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

[Daniel Vetter]

On Thu, Apr 08, 2021 at 06:34:48PM +0900, David Stevens wrote:
> 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>

Is there also an igt patch to enforce this in the drm_syncobj testcases?
Would be really good to have that too.
-Daniel

> ---
>  drivers/dma-buf/dma-fence.c   | 33 ++++++++++++++++++++++++++++++++-
>  drivers/gpu/drm/drm_syncobj.c | 28 ++++++++++++++++++++--------
>  include/linux/dma-fence.h     |  1 +
>  3 files changed, 53 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index d64fc03929be..6081eb962490 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -26,6 +26,11 @@ EXPORT_TRACEPOINT_SYMBOL(dma_fence_signaled);
>  static DEFINE_SPINLOCK(dma_fence_stub_lock);
>  static struct dma_fence dma_fence_stub;
>  
> +struct drm_fence_private_stub {
> +	struct dma_fence base;
> +	spinlock_t lock;
> +};
> +
>  /*
>   * fence context counter: each execution context should have its own
>   * fence context, this allows checking if fences belong to the same
> @@ -123,7 +128,9 @@ 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 +148,30 @@ 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 drm_fence_private_stub *fence;
> +
> +	fence = kzalloc(sizeof(*fence), GFP_KERNEL);
> +	if (fence == NULL)
> +		return ERR_PTR(-ENOMEM);
> +
> +	spin_lock_init(&fence->lock);
> +	dma_fence_init(&fence->base,
> +		       &dma_fence_stub_ops,
> +		       &fence->lock,
> +		       0, 0);
> +	dma_fence_signal(&fence->base);
> +
> +	return &fence->base;
> +}
> +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..c6125e57ae37 100644
> --- a/drivers/gpu/drm/drm_syncobj.c
> +++ b/drivers/gpu/drm/drm_syncobj.c
> @@ -350,12 +350,15 @@ 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);
> +       drm_syncobj_replace_fence(syncobj, fence);
> +       dma_fence_put(fence);
> +       return 0;
>  }
>  
>  /* 5s default for wait submission */
> @@ -469,6 +472,7 @@ 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 +483,13 @@ 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 +1331,11 @@ 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
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch