[PATCH v3] drm/syncobj: use newly allocated stub fences

From: David Stevens
Date: Thu Apr 08 2021 - 05:54:42 EST


From: David Stevens <stevensd@xxxxxxxxxxxx>

Allocate a new private stub fence in drm_syncobj_assign_null_handle,
instead of using a static stub fence.

When userspace creates a fence with DRM_SYNCOBJ_CREATE_SIGNALED or when
userspace signals a fence via DRM_IOCTL_SYNCOBJ_SIGNAL, the timestamp
obtained when the fence is exported and queried with SYNC_IOC_FILE_INFO
should match when the fence's status was changed from the perspective of
userspace, which is during the respective ioctl.

When a static stub fence started being used in by these ioctls, this
behavior changed. Instead, the timestamp returned by SYNC_IOC_FILE_INFO
became the first time anything used the static stub fence, which has no
meaning to userspace.

Signed-off-by: David Stevens <stevensd@xxxxxxxxxxxx>
---
v2 -> v3:
- reuse the static stub spinlock
v1 -> v2:
- checkpatch style fixes

drivers/dma-buf/dma-fence.c | 27 ++++++++++++++++++++++++++-
drivers/gpu/drm/drm_syncobj.c | 25 +++++++++++++++++++------
include/linux/dma-fence.h | 1 +
3 files changed, 46 insertions(+), 7 deletions(-)

diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index d64fc03929be..ce0f5eff575d 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -123,7 +123,9 @@ static const struct dma_fence_ops dma_fence_stub_ops = {
/**
* dma_fence_get_stub - return a signaled fence
*
- * Return a stub fence which is already signaled.
+ * Return a stub fence which is already signaled. The fence's
+ * timestamp corresponds to the first time after boot this
+ * function is called.
*/
struct dma_fence *dma_fence_get_stub(void)
{
@@ -141,6 +143,29 @@ struct dma_fence *dma_fence_get_stub(void)
}
EXPORT_SYMBOL(dma_fence_get_stub);

+/**
+ * dma_fence_allocate_private_stub - return a private, signaled fence
+ *
+ * Return a newly allocated and signaled stub fence.
+ */
+struct dma_fence *dma_fence_allocate_private_stub(void)
+{
+ struct dma_fence *fence;
+
+ fence = kzalloc(sizeof(*fence), GFP_KERNEL);
+ if (fence == NULL)
+ return ERR_PTR(-ENOMEM);
+
+ dma_fence_init(fence,
+ &dma_fence_stub_ops,
+ &dma_fence_stub_lock,
+ 0, 0);
+ dma_fence_signal(fence);
+
+ return fence;
+}
+EXPORT_SYMBOL(dma_fence_allocate_private_stub);
+
/**
* dma_fence_context_alloc - allocate an array of fence contexts
* @num: amount of contexts to allocate
diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
index 349146049849..a54aa850d143 100644
--- a/drivers/gpu/drm/drm_syncobj.c
+++ b/drivers/gpu/drm/drm_syncobj.c
@@ -350,12 +350,16 @@ EXPORT_SYMBOL(drm_syncobj_replace_fence);
*
* Assign a already signaled stub fence to the sync object.
*/
-static void drm_syncobj_assign_null_handle(struct drm_syncobj *syncobj)
+static int drm_syncobj_assign_null_handle(struct drm_syncobj *syncobj)
{
- struct dma_fence *fence = dma_fence_get_stub();
+ struct dma_fence *fence = dma_fence_allocate_private_stub();
+
+ if (IS_ERR(fence))
+ return PTR_ERR(fence);

drm_syncobj_replace_fence(syncobj, fence);
dma_fence_put(fence);
+ return 0;
}

/* 5s default for wait submission */
@@ -469,6 +473,7 @@ EXPORT_SYMBOL(drm_syncobj_free);
int drm_syncobj_create(struct drm_syncobj **out_syncobj, uint32_t flags,
struct dma_fence *fence)
{
+ int ret;
struct drm_syncobj *syncobj;

syncobj = kzalloc(sizeof(struct drm_syncobj), GFP_KERNEL);
@@ -479,8 +484,13 @@ int drm_syncobj_create(struct drm_syncobj **out_syncobj, uint32_t flags,
INIT_LIST_HEAD(&syncobj->cb_list);
spin_lock_init(&syncobj->lock);

- if (flags & DRM_SYNCOBJ_CREATE_SIGNALED)
- drm_syncobj_assign_null_handle(syncobj);
+ if (flags & DRM_SYNCOBJ_CREATE_SIGNALED) {
+ ret = drm_syncobj_assign_null_handle(syncobj);
+ if (ret < 0) {
+ drm_syncobj_put(syncobj);
+ return ret;
+ }
+ }

if (fence)
drm_syncobj_replace_fence(syncobj, fence);
@@ -1322,8 +1332,11 @@ drm_syncobj_signal_ioctl(struct drm_device *dev, void *data,
if (ret < 0)
return ret;

- for (i = 0; i < args->count_handles; i++)
- drm_syncobj_assign_null_handle(syncobjs[i]);
+ for (i = 0; i < args->count_handles; i++) {
+ ret = drm_syncobj_assign_null_handle(syncobjs[i]);
+ if (ret < 0)
+ break;
+ }

drm_syncobj_array_free(syncobjs, args->count_handles);

diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
index 9f12efaaa93a..6ffb4b2c6371 100644
--- a/include/linux/dma-fence.h
+++ b/include/linux/dma-fence.h
@@ -587,6 +587,7 @@ static inline signed long dma_fence_wait(struct dma_fence *fence, bool intr)
}

struct dma_fence *dma_fence_get_stub(void);
+struct dma_fence *dma_fence_allocate_private_stub(void);
u64 dma_fence_context_alloc(unsigned num);

#define DMA_FENCE_TRACE(f, fmt, args...) \
--
2.31.0.208.g409f899ff0-goog