[PATCH v4 02/40] drm/gpuvm: Allow VAs to hold soft reference to BOs

[Rob Clark <robdclark@gmail.com>]

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).

Signed-off-by: Rob Clark <robdclark@chromium.org>
---
 drivers/gpu/drm/drm_gpuvm.c |  8 ++++++--
 include/drm/drm_gpuvm.h     | 12 ++++++++++--
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_gpuvm.c b/drivers/gpu/drm/drm_gpuvm.c
index 1e89a98caad4..f1d521dc1fb0 100644
--- a/drivers/gpu/drm/drm_gpuvm.c
+++ b/drivers/gpu/drm/drm_gpuvm.c
@@ -1482,7 +1482,9 @@ 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);
@@ -1504,6 +1506,7 @@ 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);
@@ -1522,7 +1525,8 @@ 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);
 }
 
 /**
diff --git a/include/drm/drm_gpuvm.h b/include/drm/drm_gpuvm.h
index 00d4e43b76b6..13ab087a45fa 100644
--- a/include/drm/drm_gpuvm.h
+++ b/include/drm/drm_gpuvm.h
@@ -205,10 +205,18 @@ 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.
+	 */
+	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 +659,7 @@ 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;
 
-- 
2.49.0