asahi,agx: Fix txf sampler

Bizarrely, the clamps/wrap modes are respected so we need to set them
appropriately for correct out-of-bounds behaviour (returning all zero). That in
turn means we can't use whatever sampler is already there, instead we need to
allocate a dedicated sampler just for txf. Good news is we have an extra sampler
state register available for the purpose.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24258>

src/asahi/compiler/agx_compile.c

@@ -774,6 +774,23 @@ agx_translate_bindless_handle(agx_builder *b, nir_src *handle, agx_index *base)
    return agx_emit_extract(b, agx_src_index(handle), 1);
 }
 
+/*
+ * Contrary to NIR, in the hardware txf requires a special sampler. The sampler
+ * cannot be arbitrary, since the hardware honours the clamps so particular
+ * configuration is required for correct out-of-bounds behaviour for txf. This
+ * helper gets the shader's txf sampler, allocating one if needed.
+ */
+static agx_index
+agx_txf_sampler(agx_context *ctx)
+{
+   if (!ctx->out->uses_txf) {
+      ctx->out->txf_sampler = BITSET_LAST_BIT(ctx->nir->info.samplers_used);
+      ctx->out->uses_txf = true;
+   }
+
+   return agx_immediate(ctx->out->txf_sampler);
+}
+
 static agx_instr *
 agx_emit_image_store(agx_builder *b, nir_intrinsic_instr *instr)
 {
@@ -1432,11 +1449,8 @@ agx_emit_tex(agx_builder *b, nir_tex_instr *instr)
 
    bool txf = (instr->op == nir_texop_txf || instr->op == nir_texop_txf_ms);
 
-   /* txf loads a texture without an associated sampler, but in the hardware
-    * there is an associated load of a sampler. This requires that the driver
-    * upload a dummy sampler.
-    */
-   b->shader->out->needs_dummy_sampler |= txf;
+   if (txf)
+      sampler = agx_txf_sampler(b->shader);
 
    for (unsigned i = 0; i < instr->num_srcs; ++i) {
       agx_index index = agx_src_index(&instr->src[i].src);

src/asahi/compiler/agx_compile.h

@@ -125,8 +125,9 @@ struct agx_shader_info {
    /* Shader is incompatible with triangle merging */
    bool disable_tri_merging;
 
-   /* Shader needs a dummy sampler (for txf reads) */
-   bool needs_dummy_sampler;
+   /* Shader uses txf, requiring a workaround sampler in the given location */
+   bool uses_txf;
+   unsigned txf_sampler;
 
    /* Number of bindful textures, images used */
    unsigned nr_bindful_textures, nr_bindful_images;

src/asahi/lib/agx_helpers.h

@@ -12,7 +12,7 @@
 static inline enum agx_sampler_states
 agx_translate_sampler_state_count(unsigned count, bool extended)
 {
-   assert(count <= 16 && "max 16 sampler state registers supported");
+   assert(count <= 17 && "max 17 sampler state registers supported");
 
    if (count == 0) {
       return AGX_SAMPLER_STATES_0;

src/gallium/drivers/asahi/agx_state.c

@@ -1973,12 +1973,12 @@ static unsigned
 sampler_count(struct agx_context *ctx, struct agx_compiled_shader *cs,
               enum pipe_shader_type stage)
 {
-   unsigned nr_samplers = ctx->stage[stage].sampler_count;
+   unsigned sampler_count = ctx->stage[stage].sampler_count;
 
-   if (cs->info.needs_dummy_sampler)
-      nr_samplers = MAX2(nr_samplers, 1);
+   if (cs->info.uses_txf)
+      sampler_count = MAX2(sampler_count, cs->info.txf_sampler + 1);
 
-   return nr_samplers;
+   return sampler_count;
 }
 
 static inline enum agx_sampler_states
@@ -2107,31 +2107,35 @@ agx_build_pipeline(struct agx_batch *batch, struct agx_compiled_shader *cs,
 
    /* TODO: Dirty track me to save some CPU cycles and maybe improve caching */
    uint8_t *out_sampler = T_samp.cpu;
-   if (nr_samplers && ctx->stage[stage].sampler_count == 0) {
-      /* Configuration is irrelevant for the dummy sampler */
-      agx_pack(out_sampler, SAMPLER, cfg)
-         ;
-   } else {
-      for (unsigned i = 0; i < nr_samplers; ++i) {
-         struct agx_sampler_state *sampler = ctx->stage[stage].samplers[i];
-         struct agx_sampler_packed *out =
-            (struct agx_sampler_packed *)out_sampler;
+   for (unsigned i = 0; i < nr_samplers; ++i) {
+      struct agx_sampler_state *sampler = ctx->stage[stage].samplers[i];
+      struct agx_sampler_packed *out = (struct agx_sampler_packed *)out_sampler;
 
-         if (sampler) {
-            *out = sampler->desc;
+      if (cs->info.uses_txf && i == cs->info.txf_sampler) {
+         agx_pack(out, SAMPLER, cfg) {
+            /* Allow mipmapping. This is respected by txf, weirdly. */
+            cfg.mip_filter = AGX_MIP_FILTER_NEAREST;
 
-            if (custom_borders) {
-               memcpy(out_sampler + AGX_SAMPLER_LENGTH, &sampler->border,
-                      AGX_BORDER_LENGTH);
-            } else {
-               assert(!sampler->uses_custom_border && "invalid combination");
-            }
-         } else {
-            memset(out, 0, sampler_length);
+            /* Out-of-bounds reads must return 0 */
+            cfg.wrap_s = AGX_WRAP_CLAMP_TO_BORDER;
+            cfg.wrap_t = AGX_WRAP_CLAMP_TO_BORDER;
+            cfg.wrap_r = AGX_WRAP_CLAMP_TO_BORDER;
+            cfg.border_colour = AGX_BORDER_COLOUR_TRANSPARENT_BLACK;
          }
+      } else if (sampler) {
+         *out = sampler->desc;
 
-         out_sampler += sampler_length;
+         if (custom_borders) {
+            memcpy(out_sampler + AGX_SAMPLER_LENGTH, &sampler->border,
+                   AGX_BORDER_LENGTH);
+         } else {
+            assert(!sampler->uses_custom_border && "invalid combination");
+         }
+      } else {
+         memset(out, 0, sampler_length);
       }
+
+      out_sampler += sampler_length;
    }
 
    struct agx_usc_builder b =