drm/i915: Avoid the gpu reset vs. modeset deadlock

... using the biggest hammer we have. This is essentially a weaponized
version of the timeout-based wedging Chris added in

commit 36703e79
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Thu Jun 22 11:56:25 2017 +0100

    drm/i915: Break modeset deadlocks on reset

Because defense-in-depth is good it's good to still have both. Also
note that with the locking change we can now restrict this a lot (old
gpus and special testing only), so this doesn't kill the TDR benefits
on at least anything remotely modern.

And futuremore with a few tricks it should be possible to make a much
more educated guess about whether an atomic commit is stuck waiting on
the gpu (atomic_t counting the pending i915_sw_fence used by the
atomic modeset code should do it), so we can improve this.

But for now just start with something that is guaranteed to recover
faster, for much better CI througput.

This defacto reverts TDR on these platforms, but there's not really a
single commit to specify as the sole offender.

v2: Add a debug message to explain what's going on. We can't DRM_ERROR
because that spams CI. And the timeout based fallback still prints a
DRM_ERROR, in case something goes wrong.

v3: Fix comment layout (Michel)

Fixes: 4680816b ("drm/i915: Wait first for submission, before waiting for request completion")
Fixes: 221fe799 ("drm/i915: Perform a direct reset of the GPU from the waiter")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> (v2)
Cc: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> (v2)
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170808080828.23650-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/i915/intel_display.c

@@ -3681,6 +3681,13 @@ void intel_prepare_reset(struct drm_i915_private *dev_priv)
 	    !gpu_reset_clobbers_display(dev_priv))
 		return;
 
+	/* We have a modeset vs reset deadlock, defensively unbreak it.
+	 *
+	 * FIXME: We can do a _lot_ better, this is just a first iteration.
+	 */
+	i915_gem_set_wedged(dev_priv);
+	DRM_DEBUG_DRIVER("Wedging GPU to avoid deadlocks with pending modeset updates\n");
+
 	/*
 	 * Need mode_config.mutex so that we don't
 	 * trample ongoing ->detect() and whatnot.