diff mbox series

[v5,02/40] drm/gpuvm: Allow VAs to hold soft reference to BOs

Message ID 20250519175348.11924-3-robdclark@gmail.com
State New
Headers show
Series drm/msm: sparse / "VM_BIND" support | expand

Commit Message

Rob Clark May 19, 2025, 5:51 p.m. UTC 
From: Rob Clark <robdclark@chromium.org>

Eases migration for drivers where VAs don't hold hard references to
their associated BO, avoiding reference loops.

In particular, msm uses soft references to optimistically keep around
mappings until the BO is distroyed.  Which obviously won't work if the
VA (the mapping) is holding a reference to the BO.

By making this a per-VM flag, we can use normal hard-references for
mappings in a "VM_BIND" managed VM, but soft references in other cases,
such as kernel-internal VMs (for display scanout, etc).

Cc: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>
---
 drivers/gpu/drm/drm_gpuvm.c | 37 ++++++++++++++++++++++++++++++++-----
 include/drm/drm_gpuvm.h     | 19 +++++++++++++++++--
 2 files changed, 49 insertions(+), 7 deletions(-)
diff mbox series

Patch

diff --git a/drivers/gpu/drm/drm_gpuvm.c b/drivers/gpu/drm/drm_gpuvm.c
index 1e89a98caad4..892b62130ff8 100644
--- a/drivers/gpu/drm/drm_gpuvm.c
+++ b/drivers/gpu/drm/drm_gpuvm.c
@@ -1125,6 +1125,8 @@  __drm_gpuvm_prepare_objects(struct drm_gpuvm *gpuvm,
 	LIST_HEAD(extobjs);
 	int ret = 0;
 
+	WARN_ON(gpuvm->flags & DRM_GPUVM_VA_WEAK_REF);
+
 	for_each_vm_bo_in_list(gpuvm, extobj, &extobjs, vm_bo) {
 		ret = exec_prepare_obj(exec, vm_bo->obj, num_fences);
 		if (ret)
@@ -1145,6 +1147,8 @@  drm_gpuvm_prepare_objects_locked(struct drm_gpuvm *gpuvm,
 	struct drm_gpuvm_bo *vm_bo;
 	int ret = 0;
 
+	WARN_ON(gpuvm->flags & DRM_GPUVM_VA_WEAK_REF);
+
 	drm_gpuvm_resv_assert_held(gpuvm);
 	list_for_each_entry(vm_bo, &gpuvm->extobj.list, list.entry.extobj) {
 		ret = exec_prepare_obj(exec, vm_bo->obj, num_fences);
@@ -1386,6 +1390,7 @@  drm_gpuvm_validate_locked(struct drm_gpuvm *gpuvm, struct drm_exec *exec)
 	struct drm_gpuvm_bo *vm_bo, *next;
 	int ret = 0;
 
+	WARN_ON(gpuvm->flags & DRM_GPUVM_VA_WEAK_REF);
 	drm_gpuvm_resv_assert_held(gpuvm);
 
 	list_for_each_entry_safe(vm_bo, next, &gpuvm->evict.list,
@@ -1482,7 +1487,9 @@  drm_gpuvm_bo_create(struct drm_gpuvm *gpuvm,
 
 	vm_bo->vm = drm_gpuvm_get(gpuvm);
 	vm_bo->obj = obj;
-	drm_gem_object_get(obj);
+
+	if (!(gpuvm->flags & DRM_GPUVM_VA_WEAK_REF))
+		drm_gem_object_get(obj);
 
 	kref_init(&vm_bo->kref);
 	INIT_LIST_HEAD(&vm_bo->list.gpuva);
@@ -1504,16 +1511,22 @@  drm_gpuvm_bo_destroy(struct kref *kref)
 	const struct drm_gpuvm_ops *ops = gpuvm->ops;
 	struct drm_gem_object *obj = vm_bo->obj;
 	bool lock = !drm_gpuvm_resv_protected(gpuvm);
+	bool unref = !(gpuvm->flags & DRM_GPUVM_VA_WEAK_REF);
 
 	if (!lock)
 		drm_gpuvm_resv_assert_held(gpuvm);
 
+	if (kref_read(&obj->refcount) > 0) {
+		drm_gem_gpuva_assert_lock_held(obj);
+	} else {
+		WARN_ON(!(gpuvm->flags & DRM_GPUVM_VA_WEAK_REF));
+		WARN_ON(!list_empty(&vm_bo->list.entry.evict));
+		WARN_ON(!list_empty(&vm_bo->list.entry.extobj));
+	}
+
 	drm_gpuvm_bo_list_del(vm_bo, extobj, lock);
 	drm_gpuvm_bo_list_del(vm_bo, evict, lock);
 
-	if (kref_read(&obj->refcount) > 0)
-		drm_gem_gpuva_assert_lock_held(obj);
-
 	list_del(&vm_bo->list.entry.gem);
 
 	if (ops && ops->vm_bo_free)
@@ -1522,7 +1535,8 @@  drm_gpuvm_bo_destroy(struct kref *kref)
 		kfree(vm_bo);
 
 	drm_gpuvm_put(gpuvm);
-	drm_gem_object_put(obj);
+	if (unref)
+		drm_gem_object_put(obj);
 }
 
 /**
@@ -1678,6 +1692,12 @@  drm_gpuvm_bo_extobj_add(struct drm_gpuvm_bo *vm_bo)
 	if (!lock)
 		drm_gpuvm_resv_assert_held(gpuvm);
 
+	/* If the vm_bo doesn't hold a hard reference to the obj, then the
+	 * driver is responsible for object tracking.
+	 */
+	if (gpuvm->flags & DRM_GPUVM_VA_WEAK_REF)
+		return;
+
 	if (drm_gpuvm_is_extobj(gpuvm, vm_bo->obj))
 		drm_gpuvm_bo_list_add(vm_bo, extobj, lock);
 }
@@ -1699,6 +1719,13 @@  drm_gpuvm_bo_evict(struct drm_gpuvm_bo *vm_bo, bool evict)
 	bool lock = !drm_gpuvm_resv_protected(gpuvm);
 
 	dma_resv_assert_held(obj->resv);
+
+	/* If the vm_bo doesn't hold a hard reference to the obj, then the
+	 * driver must track evictions on it's own.
+	 */
+	if (gpuvm->flags & DRM_GPUVM_VA_WEAK_REF)
+		return;
+
 	vm_bo->evicted = evict;
 
 	/* Can't add external objects to the evicted list directly if not using
diff --git a/include/drm/drm_gpuvm.h b/include/drm/drm_gpuvm.h
index 2a9629377633..652e0fb66413 100644
--- a/include/drm/drm_gpuvm.h
+++ b/include/drm/drm_gpuvm.h
@@ -205,10 +205,25 @@  enum drm_gpuvm_flags {
 	 */
 	DRM_GPUVM_RESV_PROTECTED = BIT(0),
 
+	/**
+	 * @DRM_GPUVM_VA_WEAK_REF:
+	 *
+	 * Flag indicating that the &drm_gpuva (or more correctly, the
+	 * &drm_gpuvm_bo) only holds a weak reference to the &drm_gem_object.
+	 * This mode is intended to ease migration to drm_gpuvm for drivers
+	 * where the GEM object holds a referece to the VA, rather than the
+	 * other way around.
+	 *
+	 * In this mode, drm_gpuvm does not track evicted or external objects.
+	 * It is intended for legacy mode, where the needed objects are attached
+	 * to the command submission ioctl, therefore this tracking is unneeded.
+	 */
+	DRM_GPUVM_VA_WEAK_REF = BIT(1),
+
 	/**
 	 * @DRM_GPUVM_USERBITS: user defined bits
 	 */
-	DRM_GPUVM_USERBITS = BIT(1),
+	DRM_GPUVM_USERBITS = BIT(2),
 };
 
 /**
@@ -651,7 +666,7 @@  struct drm_gpuvm_bo {
 
 	/**
 	 * @obj: The &drm_gem_object being mapped in @vm. This is a reference
-	 * counted pointer.
+	 * counted pointer, unless the &DRM_GPUVM_VA_WEAK_REF flag is set.
 	 */
 	struct drm_gem_object *obj;