Re: [PATCH v3 1/4] dma-buf: add support for virtio exported objects

[David Stevens <stevensd@chromium.org>]

On Thu, May 14, 2020 at 9:30 PM Daniel Vetter <daniel@ffwll.ch> wrote:
> On Thu, May 14, 2020 at 05:19:40PM +0900, David Stevens wrote:
> > Sorry for the duplicate reply, didn't notice this until now.
> >
> > > Just storing
> > > the uuid should be doable (assuming this doesn't change during the
> > > lifetime of the buffer), so no need for a callback.
> >
> > Directly storing the uuid doesn't work that well because of
> > synchronization issues. The uuid needs to be shared between multiple
> > virtio devices with independent command streams, so to prevent races
> > between importing and exporting, the exporting driver can't share the
> > uuid with other drivers until it knows that the device has finished
> > registering the uuid. That requires a round trip to and then back from
> > the device. Using a callback allows the latency from that round trip
> > registration to be hidden.
>
> Uh, that means you actually do something and there's locking involved.
> Makes stuff more complicated, invariant attributes are a lot easier
> generally. Registering that uuid just always doesn't work, and blocking
> when you're exporting?

Registering the id at creation and blocking in gem export is feasible,
but it doesn't work well for systems with a centralized buffer
allocator that doesn't support batch allocations (e.g. gralloc). In
such a system, the round trip latency would almost certainly be
included in the buffer allocation time. At least on the system I'm
working on, I suspect that would add 10s of milliseconds of startup
latency to video pipelines (although I haven't benchmarked the
difference). Doing the blocking as late as possible means most or all
of the latency can be hidden behind other pipeline setup work.

In terms of complexity, I think the synchronization would be basically
the same in either approach, just in different locations. All it would
do is alleviate the need for a callback to fetch the UUID.

-David