[Intel-gfx] [CI 22/25] drm/i915: Track the previous pinned context inside the request

[Chris Wilson]

On Thu, Apr 28, 2016 at 07:15:23PM +0100, Dave Gordon wrote:
> On 28/04/16 09:56, Chris Wilson wrote:
> >As the contexts are accessed by the hardware until the switch is completed
> >to a new context, the hardware may still be writing to the context object
> >after the breadcrumb is visible. We must not unpin/unbind/prune that
> >object whilst still active and so we keep the previous context pinned until
> >the following request. We can generalise the tracking we already do via
> >the engine->last_context and move it to the request so that it works
> >equally for execlists and GuC.
> >
> >v2: Drop the execlists double pin as that exposes a race inside the lrc
> >irq handler as it tries to access the context after it may be retired.
> >
> >Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> >Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> >Reviewed-by: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> >---
> 
> Hmmm .. not sure this is going to play well with request reordering
> in the scheduler, or with preemption! I think it makes it a bit more
> difficult; perhaps we will just have to clear it for all preempted
> requests.

This field is maintained by whoever queues the requests to hardware. It
simplifies greatly the issue of tracking when the context is no longer
being written to by hardware.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre