[Intel-gfx] [PATCH] [CFT] drm/i915: Only set the down rps limit when at the loweset frequency

[Daniel Vetter]

The power docs say that when the gt leaves rc6, it is in the lowest
frequency and only about 25 usec later will switch to the frequency
selected in GEN6_RPNSWREQ. If the downclock limit expires in that
window and the down limit is set to the lowest possible frequency, the
hw will not send the down interrupt. Which leads to a too high gpu
clock and wasted power.

Chris Wilson already worked on this with

commit 7b9e0ae6da0a7eaf2680a1a788f08df123724f3b
Author: Chris Wilson <chris at chris-wilson.co.uk>
Date:   Sat Apr 28 08:56:39 2012 +0100

    drm/i915: Always update RPS interrupts thresholds along with
    frequency

but got the logic inverted: The current code set the down limit as
long as we haven't reached it. Instead of only once with reached the
lowest frequency.

Note that we can't always set the downclock limit to 0, because
otherwise the hw will keep on bugging us with downclock request irqs
once the lowest level is reached.

For similar reasons also always set the upclock limit, otherwise the
hw might poke us again with interrupts.

Signed-Off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/i915/intel_pm.c |   15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
index d0ce2a5..c2a7c9f 100644
--- a/drivers/gpu/drm/i915/intel_pm.c
+++ b/drivers/gpu/drm/i915/intel_pm.c
@@ -2275,13 +2275,18 @@ void gen6_set_rps(struct drm_device *dev, u8 val)
 	limits = 0;
 	if (val >= dev_priv->max_delay)
 		val = dev_priv->max_delay;
-	else
-		limits |= dev_priv->max_delay << 24;
-
-	if (val <= dev_priv->min_delay)
+	limits |= dev_priv->max_delay << 24;
+
+	/* Only set the down limit when we've reached the lowest level to avoid
+	 * getting more interrupts, otherwise leave this clear. This prevents a
+	 * race in the hw when coming out of rc6: There's a tiny window where
+	 * the hw runs at the minimal clock before selecting the desired
+	 * frequency, if the down threshold expires in that window we will not
+	 * receive a down interrupt. */
+	if (val <= dev_priv->min_delay) {
 		val = dev_priv->min_delay;
-	else
 		limits |= dev_priv->min_delay << 16;
+	}
 
 	if (val == dev_priv->cur_delay)
 		return;
-- 
1.7.10.4