drm/i915/psr: Make idle_frames sensible again

This reverts

commit dfaf37baa07513d2c37afff79978807d2d10221a
Author: Rodrigo Vivi <rodrigo.vivi@intel.com>
Date:   Mon Dec 7 14:45:20 2015 -0800

    drm/i915: Fix idle_frames counter.

and

commit 97173eaf5f33b1e85efdb06d593d333480b60bf3
Author: Rodrigo Vivi <rodrigo.vivi@intel.com>
Date:   Tue Jul 7 16:28:55 2015 -0700

    drm/i915: PSR: Increase idle_frames

and implements

commit d44b4dcbd1b44737462b77971d216d21a9413341
Author: Rodrigo Vivi <rodrigo.vivi@intel.com>
Date:   Fri Nov 14 08:52:31 2014 -0800

    drm/i915: HSW/BDW PSR Set idle_frames = VBT + 1

without the hack to use 2 idle frames when VBT says 1. We keep the + 1
just for safety, although I haven't really figured out why that one
exists.

It's nonsense. idle_frames = number of frames where the screen is
entirely idle before we think about entering PSR.

idle_patter = part of link training, and we probably totally butchered
link training because we told the hw to entirely skip it. No wonder
PSR occasionally just fell over.

I suspect the reason we've increased idle frames is that it makes PSR
entry slightly less likely, and more likely to happen in a quite
system, which probably increased the changes the panel came back up
without link training. The proper fix is to implement link training
for PSR.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Sonika Jindal <sonika.jindal@intel.com>
Cc: Durgadoss R <durgadoss.r@intel.com>
Cc: "Pandiyan, Dhinakaran" <dhinakaran.pandiyan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463590036-17824-3-git-send-email-daniel.vetter@ffwll.ch

drivers/gpu/drm/i915/intel_psr.c

@@ -272,14 +272,14 @@ static void hsw_psr_enable_source(struct intel_dp *intel_dp)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 
 	uint32_t max_sleep_time = 0x1f;
-	/*
-	 * Let's respect VBT in case VBT asks a higher idle_frame value.
-	 * Let's use 6 as the minimum to cover all known cases including
-	 * the off-by-one issue that HW has in some cases. Also there are
-	 * cases where sink should be able to train
-	 * with the 5 or 6 idle patterns.
+	/* Lately it was identified that depending on panel idle frame count
+	 * calculated at HW can be off by 1. So let's use what came
+	 * from VBT + 1.
+	 * There are also other cases where panel demands at least 4
+	 * but VBT is not being set. To cover these 2 cases lets use
+	 * at least 5 when VBT isn't set to be on the safest side.
 	 */
-	uint32_t idle_frames = max(6, dev_priv->vbt.psr.idle_frames);
+	uint32_t idle_frames = dev_priv->vbt.psr.idle_frames + 1;
 	uint32_t val = EDP_PSR_ENABLE;
 
 	val |= max_sleep_time << EDP_PSR_MAX_SLEEP_TIME_SHIFT;