[Intel-gfx] [PATCH v2 07/15] drm/i915: Protect the request->global_seqno with the engine->timeline lock

[Chris Wilson]

On Wed, Feb 22, 2017 at 12:29:00PM +0000, Tvrtko Ursulin wrote:
> 
> On 22/02/2017 11:46, Chris Wilson wrote:
> >A request is assigned a global seqno only when it is on the hardware
> >execution queue. The global seqno can be used to maintain a list of
> >requests on the same engine in retirement order, for example for
> >constructing a priority queue for waiting. Prior to its execution, or
> >if it is subsequently removed in the event of preemption, its global
> >seqno is zero. As both insertion and removal from the execution queue
> >may operate in IRQ context, it is not guarded by the usual struct_mutex
> >BKL. Instead those relying on the global seqno must be prepared for its
> >value to change between reads. Only when the request is complete can
> >the global seqno be stable (due to the memory barriers on submitting
> >the commands to the hardware to write the breadcrumb, if the HWS shows
> >that it has passed the global seqno and the global seqno is unchanged
> >after the read, it is indeed complete).
> 
> Missed some questions I've raised on this one in the previous round.

Just missed it when scanning for opens from the threads.
 
> I never got round re-reading it if you were waiting for that by any chance.

Looks like it is one you have to read after seeing where the series
goes.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre