util: add u_tristate data structure

rip the pan_tristate enum from panfrost and move it to common.

I've often wanted some sort of "yes/no/maybe" or "true/false/unknown" or
"always/never/sometimes" data structure. This adds a common one that is
hopefully neutral enough to cover all of the above.

Asahi will use this.

The Intel drivers could be ported to this (brw_sometimes/elk_sometimes), it
should be straightforward but I don't want to do that without being able to
easily test those changes.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32224>

src/util/meson.build

@@ -153,6 +153,7 @@ files_mesa_util = files(
   'u_cpu_detect.h',
   'u_printf.c',
   'u_printf.h',
+  'u_tristate.h',
   'u_worklist.c',
   'u_worklist.h',
   'vl_vlc.h',

src/util/u_tristate.h

@@ -0,0 +1,72 @@
+/*
+ * Copyright 2024 Valve Corporation
+ * Copyright 2022 Collabora Ltd
+ * SPDX-License-Identifier: MIT
+ */
+
+#include "util/macros.h"
+#include <stdbool.h>
+
+#ifndef U_TRISTATE_H
+#define U_TRISTATE_H
+
+/*
+ * Simple tri-state data structure.
+ *
+ * The tri-state can be set to a boolean or unset. The semantics of "unset"
+ * depend on the application, it could be either "don't care" or "maybe".
+ */
+enum u_tristate {
+   U_TRISTATE_UNSET,
+   U_TRISTATE_NO,
+   U_TRISTATE_YES,
+};
+
+/*
+ * Construct a tristate from an immediate value.
+ */
+static inline enum u_tristate
+u_tristate_make(bool value)
+{
+   return value ? U_TRISTATE_YES : U_TRISTATE_NO;
+}
+
+/*
+ * Try to set a tristate value to a specific boolean value, returning whether
+ * the operation is successful.
+ */
+static inline bool
+u_tristate_set(enum u_tristate *state, bool value)
+{
+   switch (*state) {
+   case U_TRISTATE_UNSET:
+      *state = u_tristate_make(value);
+      return true;
+
+   case U_TRISTATE_NO:
+      return (value == false);
+
+   case U_TRISTATE_YES:
+      return (value == true);
+
+   default:
+      unreachable("Invalid tristate value");
+   }
+}
+
+/*
+ * Invert a tristate, returning the new value.
+ */
+static inline enum u_tristate
+u_tristate_invert(enum u_tristate tri)
+{
+   switch (tri) {
+   case U_TRISTATE_UNSET: return U_TRISTATE_UNSET;
+   case U_TRISTATE_YES: return U_TRISTATE_NO;
+   case U_TRISTATE_NO: return U_TRISTATE_YES;
+   }
+
+   unreachable("invalid tristate");
+}
+
+#endif