[PATCH] drm/syncobj: Allow wait for submit and signal behavior (v2)

[Christian König]

Am 10.08.2017 um 01:53 schrieb Jason Ekstrand:
> On Wed, Aug 9, 2017 at 3:41 PM, Chris Wilson <chris at chris-wilson.co.uk 
> <mailto:chris at chris-wilson.co.uk>> wrote:
>
>     Quoting Chris Wilson (2017-08-09 18:57:44)
>     > So we are taking a snapshot here. It looks like this could have been
>     > done using a dma_fence_array + dma_fence_proxy for capturing the
>     future
>     > fence.
>
>     A quick sketch of this idea looks like:
>
>      void drm_syncobj_replace_fence(struct drm_syncobj *syncobj,
>                                    struct dma_fence *fence)
>      {
>     -       struct dma_fence *old_fence;
>     +       unsigned long flags;
>
>     -       if (fence)
>     -               dma_fence_get(fence);
>     -       old_fence = xchg(&syncobj->fence, fence);
>     -
>     -       dma_fence_put(old_fence);
>     +       spin_lock_irqsave(&syncobj->lock, flags);
>     +       dma_fence_replace_proxy(&syncobj->fence, fence);
>     +       spin_unlock_irqrestore(&syncobj->lock, flags);
>      }
>
>     +int
>     +drm_syncobj_wait_ioctl(struct drm_device *dev, void *data,
>     +                      struct drm_file *file_private)
>     +{
>     +       struct drm_syncobj_wait *args = data;
>     +       struct dma_fence **fences;
>     +       struct dma_fence_array *array;
>     +       unsigned long timeout;
>     +       unsigned int count;
>     +       u32 *handles;
>     +       int ret = 0;
>     +       u32 i;
>     +
>     +       BUILD_BUG_ON(sizeof(*fences) < sizeof(*handles));
>     +
>     +       if (!drm_core_check_feature(dev, DRIVER_SYNCOBJ))
>     +               return -ENODEV;
>     +
>     +       if (args->flags != 0 && args->flags !=
>     DRM_SYNCOBJ_WAIT_FLAGS_WAIT_ALL)
>     +               return -EINVAL;
>     +
>     +       count = args->count_handles;
>     +       if (!count)
>     +               return -EINVAL;
>     +
>     +       /* Get the handles from userspace */
>     +       fences = kmalloc_array(count,
>     +                              sizeof(struct dma_fence *),
>     +                              __GFP_NOWARN | GFP_KERNEL);
>     +       if (!fences)
>     +               return -ENOMEM;
>     +
>     +       handles = (void *)fences + count * (sizeof(*fences) -
>     sizeof(*handles));
>     +       if (copy_from_user(handles,
>     + u64_to_user_ptr(args->handles),
>     +                          sizeof(u32) * count)) {
>     +               ret = -EFAULT;
>     +               goto err;
>     +       }
>     +
>     +       for (i = 0; i < count; i++) {
>     +               struct drm_syncobj *s;
>     +
>     +               ret = -ENOENT;
>     +               s = drm_syncobj_find(file_private, handles[i]);
>     +               if (s) {
>     +                       ret = 0;
>     +                       spin_lock_irq(&s->lock);
>     +                       if (!s->fence) {
>     +                               if (args->flags &
>     DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT)
>     +                                       s->fence =
>     dma_fence_create_proxy();
>     +                               else
>     +                                       ret = -EINVAL;
>     +                       }
>     +                       if (s->fence)
>     +                               fences[i] = dma_fence_get(s->fence);
>     +                       spin_unlock_irq(&s->lock);
>     +               }
>     +               if (ret) {
>     +                       count = i;
>     +                       goto err_fences;
>     +               }
>     +       }
>     +
>     +       array = dma_fence_array_create(count, fences, 0, 0,
>     +                                      !(args->flags &
>     DRM_SYNCOBJ_WAIT_FLAGS_WAIT_ALL));
>     +       if (!array) {
>     +               ret = -ENOMEM;
>     +               goto err_fences;
>     +       }
>     +
>     +       timeout = drm_timeout_abs_to_jiffies(args->timeout_nsec);
>     +       timeout = dma_fence_wait_timeout(&array->base, true, timeout);
>     +       args->first_signaled = array->first_signaled;
>     +       dma_fence_put(&array->base);
>     +
>     +       return timeout < 0 ? timeout : 0;
>     +
>     +err_fences:
>     +       for (i = 0; i < count; i++)
>     +               dma_fence_put(fences[i]);
>     +err:
>     +       kfree(fences);
>     +       return ret;
>     +}
>
>     The key advantage is that this translate the ioctl into a
>     dma-fence-array
>     which already has to deal with the mess, sharing the burden. (But it
>     does require a trivial patch to dma-fence-array to record the first
>     signaled fence.)
>
>     However, this installs the proxy into syncobj->fence with the result
>     that any concurrent wait also become a WAIT_FOR_SUBMIT. The behaviour
>     of drm_syncobj is then quite inconsistent, sometimes it will wait
>     for a
>     future fence, sometimes it will report an error.
>
>
> Yeah, that's not good.  I thought about a variety of solutions to try 
> and re-use more core dma_fence code.  Ultimately I chose the current 
> one because it takes a snapshot of the syncobjs and then, from that 
> point forward, it's consistent with its snapshot.  Nothing I was able 
> to come up with based on core dma_fence wait routines does that.

As Chris pointed out, that's really not a good idea.

Most of the time we need the behavior of reporting an error and only 
when the flag is given wait until some fence shows up.

In general I suggest that we only support this use case in the form of a 
wait_event_interruptible() on setting the first fence on an object.

Waiting on the first one of multiple objects wouldn't be possible (you 
would always wait till all objects have fences), but I think that this 
is acceptable.

The big advantage of the wait_event_interruptible() interface is that 
your condition checking and process handling is bullet prove, as far as 
I have seen so far your code is a bit buggy in that direction.

Christian.

>
> --Jason
>
>
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