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

[Christian König]

Am 22.09.2016 um 16:11 schrieb Christian König:
> 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.

Sorry hitting send to soon. That should read "because of the extreme 
overhead".

Christian.

>
> Exposing this poll function to userspace without enabling signaling is 
> a clear NAK from my side.
>
> Christian.
>
>>
>> Gustavo
>>
>