[PATCH] dma-buf/fence-array: get signaled state when signaling is disabled

[Gustavo Padovan]

2016-09-23 Christian König <christian.koenig at amd.com>:

> Am 23.09.2016 um 13:30 schrieb Gustavo Padovan:
> > 2016-09-22 Christian König <christian.koenig at amd.com>:
> > 
> > > Am 22.09.2016 um 13:16 schrieb Gustavo Padovan:
> > > > 2016-09-22 Christian König <christian.koenig at amd.com>:
> > > > 
> > > > > Dropping the rest of the patch, cause that really doesn't make sense any
> > > > > more.
> > > > > 
> > > > > Am 22.09.2016 um 12:40 schrieb Gustavo Padovan:
> > > > > > > E.g. for example it is illegal to do something like
> > > > > > > > "while(!fence_is_signaled(f)) sleep();" without enabling signaling before
> > > > > > > > doing this.
> > > > > > > > 
> > > > > > > > Could just be a misunderstanding, but the comments on your patch actually
> > > > > > > > sounds a bit like somebody is trying to do exactly that.
> > > > > > I think the usecase in mind here is poll(fd, timeout=0)
> > > > > Exactly as I feared. Even if userspace polls with timeout=0 you still need
> > > > > to call enable_signaling().
> > > > > 
> > > > > Otherwise you can run into a situation where userspace only uses timeout=0
> > > > > and so never activates the signaling check in the driver.
> > > > > 
> > > > > This would in turn result in an endless loop on implementations where the
> > > > > driver never signals fences on their own.
> > > > Polling is optional, userspace may never call it. And DRM/KMS or GPU
> > > > drivers will be doing fence_wait() themselves so signaling is enabled at
> > > > some point.
> > > No they won't. We have an use case where we clearly want to avoid that as
> > > much as possible because and so the driver never calls enable_signaling() on
> > > it's own.
> > > 
> > > Exposing this poll function to userspace without enabling signaling is a
> > > clear NAK from my side.
> > Okay. So you are NAK'ing the does_not_pool_wait change. Should we revert
> > that one then? It is already broken.
> 
> Yeah, that would probably a good idea. The AMD driver changes which really
> rely on this aren't upstream yet, so if you point me to the commit hash I
> could revert that as well when we send that out.
> 
> On the other hand the idea behind fence_is_signaled() is really that you
> check the status multiple times after enabling signaling. So I would prefer
> if you would revert this change preliminary.
> 
> Double checking this patch (and thinking about it a bit more) reveals that
> it is most likely correct. So feel free to commit this one if it is still
> needed for something.

It is this patch:

ecebca7 dma-buf/sync-file: Avoid enable fence signaling if poll(.timeout=0)

But if we revert it as you are a proposing we don't need my patch here
anymore. However we would need to revert it now because it is broken.
Shall I send a revert part?

Gustavo