Re: [PATCH v4 05/14] dma-buf/sync_file: Add SET_DEADLINE ioctl

[Pekka Paalanen <ppaalanen@gmail.com>]

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On Mon, 20 Feb 2023 08:09:04 -0800
Rob Clark <robdclark@gmail.com> wrote:

> On Mon, Feb 20, 2023 at 12:27 AM Christian König
> <christian.koenig@amd.com> wrote:
> >
> > Am 18.02.23 um 22:15 schrieb Rob Clark:  
> > > From: Rob Clark <robdclark@chromium.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.
> > >
> > > The igt tests can be found at:
> > >
> > > https://gitlab.freedesktop.org/robclark/igt-gpu-tools/-/commits/fence-deadline
> > >
> > > v2: Clarify the timebase, add link to igt tests
> > >
> > > Signed-off-by: Rob Clark <robdclark@chromium.org>
> > > ---
> > >   drivers/dma-buf/sync_file.c    | 19 +++++++++++++++++++
> > >   include/uapi/linux/sync_file.h | 22 ++++++++++++++++++++++
> > >   2 files changed, 41 insertions(+)
> > >
> > > diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
> > > index af57799c86ce..fb6ca1032885 100644
> > > --- a/drivers/dma-buf/sync_file.c
> > > +++ b/drivers/dma-buf/sync_file.c
> > > @@ -350,6 +350,22 @@ 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)
> > >   {
> > > @@ -362,6 +378,9 @@ 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..c8666580816f 100644
> > > --- a/include/uapi/linux/sync_file.h
> > > +++ b/include/uapi/linux/sync_file.h
> > > @@ -67,6 +67,20 @@ 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
> > > + *
> > > + * The timebase for the deadline is CLOCK_MONOTONIC (same as vblank)
> > > + */
> > > +struct sync_set_deadline {
> > > +     __s64   tv_sec;
> > > +     __s32   tv_nsec;
> > > +     __u32   pad;  
> >
> > IIRC struct timespec defined this as time_t/long (which is horrible for
> > an UAPI because of the sizeof(long) dependency), one possible
> > alternative is to use 64bit nanoseconds from CLOCK_MONOTONIC (which is
> > essentially ktime).
> >
> > Not 100% sure if there is any preferences documented, but I think the
> > later might be better.  
> 
> The original thought is that this maps directly to clock_gettime()
> without extra conversion needed, and is similar to other pre-ktime_t
> UAPI.  But OTOH if userspace wants to add an offset, it is maybe
> better to convert completely to ns in userspace and use a u64 (as that
> is what ns_to_ktime() uses).. (and OFC whatever decision here also
> applies to the syncobj wait ioctls)
> 
> I'm leaning towards u64 CLOCK_MONOTONIC ns if no one has a good
> argument against that.

No, no good argument against that, it's just different from any other
UAPI so far, but a new style has to start somewhere. It's good for 584
years after the epoch.

Just make sure the documentation is explicit on how struct timespec is
converted to and from u64 (signedness issues, overflow and whatnot).


Thanks,
pq


> 
> BR,
> -R
> 
> > Either way the patch is Acked-by: Christian König
> > <christian.koenig@amd.com> for this patch.
> >
> > Regards,
> > Christian.
> >  
> > > +};
> > > +
> > >   #define SYNC_IOC_MAGIC              '>'
> > >
> > >   /**
> > > @@ -95,4 +109,12 @@ 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 */  
> >  


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