We use the request to ensure we hold a reference to the context for the
duration that it remains in use by the ring. Each request only holds a
reference to the current context, hence we emit a request after
switching contexts with the final reference to the old context. However,
the extra interrupt caused by that request is not useful (no timing
critical function will wait for the context object), instead the overhead
of servicing the IRQ shows up in some (lightweight) benchmarks. In order
to keep the useful property of using the request to manage the context
lifetime, we want to add a dummy request that is associated with the
interrupt from the subsequent real request following the batch.

The extra interrupt was added as a side-effect of using
i915_add_request() in

commit 112522f6
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Thu May 2 16:48:07 2013 +0300

    drm/i915: put context upon switching

v2: Daniel convinced me that the request here was solely for context
lifetime tracking and that we have the active ref to keep the object
alive whilst the MI_SET_CONTEXT. So the only concern then is which
context should get the blame for MI_SET_CONTEXT failing. The old scheme
added a request for the old context so that any hang upto and including
the switch away would mark the old context as guilty. Now any hang here
implicates the new context. However since we have already gone through a
complete flush with the last context in its last request, and all that
lies in no-man's-land is an invalidate flush and the MI_SET_CONTEXT, we
should be safe in not unduly placing blame on the new context.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>