mesa: (more) correctly handle incomplete depth textures

according to GL spec, incomplete shadow samplers should return 0

this is technically possible for drivers to do using a RGBA texture in
the sense that somehow it's been working, but it's broken at the gallium-level
for what drivers should be expecting to see in such circumstances given
that such scenarios have been binding a RGBA texture to use with shadow samplers

instead, we can give drivers a fallback Z32 texture to avoid format/sampler
mismatches and complying with expected behavior

see also KHR-GL46.incomplete_texture_access.sampler for driver-specific testing

Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20817>

src/mesa/main/mtypes.h

@@ -2413,7 +2413,7 @@ struct gl_shared_state
    struct gl_texture_object *DefaultTex[NUM_TEXTURE_TARGETS];
 
    /** Fallback texture used when a bound texture is incomplete */
-   struct gl_texture_object *FallbackTex[NUM_TEXTURE_TARGETS];
+   struct gl_texture_object *FallbackTex[NUM_TEXTURE_TARGETS][2]; /**< [color, depth] */
 
    /**
     * \name Thread safety and statechange notification for texture

src/mesa/main/shared.c

@@ -335,8 +335,10 @@ free_shared_state(struct gl_context *ctx, struct gl_shared_state *shared)
 
    /* Free the dummy/fallback texture objects */
    for (i = 0; i < NUM_TEXTURE_TARGETS; i++) {
-      if (shared->FallbackTex[i])
+      for (unsigned j = 0; j < ARRAY_SIZE(shared->FallbackTex[0]); j++) {
-         _mesa_delete_texture_object(ctx, shared->FallbackTex[i]);
+         if (shared->FallbackTex[i][j])
+            _mesa_delete_texture_object(ctx, shared->FallbackTex[i][j]);
+      }
    }
 
    /*

src/mesa/main/texobj.c

@@ -974,14 +974,15 @@ _mesa_dirty_texobj(struct gl_context *ctx, struct gl_texture_object *texObj)
 
 /**
  * Return pointer to a default/fallback texture of the given type/target.
- * The texture is an RGBA texture with all texels = (0,0,0,1).
+ * The texture is an RGBA texture with all texels = (0,0,0,1) OR
+ * a depth texture that returns 0.
  * That's the value a GLSL sampler should get when sampling from an
  * incomplete texture.
  */
 struct gl_texture_object *
-_mesa_get_fallback_texture(struct gl_context *ctx, gl_texture_index tex)
+_mesa_get_fallback_texture(struct gl_context *ctx, gl_texture_index tex, bool is_depth)
 {
-   if (!ctx->Shared->FallbackTex[tex]) {
+   if (!ctx->Shared->FallbackTex[tex][is_depth]) {
       /* create fallback texture now */
       const GLsizei width = 1, height = 1;
       GLsizei depth = 1;
@@ -1067,6 +1068,11 @@ _mesa_get_fallback_texture(struct gl_context *ctx, gl_texture_index tex)
       texObj->Sampler.Attrib.state.min_mip_filter = PIPE_TEX_MIPFILTER_NONE;
       texObj->Sampler.Attrib.state.mag_img_filter = PIPE_TEX_FILTER_NEAREST;
 
+      if (is_depth)
+         texFormat = st_ChooseTextureFormat(ctx, target,
+                                            GL_DEPTH_COMPONENT, GL_DEPTH_COMPONENT,
+                                            GL_NONE);
+      else
          texFormat = st_ChooseTextureFormat(ctx, target,
                                             GL_RGBA, GL_RGBA,
                                             GL_UNSIGNED_BYTE);
@@ -1083,8 +1089,13 @@ _mesa_get_fallback_texture(struct gl_context *ctx, gl_texture_index tex)
                                     (dims > 1) ? height : 1,
                                     (dims > 2) ? depth : 1,
                                     0, /* border */
-                                    GL_RGBA, texFormat);
+                                    is_depth ? GL_DEPTH_COMPONENT : GL_RGBA, texFormat);
          _mesa_update_texture_object_swizzle(ctx, texObj);
+         if (is_depth)
+            st_TexImage(ctx, dims, texImage,
+                        GL_DEPTH_COMPONENT, GL_FLOAT, texel,
+                        &ctx->DefaultPacking);
+         else
             st_TexImage(ctx, dims, texImage,
                         GL_RGBA, GL_UNSIGNED_BYTE, texel,
                         &ctx->DefaultPacking);
@@ -1094,13 +1105,13 @@ _mesa_get_fallback_texture(struct gl_context *ctx, gl_texture_index tex)
       assert(texObj->_BaseComplete);
       assert(texObj->_MipmapComplete);
 
-      ctx->Shared->FallbackTex[tex] = texObj;
+      ctx->Shared->FallbackTex[tex][is_depth] = texObj;
 
       /* Complete the driver's operation in case another context will also
        * use the same fallback texture. */
       st_glFinish(ctx);
    }
-   return ctx->Shared->FallbackTex[tex];
+   return ctx->Shared->FallbackTex[tex][is_depth];
 }
 
 

src/mesa/main/texobj.h

@@ -186,7 +186,7 @@ extern void
 _mesa_dirty_texobj(struct gl_context *ctx, struct gl_texture_object *texObj);
 
 extern struct gl_texture_object *
-_mesa_get_fallback_texture(struct gl_context *ctx, gl_texture_index tex);
+_mesa_get_fallback_texture(struct gl_context *ctx, gl_texture_index tex, bool is_depth);
 
 extern GLuint
 _mesa_total_texture_memory(struct gl_context *ctx);

src/mesa/main/texstate.c

@@ -704,7 +704,7 @@ update_single_program_texture(struct gl_context *ctx, struct gl_program *prog,
     * Mesa implements this by creating a hidden texture object with a pixel of
     * that value.
     */
-   texObj = _mesa_get_fallback_texture(ctx, target_index);
+   texObj = _mesa_get_fallback_texture(ctx, target_index, !!(prog->ShadowSamplers & BITFIELD_BIT(unit)));
    assert(texObj);
 
    return texObj;
@@ -870,7 +870,7 @@ fix_missing_textures_for_atifs(struct gl_context *ctx,
 
       if (!ctx->Texture.Unit[unit]._Current) {
          struct gl_texture_object *texObj =
-            _mesa_get_fallback_texture(ctx, target_index);
+            _mesa_get_fallback_texture(ctx, target_index, false);
          _mesa_reference_texobj(&ctx->Texture.Unit[unit]._Current, texObj);
          BITSET_SET(enabled_texture_units, unit);
          ctx->Texture._MaxEnabledTexImageUnit =

src/mesa/state_tracker/st_context.c

@@ -944,12 +944,14 @@ st_destroy_context(struct st_context *st)
     * context.
     */
    for (unsigned i = 0; i < NUM_TEXTURE_TARGETS; i++) {
+      for (unsigned j = 0; j < ARRAY_SIZE(ctx->Shared->FallbackTex[0]); j++) {
          struct gl_texture_object *stObj =
-         ctx->Shared->FallbackTex[i];
+            ctx->Shared->FallbackTex[i][j];
          if (stObj) {
             st_texture_release_context_sampler_view(st, stObj);
          }
       }
+   }
 
    st_release_program(st, &st->fp);
    st_release_program(st, &st->gp);