Re: i915_chipset_val()

[Konstantin Belousov <kostikbel@gmail.com>]

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On Fri, Jul 08, 2011 at 10:42:02AM +0100, Chris Wilson wrote:
> On Fri, 8 Jul 2011 12:26:51 +0300, Konstantin Belousov <kostikbel@gmail.com> wrote:
> > i915_chipset_val saves the jiffies count of the invocation in last_time1.
> > Then, on the next call, the diff between current jiffies value and
> > last_time1 is used as divisor.
> > 
> > I have a suspicious that two rapid calls to i915_chipset_val() may result
> > in division by zero. This looks as user-controllable action, since
> > debugfs, if configured, would export i915_chipset_val() as emon status.
> > 
> > I did not tested the Linux, but in the (ported) code I am able to get
> > into the described situation. As a workaround, I cached the previous
> > return value from i915_chipset_val() and return it instead of doing
> > the calculation if consequtive calls are close enough.
> > 
> > What do you think ? (patch is not directly applicable to Linux).
> 
> You're right. But I would go even further and say that if the difference
> is less than say 10ms then we have not accumulated enough samples for the
> calculation to be stable.
> 
> >  unsigned long i915_mch_val(struct drm_i915_private *dev_priv)
> > diff --git a/sys/dev/drm/i915_drv.h b/sys/dev/drm/i915_drv.h
> > index 5e6340b..36d066a 100644
> > --- a/sys/dev/drm/i915_drv.h
> > +++ b/sys/dev/drm/i915_drv.h
> > @@ -579,6 +579,7 @@ typedef struct drm_i915_private {
> >  
> >  	u64 last_count1;
> >  	unsigned long last_time1;
> > +	unsigned long last_chipset_val;
> >  	u64 last_count2;
> >  	struct timespec last_time2;
> >  	unsigned long gfx_power;
> 
> We need to thank Jesse for such informative variable names. Not even a
> comment to mention that they are part of IPS.

I agree, I did the following, relative to the previous patch

diff --git a/sys/dev/drm/i915_dma.c b/sys/dev/drm/i915_dma.c
index 773a171..07100f4 100644
--- a/sys/dev/drm/i915_dma.c
+++ b/sys/dev/drm/i915_dma.c
@@ -1592,9 +1592,9 @@ unsigned long i915_chipset_val(struct drm_i915_private *dev_priv)
 	 * sysctl(8) reads the value of sysctl twice in rapid
 	 * succession.  There is high chance that it happens in the
 	 * same timer tick.  Use the cached value to not divide by
-	 * zero.
+	 * zero and give the hw a chance to gather more samples.
 	 */
-	if (diff1 == 0)
+	if (diff1 <= 10)
 		return (dev_priv->last_chipset_val);
 
 	count1 = I915_READ(DMIEC);

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