gallium: add pipe_screen::num_contexts for skipping mutex locking in util_range

Reviewed-by: Zoltán Böszörményi <zboszor@gmail.com>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9356>

src/gallium/auxiliary/util/u_range.h

@@ -36,6 +36,8 @@
 
 #include "os/os_thread.h"
 #include "pipe/p_state.h"
+#include "pipe/p_screen.h"
+#include "util/u_atomic.h"
 #include "util/u_math.h"
 #include "util/simple_mtx.h"
 
@@ -61,7 +63,8 @@ util_range_add(struct pipe_resource *resource, struct util_range *range,
                unsigned start, unsigned end)
 {
    if (start < range->start || end > range->end) {
-      if (resource->flags & PIPE_RESOURCE_FLAG_SINGLE_THREAD_USE) {
+      if (resource->flags & PIPE_RESOURCE_FLAG_SINGLE_THREAD_USE ||
+          p_atomic_read(&resource->screen->num_contexts) == 1) {
          range->start = MIN2(start, range->start);
          range->end = MAX2(end, range->end);
       } else {

src/gallium/include/pipe/p_screen.h

@@ -68,6 +68,12 @@ struct u_transfer_helper;
  * context.
  */
 struct pipe_screen {
+   /**
+    * Atomically incremented by drivers to track the number of contexts.
+    * If it's 0, it can be assumed that contexts are not tracked.
+    * Used by some places to skip locking if num_contexts == 1.
+    */
+   unsigned num_contexts;
 
    /**
     * For drivers using u_transfer_helper: