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

[Gustavo Padovan <gustavo@padovan.org>]

2016-09-22 Christian König <christian.koenig@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. 

Gustavo

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