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@xxxxxxx>:
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
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