[PATCH] drm/radeon: Add support for userspace fence waits

[Michel Dänzer]

On Die, 2012-01-31 at 22:08 +0100, Marek Olšák wrote: 
> 2012/1/31 Jerome Glisse <j.glisse at gmail.com>:
> > On Tue, Jan 31, 2012 at 06:56:01PM +0100, Michel Dänzer wrote:
> >> On Die, 2012-01-31 at 16:59 +0000, Simon Farnsworth wrote:
> >> > Userspace currently busywaits for fences to complete; on my workload, this
> >> > busywait consumes 10% of the available CPU time.
> >> >
> >> > Provide an ioctl so that userspace can wait for an EOP interrupt that
> >> > corresponds to a previous EVENT_WRITE_EOP.
> >> >
> >> > Signed-off-by: Simon Farnsworth <simon.farnsworth at onelan.co.uk>
> >> > ---
> >> > I've been working on top of Jerome's tiling patches, so this doesn't apply
> >> > directly on top of current upstream kernels. I can easily rebase to another
> >> > version upon request - just point me to a git tree.
> >> >
> >> > My goal is to remove the sched_yield in Mesa's r600_fence_finish given up to
> >> > date enough kernel; I hope, though, that the interface is clean enough for
> >> > other users to extend it in the future (e.g. using compute rings).
> >>
> >> I'm afraid not: Unless I'm missing something, userspace can't know which
> >> ring the kernel submitted the CS to, and the kernel can't guess which
> >> ring userspace needs to wait for.
> >
> > iirc the plan was to add a return value to cs ioctl and add an ioctl to
> > allow to wait on this return value. ie allowing userspace to wait on
> > specific submited cs.
> 
> You don't need a new API for that, r300g already does that. It adds a
> dummy relocation and later uses GEM_WAIT_IDLE to wait for it. r600g
> can be updated to do the same thing without kernel changes (besides,
> we must support the old kernels as well, so this is a no-brainer).

One minor problem being that this doesn't support a timeout without
spinning. Shouldn't be relevant for Simon's problem though.


> What would be much more useful is to be able to wait for a fence,
> which can be in the middle of a CS. Now that's something that would
> justify changes in the kernel interface.

To take advantage of that, one would also need to change Gallium such
that it's possible to get a fence without a flush.


-- 
Earthling Michel Dänzer           |                   http://www.amd.com
Libre software enthusiast         |          Debian, X and DRI developer