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

[Christian König <deathsimple@xxxxxxxxxxx>]

Am 10.08.2017 um 01:53 schrieb Jason Ekstrand:

On Wed, Aug 9, 2017 at 3:41 PM, Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> 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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