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

[Marek Olšák]

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).

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.

Marek