gallium: add PIPE_CAP_NULL_TEXTURES

this allows drivers to indicate that they support sampling from null
textures instead of using fallback textures

for now, this is only used for depth-based fallback textures

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21072>

docs/gallium/screen.rst

@@ -640,6 +640,7 @@ The integer capabilities:
 * ``PIPE_CAP_DEVICE_PROTECTED_CONTEXT``: Whether the device supports protected / encrypted context which can manipulate protected / encrypted content (some devices might need protected contexts to access protected content, whereas ``PIPE_CAP_DEVICE_PROTECTED_SURFACE`` does not require any particular context to do so).
 * ``PIPE_CAP_ALLOW_GLTHREAD_BUFFER_SUBDATA_OPT``: Whether to allow glthread to convert glBufferSubData to glCopyBufferSubData. This may improve or worsen performance depending on your driver.
 * ``PIPE_CAP_VALIDATE_ALL_DIRTY_STATES`` : Whether state validation must also validate the state changes for resources types used in the previous shader but not in the current shader.
+* ``PIPE_CAP_NULL_TEXTURES`` : Whether the driver supports sampling from NULL textures.
 
 .. _pipe_capf:
 

src/gallium/auxiliary/util/u_screen.c

@@ -524,6 +524,7 @@ u_pipe_screen_get_param_defaults(struct pipe_screen *pscreen,
       return 64;
 
    case PIPE_CAP_VALIDATE_ALL_DIRTY_STATES:
+   case PIPE_CAP_NULL_TEXTURES:
       return 0;
 
    default:

src/gallium/include/pipe/p_defines.h

@@ -1015,6 +1015,7 @@ enum pipe_cap
    /** For EGL_EXT_protected_content */
    PIPE_CAP_DEVICE_PROTECTED_CONTEXT,
    PIPE_CAP_ALLOW_GLTHREAD_BUFFER_SUBDATA_OPT,
+   PIPE_CAP_NULL_TEXTURES,
 
    PIPE_CAP_VALIDATE_ALL_DIRTY_STATES,
    PIPE_CAP_LAST,

src/mesa/main/mtypes.h

@@ -946,6 +946,8 @@ struct gl_texture_object
    GLboolean External;
    GLubyte RequiredTextureImageUnits;
 
+   GLboolean NullTexture; /**< this texture is incomplete and should be passed to the driver as NULL */
+
    /** GL_EXT_memory_object */
    GLenum16 TextureTiling;
 

src/mesa/main/texobj.c

@@ -1091,14 +1091,18 @@ _mesa_get_fallback_texture(struct gl_context *ctx, gl_texture_index tex, bool is
                                     0, /* border */
                                     is_depth ? GL_DEPTH_COMPONENT : GL_RGBA, texFormat);
          _mesa_update_texture_object_swizzle(ctx, texObj);
-         if (is_depth)
+         if (ctx->st->can_null_texture && is_depth) {
-            st_TexImage(ctx, dims, texImage,
+            texObj->NullTexture = GL_TRUE;
-                        GL_DEPTH_COMPONENT, GL_FLOAT, texel,
+         } else {
-                        &ctx->DefaultPacking);
+            if (is_depth)
-         else
+               st_TexImage(ctx, dims, texImage,
-            st_TexImage(ctx, dims, texImage,
+                           GL_DEPTH_COMPONENT, GL_FLOAT, texel,
-                        GL_RGBA, GL_UNSIGNED_BYTE, texel,
+                           &ctx->DefaultPacking);
-                        &ctx->DefaultPacking);
+            else
+               st_TexImage(ctx, dims, texImage,
+                           GL_RGBA, GL_UNSIGNED_BYTE, texel,
+                           &ctx->DefaultPacking);
+         }
       }
 
       _mesa_test_texobj_completeness(ctx, texObj);
@@ -1109,7 +1113,8 @@ _mesa_get_fallback_texture(struct gl_context *ctx, gl_texture_index tex, bool is
 
       /* Complete the driver's operation in case another context will also
        * use the same fallback texture. */
-      st_glFinish(ctx);
+      if (!ctx->st->can_null_texture || !is_depth)
+         st_glFinish(ctx);
    }
    return ctx->Shared->FallbackTex[tex][is_depth];
 }

src/mesa/state_tracker/st_cb_texture.c

@@ -3085,7 +3085,7 @@ st_finalize_texture(struct gl_context *ctx,
 
    /* May need to create a new gallium texture:
     */
-   if (!tObj->pt) {
+   if (!tObj->pt && !tObj->NullTexture) {
       GLuint bindings = default_bindings(st, firstImageFormat);
 
       tObj->pt = st_texture_create(st,
@@ -3115,7 +3115,7 @@ st_finalize_texture(struct gl_context *ctx,
 
          /* Need to import images in main memory or held in other textures.
           */
-         if (stImage && tObj->pt != stImage->pt) {
+         if (stImage && !tObj->NullTexture && tObj->pt != stImage->pt) {
             GLuint height;
             GLuint depth;
 

src/mesa/state_tracker/st_context.c

@@ -642,6 +642,9 @@ st_create_context_priv(struct gl_context *ctx, struct pipe_context *pipe,
    st->validate_all_dirty_states =
       screen->get_param(screen, PIPE_CAP_VALIDATE_ALL_DIRTY_STATES)
       ? true : false;
+   st->can_null_texture =
+      screen->get_param(screen, PIPE_CAP_NULL_TEXTURES)
+      ? true : false;
 
    util_throttle_init(&st->throttle,
                       screen->get_param(screen,

src/mesa/state_tracker/st_context.h

@@ -207,6 +207,7 @@ struct st_context
    boolean has_hw_atomics;
 
    boolean validate_all_dirty_states;
+   boolean can_null_texture;
 
    /* driver supports scissored clears */
    boolean can_scissor_clear;