EPOLL for drm_syncfile (was Re: [PATCH 4/4] RFC: dma-buf: Add an API for importing sync files (v6))

[Daniel Stone]

Hi Christian,

On Sun, 23 May 2021 at 18:16, Christian König <christian.koenig at amd.com> wrote:
> Am 22.05.21 um 22:05 schrieb Daniel Stone:
> > Anyway, the problem with syncobj is that the ioctl to wait for a
> > sync_file to materialise for a given timeline point only allows us to
> > block with a timeout; this is a non-starter, because we need something
> > which fits into epoll. The most optimal case is returning a new
> > eventfd or similar which signals when a given timeline point becomes
> > available or signaled, but in extremis a syncobj FD becoming readable
> > when any activity which would change the result of any zero-timeout
> > wait on that syncobj is more or less workable.
>
> I think the tricky part is to epoll for a certain value.
>
> Not sure how eventfd is supposed to work, but IIRC we don't have the
> functionality to poll for a certain value/offset etc to become available.
>
> We could of course create a separate fd for each requested value to poll
> for thought, but that sounds like a bit much overhead to me.

Yeah, I understand the point; something like an eventfd is exactly
what you think, that we would have to materialise a new FD for it. On
the other hand, as soon as the sync point becomes available, the
winsys will want to immediately extract a sync_file from it, so I
don't think FD amplification is a big issue. If it looks like being a
problem in practice, we could look at providing a FD which starts off
inert, and only later becomes a sync_file after it polls readable, but
that sounds like something we really want to avoid if we can.

Cheers,
Daniel