linux-kernel - Re: [PATCH v3 8/9] dma-buf/sync_file: Add SET

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Message-ID: <3a0ef667-a697-50ab-bc2d-07001c6a9889@amd.com>
Date:   Mon, 27 Sep 2021 10:53:28 +0200
From:   Christian König <christian.koenig@....com>
To:     Pekka Paalanen <ppaalanen@...il.com>,
        Rob Clark <robdclark@...il.com>
Cc:     dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
        Daniel Vetter <daniel@...ll.ch>,
        Christian König <ckoenig.leichtzumerken@...il.com>,
        Michel Dänzer <michel@...nzer.net>,
        Rob Clark <robdclark@...omium.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Gustavo Padovan <gustavo@...ovan.org>,
        "open list:SYNC FILE FRAMEWORK" <linux-media@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 8/9] dma-buf/sync_file: Add SET_DEADLINE ioctl

Am 27.09.21 um 10:42 schrieb Pekka Paalanen:
> On Fri,  3 Sep 2021 11:47:59 -0700
> Rob Clark <robdclark@...il.com> wrote:
>
>> From: Rob Clark <robdclark@...omium.org>
>>
>> The initial purpose is for igt tests, but this would also be useful for
>> compositors that wait until close to vblank deadline to make decisions
>> about which frame to show.
>>
>> Signed-off-by: Rob Clark <robdclark@...omium.org>
>> ---
>>   drivers/dma-buf/sync_file.c    | 19 +++++++++++++++++++
>>   include/uapi/linux/sync_file.h | 20 ++++++++++++++++++++
>>   2 files changed, 39 insertions(+)
>>
>> diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
>> index 394e6e1e9686..f295772d5169 100644
>> --- a/drivers/dma-buf/sync_file.c
>> +++ b/drivers/dma-buf/sync_file.c
>> @@ -459,6 +459,22 @@ static long sync_file_ioctl_fence_info(struct sync_file *sync_file,
>>   	return ret;
>>   }
>>   
>> +static int sync_file_ioctl_set_deadline(struct sync_file *sync_file,
>> +					unsigned long arg)
>> +{
>> +	struct sync_set_deadline ts;
>> +
>> +	if (copy_from_user(&ts, (void __user *)arg, sizeof(ts)))
>> +		return -EFAULT;
>> +
>> +	if (ts.pad)
>> +		return -EINVAL;
>> +
>> +	dma_fence_set_deadline(sync_file->fence, ktime_set(ts.tv_sec, ts.tv_nsec));
>> +
>> +	return 0;
>> +}
>> +
>>   static long sync_file_ioctl(struct file *file, unsigned int cmd,
>>   			    unsigned long arg)
>>   {
>> @@ -471,6 +487,9 @@ static long sync_file_ioctl(struct file *file, unsigned int cmd,
>>   	case SYNC_IOC_FILE_INFO:
>>   		return sync_file_ioctl_fence_info(sync_file, arg);
>>   
>> +	case SYNC_IOC_SET_DEADLINE:
>> +		return sync_file_ioctl_set_deadline(sync_file, arg);
>> +
>>   	default:
>>   		return -ENOTTY;
>>   	}
>> diff --git a/include/uapi/linux/sync_file.h b/include/uapi/linux/sync_file.h
>> index ee2dcfb3d660..f67d4ffe7566 100644
>> --- a/include/uapi/linux/sync_file.h
>> +++ b/include/uapi/linux/sync_file.h
>> @@ -67,6 +67,18 @@ struct sync_file_info {
>>   	__u64	sync_fence_info;
>>   };
>>   
>> +/**
>> + * struct sync_set_deadline - set a deadline on a fence
>> + * @tv_sec:	seconds elapsed since epoch
>> + * @tv_nsec:	nanoseconds elapsed since the time given by the tv_sec
>> + * @pad:	must be zero
> Hi Rob,
>
> I think you need to specify which clock this timestamp must be in.
>
> Which epoch? Sounds a bit like CLOCK_REALTIME to me which would not
> make sense.

Most likely CLOCK_MONOTONIC I think since that's what ktime is based on 
as well IIRC.

My recollection might be wrong but I think Daniel documented somewhere 
that an absolut 64bit nanosecond value should be used for timeouts in 
IOCTLs which is sufficient for ~500 years uptime.

So the separation between seconds and nanoseconds might be redundant.

Regards,
Christian.

>
> Also I cannot guess how a compositor should be using this, so
> explaining the expected usage would be really good, with reasons for
> why should userspace bother.
>
>
> Thanks,
> pq
>
>> + */
>> +struct sync_set_deadline {
>> +	__s64	tv_sec;
>> +	__s32	tv_nsec;
>> +	__u32	pad;
>> +};
>> +
>>   #define SYNC_IOC_MAGIC		'>'
>>   
>>   /**
>> @@ -95,4 +107,12 @@ struct sync_file_info {
>>    */
>>   #define SYNC_IOC_FILE_INFO	_IOWR(SYNC_IOC_MAGIC, 4, struct sync_file_info)
>>   
>> +
>> +/**
>> + * DOC: SYNC_IOC_SET_DEADLINE - set a deadline on a fence
>> + *
>> + * Allows userspace to set a deadline on a fence, see dma_fence_set_deadline()
>> + */
>> +#define SYNC_IOC_SET_DEADLINE	_IOW(SYNC_IOC_MAGIC, 5, struct sync_set_deadline)
>> +
>>   #endif /* _UAPI_LINUX_SYNC_H */