Re: [RFC PATCH 7/7] drm/prime: Support explicit fence on export

[Lauri Peltonen <lpeltonen@nvidia.com>]

On Mon, Sep 29, 2014 at 09:46:48AM +0200, Daniel Vetter wrote:
> On Fri, Sep 26, 2014 at 01:00:12PM +0300, Lauri Peltonen wrote:
> > Allow user space to provide an explicit sync fence fd when exporting
> > a dma-buf from gem handle.  The fence will be stored as the explicit
> > fence to the reservation object.
> > 
> > Signed-off-by: Lauri Peltonen <lpeltonen@nvidia.com>
> 
> All existing userspace treats dma_bufs as long-lived objects. Well, all
> the userspace that expects implicit syncing, afaik Android shovels lots of
> dma-bufs around all the time (since ion is using them as it's native
> buffer handles).
> 
> So adding an exclusive fence once at export time isn't going to be
> terribly useful.
> 
> So I think a better approach would be to add this as ioctls to the dma-buf
> fd itself. Then you can also add a "give me the fence(s) for this dma_buf"
> ioctl, which is useful for interop the other way round (i.e. implicit ->
> explicit).

Yes, I like this.  I thought that one could support long-lived dma-bufs by
doing a "re-export" when a new fence needs to be attached, but your model is
indeed much nicer!


On Sat, Sep 27, 2014 at 08:49:39AM +0200, Maarten Lankhorst wrote:
> This is never true. A default resv gets allocated, see dma_buf's create
> function.

Ah, ok.  I'll keep that in mind when writing new versions. :-)


Thanks,
Lauri