pan/{mdg,bi}: Always use sampler 0 for txf

Now that we upload workaround samplers for txf, sampler 0 is guaranteed to be
valid but other samplers are not. So ignore whatever the current sampler_index
value is (it's formally undefined in NIR) and use 0, which we know is valid. We
already do this on Valhall for OpenCL, just need to generalize for Midgard and
Bifrost.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Italo Nicola <italonicola@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22223>

src/panfrost/compiler/bifrost_compile.c

@@ -3534,9 +3534,7 @@ bi_emit_tex_valhall(bi_builder *b, nir_tex_instr *instr)
 
    /* 32-bit indices to be allocated as consecutive staging registers */
    bi_index sregs[VALHALL_TEX_SREG_COUNT] = {};
-
-   bool has_sampler = nir_tex_instr_need_sampler(instr);
-   bi_index sampler = bi_imm_u32(has_sampler ? instr->sampler_index : 0);
+   bi_index sampler = bi_imm_u32(instr->sampler_index);
    bi_index texture = bi_imm_u32(instr->texture_index);
    uint32_t tables = (PAN_TABLE_SAMPLER << 11) | (PAN_TABLE_TEXTURE << 27);
 
@@ -3779,6 +3777,14 @@ bi_is_simple_tex(nir_tex_instr *instr)
 static void
 bi_emit_tex(bi_builder *b, nir_tex_instr *instr)
 {
+   /* If txf is used, we assume there is a valid sampler bound at index 0. Use
+    * it for txf operations, since there may be no other valid samplers. This is
+    * a workaround: txf does not require a sampler in NIR (so sampler_index is
+    * undefined) but we need one in the hardware. This is ABI with the driver.
+    */
+   if (!nir_tex_instr_need_sampler(instr))
+      instr->sampler_index = 0;
+
    if (b->shader->arch >= 9)
       bi_emit_tex_valhall(b, instr);
    else if (bi_is_simple_tex(instr))

src/panfrost/midgard/midgard_compile.c

@@ -2334,6 +2334,14 @@ emit_texop_native(compiler_context *ctx, nir_tex_instr *instr,
    int texture_index = instr->texture_index;
    int sampler_index = instr->sampler_index;
 
+   /* If txf is used, we assume there is a valid sampler bound at index 0. Use
+    * it for txf operations, since there may be no other valid samplers. This is
+    * a workaround: txf does not require a sampler in NIR (so sampler_index is
+    * undefined) but we need one in the hardware. This is ABI with the driver.
+    */
+   if (!nir_tex_instr_need_sampler(instr))
+      sampler_index = 0;
+
    nir_alu_type dest_base = nir_alu_type_get_base_type(instr->dest_type);
 
    /* texture instructions support float outmods */