The unconditionally fallback to the blocking wait_for resulted in
impressive fireworks at boot-up on my snb here. Make sure if we set
the slow timeout to 0 that we never ever sleep. The tail of the
callchain was

intel_wait_for_register
-> __intel_wait_for_register_fw
  -> usleep_range
     -> BOOM

It blew up in intel_crt_detect load detection code on the
ADPA_CRT_HOTPLUG_FORCE_TRIGGER in the ADPA register.

Fixes: 0564654340e2 ("drm/i915: Acquire uncore.lock over 
intel_uncore_wait_for_register()")
Cc: Chris Wilson <[email protected]>
Cc: Michal Wajdeczko <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Jani Nikula <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
---
 drivers/gpu/drm/i915/intel_uncore.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_uncore.c 
b/drivers/gpu/drm/i915/intel_uncore.c
index aa9d3065853c..b03ad06bc3b6 100644
--- a/drivers/gpu/drm/i915/intel_uncore.c
+++ b/drivers/gpu/drm/i915/intel_uncore.c
@@ -1609,7 +1609,7 @@ int __intel_wait_for_register_fw(struct drm_i915_private 
*dev_priv,
        ret = -ETIMEDOUT;
        if (fast_timeout_us && fast_timeout_us <= 20000)
                ret = _wait_for_atomic(done, fast_timeout_us, 0);
-       if (ret)
+       if (ret && slow_timeout_ms)
                ret = wait_for(done, slow_timeout_ms);
 
        if (out_value)
-- 
2.5.5

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