Re: [PATCH] drm/i915: Keep ring->active_list and ring->requests_list consistent

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Mar 20, 2015 at 02:45:04PM +0000, Chris Wilson wrote:
> On Fri, Mar 20, 2015 at 03:32:52PM +0100, Daniel Vetter wrote:
> > But if we do that short-circuiting in ring_idle the all the requests
> > _should_ be completed. Which meanse retire_request_ring should move all
> > buffers to the inactive list, even when we do that before retiring
> > requests.
> 
> We test for the requests to be retired after we test for the buffers to
> be retired. It is very easy then for us to have active buffers as the
> seqno advanced after the buffer retirement and before the requests. That
> is (one of) the reasons why we previously sampled seqno only once when
> retiring buffers + requests.

Yeah I get that part of the race. But before we retire anything in these
callsites we call gpu_idle. And that waits for everything to complete,
except whent there are not outstanding requests (i.e. ->request_list is
empyt). So either
- ->request_list is empty in ring_idle, which means all requests should
  have completed.  Even if there are some lingering active buffers still
  around we should clean them up.
- ->request_list is not empty, in which case we do a full wait for the
  most recent request. Again all requests should have completed and we
  should be able to clean out both request and active lists.

I do see how we can get out of the retire_request functions with requests
empty but still active buffers around. But I don't understand how that's
possible with a gpu_idle in front. And thus far all traces are from places
where we do call gpu_idle first.

Or am I missing something?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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