drm/i915/gem: Clean-up outdated struct_mutex comments

The struct_mutex will be removed from the DRM subsystem, as it was a
legacy BKL that was only used by i915 driver. After review, it was
concluded that its usage was no longer necessary

This patch updates various comments in the i915/gem and i915/gt
codebase to either remove or clarify references to struct_mutex, in
order to prevent future misunderstandings.

* i915_gem_execbuffer.c: Replace reference to struct_mutex with
  vm->mutex, as noted in the eb_reserve() function, which states that
  vm->mutex handles deadlocks.
* i915_gem_object.c: Change struct_mutex by
  drm_i915_gem_object->vma.lock. i915_gem_object_unbind() in i915_gem.c
  states that this lock is who actually protects the unbind.
* i915_gem_shrinker.c: The correct lock is actually i915->mm.obj, as
  already documented in its declaration.
* i915_gem_wait.c: The existing comment already mentioned that
  struct_mutex was no longer necessary. Updated to refer to a generic
  global lock instead.
* intel_reset_types.h: Cleaned up the comment text. Updated to refer to
  a generic global lock instead.

Signed-off-by: Luiz Otavio Mello <luiz.mello@estudante.ufscar.br>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20250908131518.36625-6-luiz.mello@estudante.ufscar.br
Acked-by: Tvrtko Ursulin <tursulin@ursulin.net>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c

@@ -182,7 +182,7 @@ enum {
  * the object. Simple! ... The relocation entries are stored in user memory
  * and so to access them we have to copy them into a local buffer. That copy
  * has to avoid taking any pagefaults as they may lead back to a GEM object
- * requiring the struct_mutex (i.e. recursive deadlock). So once again we split
+ * requiring the vm->mutex (i.e. recursive deadlock). So once again we split
  * the relocation into multiple passes. First we try to do everything within an
  * atomic context (avoid the pagefaults) which requires that we never wait. If
  * we detect that we may wait, or if we need to fault, then we have to fallback

drivers/gpu/drm/i915/gem/i915_gem_object.c

@@ -459,8 +459,8 @@ static void i915_gem_free_object(struct drm_gem_object *gem_obj)
 	atomic_inc(&i915->mm.free_count);
 
 	/*
-	 * Since we require blocking on struct_mutex to unbind the freed
-	 * object from the GPU before releasing resources back to the
+	 * Since we require blocking on drm_i915_gem_object->vma.lock to unbind
+	 * the freed object from the GPU before releasing resources back to the
 	 * system, we can not do that directly from the RCU callback (which may
 	 * be a softirq context), but must instead then defer that work onto a
 	 * kthread. We use the RCU callback rather than move the freed object

drivers/gpu/drm/i915/gem/i915_gem_shrinker.c

@@ -170,7 +170,7 @@ i915_gem_shrink(struct i915_gem_ww_ctx *ww,
 	 * Also note that although these lists do not hold a reference to
 	 * the object we can safely grab one here: The final object
 	 * unreferencing and the bound_list are both protected by the
-	 * dev->struct_mutex and so we won't ever be able to observe an
+	 * i915->mm.obj_lock and so we won't ever be able to observe an
 	 * object on the bound_list with a reference count equals 0.
 	 */
 	for (phase = phases; phase->list; phase++) {
@@ -185,7 +185,7 @@ i915_gem_shrink(struct i915_gem_ww_ctx *ww,
 
 		/*
 		 * We serialize our access to unreferenced objects through
-		 * the use of the struct_mutex. While the objects are not
+		 * the use of the obj_lock. While the objects are not
 		 * yet freed (due to RCU then a workqueue) we still want
 		 * to be able to shrink their pages, so they remain on
 		 * the unbound/bound list until actually freed.

drivers/gpu/drm/i915/gem/i915_gem_wait.c

@@ -222,10 +222,10 @@ static unsigned long to_wait_timeout(s64 timeout_ns)
  *
  * The wait ioctl with a timeout of 0 reimplements the busy ioctl. With any
  * non-zero timeout parameter the wait ioctl will wait for the given number of
- * nanoseconds on an object becoming unbusy. Since the wait itself does so
- * without holding struct_mutex the object may become re-busied before this
- * function completes. A similar but shorter * race condition exists in the busy
- * ioctl
+ * nanoseconds on an object becoming unbusy. Since the wait occurs without
+ * holding a global or exclusive lock the object may become re-busied before
+ * this function completes. A similar but shorter * race condition exists
+ * in the busy ioctl
  */
 int
 i915_gem_wait_ioctl(struct drm_device *dev, void *data, struct drm_file *file)

drivers/gpu/drm/i915/gt/intel_reset_types.h

@@ -20,7 +20,7 @@ struct intel_reset {
 	 * FENCE registers).
 	 *
 	 * #I915_RESET_ENGINE[num_engines] - Since the driver doesn't need to
-	 * acquire the struct_mutex to reset an engine, we need an explicit
+	 * acquire a global lock to reset an engine, we need an explicit
 	 * flag to prevent two concurrent reset attempts in the same engine.
 	 * As the number of engines continues to grow, allocate the flags from
 	 * the most significant bits.