[PATCH] dma-fence: Make dma_fence_add_callback() fail if signaled with error

[Chris Wilson]

Quoting Ezequiel Garcia (2018-05-14 22:28:31)
> On Mon, 2018-05-14 at 18:48 +0200, Daniel Vetter wrote:
> > On Fri, May 11, 2018 at 08:27:41AM +0100, Chris Wilson wrote:
> > > Quoting Ezequiel Garcia (2018-05-09 21:14:49)
> > > > Change how dma_fence_add_callback() behaves, when the fence
> > > > has error-signaled by the time it is being add. After this commit,
> > > > dma_fence_add_callback() returns the fence error, if it
> > > > has error-signaled before dma_fence_add_callback() is called.
> > > 
> > > Why? What problem are you trying to solve? fence->error does not imply
> > > that the fence has yet been signaled, and the caller wants a callback
> > > when it is signaled.
> > 
> > On top this is incosistent, e.g. we don't do the same for any of the other
> > dma_fence interfaces. Plus there's the issue that you might alias errno
> > values with fence errno values.
> > 
> 
> Right.
> 
> > I think keeping the error codes from the functions you're calling distinct
> > from the error code of the fence itself makes a lot of sense. The first
> > tells you whether your request worked out (or why not), the second tells
> > you whether the asynchronous dma operation (gpu rendering, page flip,
> > whatever) that the dma_fence represents worked out (or why not). That's 2
> > distinct things imo.
> > 
> > Might be good to show us the driver code that needs this behaviour so we
> > can discuss how to best handle your use-case.
> > 
> 
> This change arose while discussing the in-fences support for video4linux.
> Here's the patch that calls dma_fence_add_callback https://lkml.org/lkml/2018/5/4/766.
> 
> The code snippet currently looks something like this:
> 
>         if (vb->in_fence) {
>                 ret = dma_fence_add_callback(vb->in_fence, &vb->fence_cb,
>                                 
>              vb2_qbuf_fence_cb);
>                 /* is the fence signaled? */
>                 if (ret == -ENOENT) {
>         
>                 dma_fence_put(vb->in_fence);
>                         vb->in_fence = NULL;
>                 } else if (ret)
> {
>                         goto unlock;
>                 }
>         }
> 
> In this use case, if the callback is added successfully,
> the video4linux core defers the activation of the buffer
> until the fence signals.
> 
> If the fence is signaled (currently disregarding of errors)
> then the buffer is assumed to be ready to be activated,
> and so it gets queued for hardware usage.
> 
> Giving some more thought to this, I'm not so sure what is
> the right action if a fence signaled with error. In this case,
> it appears to me that we shouldn't be using this buffer
> if its in-fence is in error, but perhaps I'm missing
> something.

What I have in mind for async errors is to skip the operation and
propagate the error onto the next fence. Mostly because those async
errors may include fatal errors such as unable to pin the backing
storage for the operation, but even "trivial" errors such as an early
operation failing means that this request is then subject to garbage-in,
garbage-out. However, for trivial errors I would just propagate the
error status (so the caller knows something went wrong if they care, but
in all likelihood no one will notice) and continue on with the glitchy
operation.
-Chris