linux-kernel - Re: [PATCH] dma_buf/sync_file: Enable signaling for fences when querying status

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Message-ID: <1ebba600-31d2-48ed-b08b-46d667d38054@amd.com>
Date: Tue, 8 Jul 2025 16:20:31 +0200
From: Christian König <christian.koenig@....com>
To: Mikko Perttunen <cyndis@...si.fi>, Sumit Semwal
 <sumit.semwal@...aro.org>, Gustavo Padovan <gustavo@...ovan.org>
Cc: linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
 Mikko Perttunen <mperttunen@...dia.com>
Subject: Re: [PATCH] dma_buf/sync_file: Enable signaling for fences when
 querying status

On 08.07.25 14:03, Mikko Perttunen wrote:
> From: Mikko Perttunen <mperttunen@...dia.com>
> 
> dma_fence_get_status is not guaranteed to return valid information
> on if the fence has been signaled or not if SW signaling has not
> been enabled for the fence. To ensure valid information is reported,
> enable SW signaling for fences before getting their status.

I don't know how often I had to rejected that patch now, we really need to improve the documentation :(

The fence info query exists to query the status *without* enabling signaling, that is the whole purpose of the function!

If you want to enable signaling *and* query the status then just poll on the sync file file descriptor with a zero timeout.

If the signaling timestamp or error code is needed then that can be retrieved after signaling through the info IOCTL. 

Regards,
Christian.

> 
> Signed-off-by: Mikko Perttunen <mperttunen@...dia.com>
> ---
>  drivers/dma-buf/sync_file.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
> index 747e377fb95417ddd506b528618a4288bea9d459..a6fd1d14dde155561b9fd2c07e6aa20dc9863a8d 100644
> --- a/drivers/dma-buf/sync_file.c
> +++ b/drivers/dma-buf/sync_file.c
> @@ -271,6 +271,8 @@ static int sync_fill_fence_info(struct dma_fence *fence,
>  	const char __rcu *timeline;
>  	const char __rcu *driver;
>  
> +	dma_fence_enable_sw_signaling(fence);
> +
>  	rcu_read_lock();
>  
>  	driver = dma_fence_driver_name(fence);
> @@ -320,6 +322,7 @@ static long sync_file_ioctl_fence_info(struct sync_file *sync_file,
>  	 * info->num_fences.
>  	 */
>  	if (!info.num_fences) {
> +		dma_fence_enable_sw_signaling(sync_file->fence);
>  		info.status = dma_fence_get_status(sync_file->fence);
>  		goto no_fences;
>  	} else {
> 
> ---
> base-commit: 58ba80c4740212c29a1cf9b48f588e60a7612209
> change-id: 20250708-syncfile-enable-signaling-a993acff1860
>