Question on UAPI for fences

[Daniel Vetter]

On Fri, Sep 12, 2014 at 10:50:49AM -0400, Jerome Glisse wrote:
> On Fri, Sep 12, 2014 at 04:43:44PM +0200, Daniel Vetter wrote:
> > On Fri, Sep 12, 2014 at 4:09 PM, Daniel Vetter <daniel at ffwll.ch> wrote:
> > > On Fri, Sep 12, 2014 at 03:23:22PM +0200, Christian König wrote:
> > >> Hello everyone,
> > >>
> > >> to allow concurrent buffer access by different engines beyond the multiple
> > >> readers/single writer model that we currently use in radeon and other
> > >> drivers we need some kind of synchonization object exposed to userspace.
> > >>
> > >> My initial patch set for this used (or rather abused) zero sized GEM buffers
> > >> as fence handles. This is obviously isn't the best way of doing this (to
> > >> much overhead, rather ugly etc...), Jerome commented on this accordingly.
> > >>
> > >> So what should a driver expose instead? Android sync points? Something else?
> > >
> > > I think actually exposing the struct fence objects as a fd, using android
> > > syncpts (or at least something compatible to it) is the way to go. Problem
> > > is that it's super-hard to get the android guys out of hiding for this :(
> > >
> > > Adding a bunch of people in the hopes that something sticks.
> > 
> > More people.
> 
> Just to re-iterate, exposing such thing while still using command stream
> ioctl that use implicit synchronization is a waste and you can only get
> the lowest common denominator which is implicit synchronization. So i do
> not see the point of such api if you are not also adding a new cs ioctl
> with explicit contract that it does not do any kind of synchronization
> (it could be almost the exact same code modulo the do not wait for
> previous cmd to complete).

I don't think we should cathegorically exclude this, since without some
partial implicit/explicit world we'll never convert over to fences. Of
course adding fences without any way to at least partially forgoe the
implicit syncing is pointless. But that might be some other user (e.g.
camera capture device) which needs explicit fences.

> Also one thing that the Android sync point does not have, AFAICT, is a
> way to schedule synchronization as part of a cs ioctl so cpu never have
> to be involve for cmd stream that deal only one gpu (assuming the driver
> and hw can do such trick).

You need to integrate the android stuff with your (new) cs ioctl, with a
input parameter for the fence fd to wait on before executing the cs and
one that gets created to signal when it's all done.

Same goes for all the other places android wants sync objects, e.g. for
synchronization before atomic flips and for signalling completion of the
same.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch