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

[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.

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

Christian.

>
> Gustavo
>