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

[Jerome Glisse]

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.

Cheers,
Jerome