drm/i915: Use atomic waits for short non-atomic ones
usleep_range is not recommended for waits shorten than 10us.
Make the wait_for_us use the atomic variant for such waits.
To do so we need to reimplement the _wait_for_atomic macro to
be safe with regards to preemption and interrupts.
v2: Reimplement _wait_for_atomic to be irq and preemption safe.
(Chris Wilson and Imre Deak)
v3: Fixed in_atomic check due rebase error.
v4: Build bug on non-constant timeouts.
v5: Compile away cpu migration code in atomic paths.
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1467114710-29989-1-git-send-email-tvrtko.ursulin@linux.intel.com
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