drm/i915/gt: Restrict forced preemption to the active context

drm/i915/gt: Restrict forced preemption to the active context

When we submit a new pair of contexts to ELSP for execution, we start a
timer by which point we expect the HW to have switched execution to the
pending contexts. If the promotion to the new pair of contexts has not
occurred, we declare the executing context to have hung and force the
preemption to take place by resetting the engine and resubmitting the
new contexts.

This can lead to an unfair situation where almost all of the preemption
timeout is consumed by the first context which just switches into the
second context immediately prior to the timer firing and triggering the
preemption reset (assuming that the timer interrupts before we process
the CS events for the context switch). The second context hasn't yet had
a chance to yield to the incoming ELSP (and send the ACk for the
promotion) and so ends up being blamed for the reset.

If we see that a context switch has occurred since setting the
preemption timeout, but have not yet received the ACK for the ELSP
promotion, rearm the preemption timer and check again. This is
especially significant if the first context was not schedulable and so
we used the shortest timer possible, greatly increasing the chance of
accidentally blaming the second innocent context.

Fixes: 3a7a92aba8fb ("drm/i915/execlists: Force preemption")
Fixes: d12acee84ffb ("drm/i915/execlists: Cancel banned contexts on schedule-out")
Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Tested-by: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: <stable@vger.kernel.org> # v5.5+
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220921135258.1714873-1-andrzej.hajda@intel.com