Re: [PATCH] Revert "drm/syncobj: use dma_fence_get_stub"

[David Stevens]

On Thu, Apr 8, 2021 at 4:03 PM Christian König <christian.koenig@xxxxxxx> wrote:
>
> Am 08.04.21 um 06:59 schrieb David Stevens:
> > From: David Stevens <stevensd@xxxxxxxxxxxx>
> >
> > This reverts commit 86bbd89d5da66fe760049ad3f04adc407ec0c4d6.
> >
> > Using the singleton stub fence in drm_syncobj_assign_null_handle means
> > that all syncobjs created in an already signaled state or any syncobjs
> > signaled by userspace will reference the singleton fence when exported
> > to a sync_file. If those sync_files are queried with SYNC_IOC_FILE_INFO,
> > then the timestamp_ns value returned will correspond to whenever the
> > singleton stub fence was first initialized. This can break the ability
> > of userspace to use timestamps of these fences, as the singleton stub
> > fence's timestamp bears no relationship to any meaningful event.
>
> And why exactly is having the timestamp of the call to
> drm_syncobj_assign_null_handle() better?

The timestamp returned by SYNC_IOC_FILE_INFO is the "timestamp of
status change in nanoseconds". If userspace signals the fence with
DRM_IOCTL_SYNCOBJ_SIGNAL, then a timestamp from
drm_syncobj_assign_null_handle corresponds to the status change. If
userspace sets DRM_SYNCOBJ_CREATE_SIGNALED when creating a fence, then
the status change happens immediately upon creation, which again
corresponds to when drm_syncobj_assign_null_handle gets called.

> Additional if you really need that please don't revert the patch.
> Instead provide a function which returns a newly initialized stub fence
> in the dma_fence.c code.

Ack.

-David

> Regards,
> Christian.
>
> >
> > Signed-off-by: David Stevens <stevensd@xxxxxxxxxxxx>
> > ---
> >   drivers/gpu/drm/drm_syncobj.c | 58 ++++++++++++++++++++++++++---------
> >   1 file changed, 44 insertions(+), 14 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
> > index 349146049849..7cc11f1a83f4 100644
> > --- a/drivers/gpu/drm/drm_syncobj.c
> > +++ b/drivers/gpu/drm/drm_syncobj.c
> > @@ -211,6 +211,21 @@ struct syncobj_wait_entry {
> >   static void syncobj_wait_syncobj_func(struct drm_syncobj *syncobj,
> >                                     struct syncobj_wait_entry *wait);
> >
> > +struct drm_syncobj_stub_fence {
> > +     struct dma_fence base;
> > +     spinlock_t lock;
> > +};
> > +
> > +static const char *drm_syncobj_stub_fence_get_name(struct dma_fence *fence)
> > +{
> > +     return "syncobjstub";
> > +}
> > +
> > +static const struct dma_fence_ops drm_syncobj_stub_fence_ops = {
> > +     .get_driver_name = drm_syncobj_stub_fence_get_name,
> > +     .get_timeline_name = drm_syncobj_stub_fence_get_name,
> > +};
> > +
> >   /**
> >    * drm_syncobj_find - lookup and reference a sync object.
> >    * @file_private: drm file private pointer
> > @@ -344,18 +359,24 @@ void drm_syncobj_replace_fence(struct drm_syncobj *syncobj,
> >   }
> >   EXPORT_SYMBOL(drm_syncobj_replace_fence);
> >
> > -/**
> > - * drm_syncobj_assign_null_handle - assign a stub fence to the sync object
> > - * @syncobj: sync object to assign the fence on
> > - *
> > - * 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 drm_syncobj_stub_fence *fence;
> >
> > -     drm_syncobj_replace_fence(syncobj, fence);
> > -     dma_fence_put(fence);
> > +     fence = kzalloc(sizeof(*fence), GFP_KERNEL);
> > +     if (fence == NULL)
> > +             return -ENOMEM;
> > +
> > +     spin_lock_init(&fence->lock);
> > +     dma_fence_init(&fence->base, &drm_syncobj_stub_fence_ops,
> > +                    &fence->lock, 0, 0);
> > +     dma_fence_signal(&fence->base);
> > +
> > +     drm_syncobj_replace_fence(syncobj, &fence->base);
> > +
> > +     dma_fence_put(&fence->base);
> > +
> > +     return 0;
> >   }
> >
> >   /* 5s default for wait submission */
> > @@ -469,6 +490,7 @@ 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 +501,13 @@ 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 +1349,11 @@ 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);
> >
>