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Subject: Re: [PATCH] drm/syncobj: use newly allocated stub fences
To:     David Stevens <stevensd@chromium.org>
Cc:     Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org
References: <20210408093448.3897988-1-stevensd@google.com>
From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>
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Date:   Thu, 8 Apr 2021 11:41:52 +0200
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Am 08.04.21 um 11:34 schrieb David Stevens:
> From: David Stevens <stevensd@chromium.org>
>
> 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@chromium.org>
> ---
>   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;
> +};
> +

You can drop this. The spinlock is only used when the fence is signaled 
to avoid races between signaling and adding a callback.

And for this the global spinlock should be perfectly sufficient. Apart 
from that looks good to me.

Regards,
Christian.

>   /*
>    * 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...) \