drm/i915: Remove the vm open count

vms are not getting properly closed. Rather than fixing that,
Remove the vm open count and instead rely on the vm refcount.

The vm open count existed solely to break the strong references the
vmas had on the vms. Now instead make those references weak and
ensure vmas are destroyed when the vm is destroyed.

Unfortunately if the vm destructor and the object destructor both
wants to destroy a vma, that may lead to a race in that the vm
destructor just unbinds the vma and leaves the actual vma destruction
to the object destructor. However in order for the object destructor
to ensure the vma is unbound it needs to grab the vm mutex. In order
to keep the vm mutex alive until the object destructor is done with
it, somewhat hackishly grab a vm_resv refcount that is released late
in the vma destruction process, when the vm mutex is no longer needed.

v2: Address review-comments from Niranjana
- Clarify that the struct i915_address_space::skip_pte_rewrite is a hack
  and should ideally be replaced in an upcoming patch.
- Remove an unneeded continue in clear_vm_list and update comment.

v3:
- Documentation update
- Commit message formatting

Co-developed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220304082641.308069-2-thomas.hellstrom@linux.intel.com

1 files changed, 13 insertions, 17 deletions

diff --git a/drivers/gpu/drm/i915/gt/intel_ggtt.c b/drivers/gpu/drm/i915/gt/intel_ggtt.c
index 8850d4e0f9cc..04191fe2ee34 100644
--- a/drivers/gpu/drm/i915/gt/intel_ggtt.c
+++ b/drivers/gpu/drm/i915/gt/intel_ggtt.c
@@ -126,7 +126,7 @@ static bool needs_idle_maps(struct drm_i915_private *i915)
 void i915_ggtt_suspend_vm(struct i915_address_space *vm)
 {
 	struct i915_vma *vma, *vn;
-	int open;
+	int save_skip_rewrite;
 
 	drm_WARN_ON(&vm->i915->drm, !vm->is_ggtt && !vm->is_dpt);
 
@@ -135,8 +135,12 @@ retry:
 
 	mutex_lock(&vm->mutex);
 
-	/* Skip rewriting PTE on VMA unbind. */
-	open = atomic_xchg(&vm->open, 0);
+	/*
+	 * Skip rewriting PTE on VMA unbind.
+	 * FIXME: Use an argument to i915_vma_unbind() instead?
+	 */
+	save_skip_rewrite = vm->skip_pte_rewrite;
+	vm->skip_pte_rewrite = true;
 
 	list_for_each_entry_safe(vma, vn, &vm->bound_list, vm_link) {
 		struct drm_i915_gem_object *obj = vma->obj;
@@ -154,16 +158,14 @@ retry:
 			 */
 			i915_gem_object_get(obj);
 
-			atomic_set(&vm->open, open);
 			mutex_unlock(&vm->mutex);
 
 			i915_gem_object_lock(obj, NULL);
-			open = i915_vma_unbind(vma);
+			GEM_WARN_ON(i915_vma_unbind(vma));
 			i915_gem_object_unlock(obj);
-
-			GEM_WARN_ON(open);
-
 			i915_gem_object_put(obj);
+
+			vm->skip_pte_rewrite = save_skip_rewrite;
 			goto retry;
 		}
 
@@ -179,7 +181,7 @@ retry:
 
 	vm->clear_range(vm, 0, vm->total);
 
-	atomic_set(&vm->open, open);
+	vm->skip_pte_rewrite = save_skip_rewrite;
 
 	mutex_unlock(&vm->mutex);
 }
@@ -773,13 +775,13 @@ static void ggtt_cleanup_hw(struct i915_ggtt *ggtt)
 {
 	struct i915_vma *vma, *vn;
 
-	atomic_set(&ggtt->vm.open, 0);
-
 	flush_workqueue(ggtt->vm.i915->wq);
 	i915_gem_drain_freed_objects(ggtt->vm.i915);
 
 	mutex_lock(&ggtt->vm.mutex);
 
+	ggtt->vm.skip_pte_rewrite = true;
+
 	list_for_each_entry_safe(vma, vn, &ggtt->vm.bound_list, vm_link) {
 		struct drm_i915_gem_object *obj = vma->obj;
 		bool trylock;
@@ -1307,16 +1309,12 @@ bool i915_ggtt_resume_vm(struct i915_address_space *vm)
 {
 	struct i915_vma *vma;
 	bool write_domain_objs = false;
-	int open;
 
 	drm_WARN_ON(&vm->i915->drm, !vm->is_ggtt && !vm->is_dpt);
 
 	/* First fill our portion of the GTT with scratch pages */
 	vm->clear_range(vm, 0, vm->total);
 
-	/* Skip rewriting PTE on VMA unbind. */
-	open = atomic_xchg(&vm->open, 0);
-
 	/* clflush objects bound into the GGTT and rebind them. */
 	list_for_each_entry(vma, &vm->bound_list, vm_link) {
 		struct drm_i915_gem_object *obj = vma->obj;
@@ -1333,8 +1331,6 @@ bool i915_ggtt_resume_vm(struct i915_address_space *vm)
 		}
 	}
 
-	atomic_set(&vm->open, open);
-
 	return write_domain_objs;
 }