nir_bcsel(b, nir_load_is_first_fan_agx(b),
+                    nir_channel(b, coefficients, 1),
+                    nir_channel(b, coefficients, 2));
 }
 
 static enum glsl_interp_mode

src/gallium/drivers/asahi/agx_nir_lower_sysvals.c

@@ -198,6 +198,9 @@ lower_intrinsic(nir_builder *b, nir_intrinsic_instr *intr,
    case nir_intrinsic_load_uvs_index_agx:
       return load_sysval_root(
          b, 1, 16, &u->uvs_index[nir_intrinsic_io_semantics(intr).location]);
+   case nir_intrinsic_load_is_first_fan_agx:
+      /* TODO: Plumb this so we can stop using geometry shaders for this case */
+      return nir_imm_false(b);
    default:
       break;
    }

src/gallium/drivers/asahi/agx_state.c

@@ -1564,6 +1564,10 @@ agx_compile_nir(struct agx_device *dev, nir_shader *nir,
       .secondary = secondary,
    };
 
+   if (nir->info.stage == MESA_SHADER_FRAGMENT) {
+      NIR_PASS(_, nir, agx_nir_lower_interpolation);
+   }
+
    /* We always use dynamic sample shading in the GL driver. Indicate that. */
    if (nir->info.stage == MESA_SHADER_FRAGMENT &&
        nir->info.fs.uses_sample_shading)
@@ -4190,21 +4194,12 @@ agx_needs_passthrough_gs(struct agx_context *ctx,
       return true;
    }
 
-   /* Experimentally, G13 does not seem to pick the right provoking vertex for
-    * triangle fans with first provoking. Inserting a GS for this case lets us
-    * use our (correct) shader-based input assembly, translating to
-    * appropriately oriented triangles and working around the hardware issue.
-    * This warrants more investigation in case we're just misconfiguring the
-    * hardware, but as tri fans are absent in Metal and GL defaults to last
-    * vertex, this is a plausible part of the hardware to be broken (or absent).
-    *
-    * Affects piglit clipflat.
-    */
+   /* TODO: Handle fans properly, we need to plumb a sysval. */
    if (info->mode == MESA_PRIM_TRIANGLE_FAN &&
        ctx->rast->base.flatshade_first &&
        ctx->stage[MESA_SHADER_FRAGMENT].shader->info.inputs_flat_shaded) {
 
-      perf_debug_ctx(ctx, "Using passthrough GS due to tri fan bug");
+      perf_debug_ctx(ctx, "Using passthrough GS due to first tri fans");
       return true;
    }