Re: [PATCH 2/2] drm/syncobj: flatten chains when importing sync_file into timeline point

[Julian Orth]

On Mon, Oct 12, 2026 at 9:51 AM Christian König
<christian.koenig@xxxxxxx> wrote:
>
> On 10/10/26 20:07, Julian Orth wrote:
> > A sync_file exported from a timeline point can contain a dma_fence_chain.
>
> No it can't, IIRC sync files are only supposed to contain raw fences or dma_fence_array containers.
>
> I think we never documented that and that's why this issue here leaked in.

When I reviewed this patch before sending it in, I asked the AI if it
would not be better to do the merging at export time. It was adamant
that it had to be at import time because of commit 519f490d by
yourself:

    dma-buf/sync-file: fix warning about fence containers

    The dma_fence_chain containers can show up in sync_files as well

>
> I'm not sure if that doesn't break anywhere else as well, so I think we should fix the exporting side instead.
>
> Alternatively we need to audit everywhere where fences from syncfiles can be imported somewhere else.
>
> Regards,
> Christian.
>
> > Importing such a sync_file into a timeline point with
> > DRM_SYNCOBJ_FD_TO_HANDLE_FLAGS_IMPORT_SYNC_FILE |
> > DRM_SYNCOBJ_FD_TO_HANDLE_FLAGS_TIMELINE passes the chain directly to
> > dma_fence_chain_init() and triggers
> >
> >   WARNING: drivers/dma-buf/dma-fence-chain.c:286 at dma_fence_chain_init
> >
> > because chain fences must not be wrapped in other chain fences.
> >
> > Flatten the fence with dma_fence_unwrap_merge() before adding it to the
> > timeline, as drm_syncobj_transfer_to_timeline() already does.
> >
> > Reproducer:
> >
> > 1. Export a sync_file from a point > 0 of a timeline syncobj with
> >    DRM_IOCTL_SYNCOBJ_HANDLE_TO_FD.
> > 2. Import it into a point > 0 of another timeline syncobj with
> >    DRM_IOCTL_SYNCOBJ_FD_TO_HANDLE.
> >
> > Fixes: c2d3a7300695 ("drm/syncobj: Extend EXPORT_SYNC_FILE for timeline syncobjs")
> > Assisted-by: Claude:claude-opus-5-5
> > Signed-off-by: Julian Orth <ju.orth@xxxxxxxxx>
> > ---
> >  drivers/gpu/drm/drm_syncobj.c | 12 +++++++++++-
> >  1 file changed, 11 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
> > index 5ca5163d0..c545af77e 100644
> > --- a/drivers/gpu/drm/drm_syncobj.c
> > +++ b/drivers/gpu/drm/drm_syncobj.c
> > @@ -742,8 +742,18 @@ static int drm_syncobj_import_sync_file_fence(struct drm_file *file_private,
> >       }
> >
> >       if (point) {
> > -             struct dma_fence_chain *chain = dma_fence_chain_alloc();
> > +             struct dma_fence_chain *chain;
> > +             struct dma_fence *tmp;
> > +
> > +             tmp = dma_fence_unwrap_merge(fence);
> > +             dma_fence_put(fence);
> > +             fence = tmp;
> > +             if (!fence) {
> > +                     drm_syncobj_put(syncobj);
> > +                     return -ENOMEM;
> > +             }
> >
> > +             chain = dma_fence_chain_alloc();
> >               if (!chain) {
> >                       ret = -ENOMEM;
> >                       goto out;
>