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).
diff --git a/sys/dev/drm/i915_dma.c b/sys/dev/drm/i915_dma.c
index a22643f..773a171 100644
--- a/sys/dev/drm/i915_dma.c
+++ b/sys/dev/drm/i915_dma.c
@@ -1588,9 +1588,14 @@ unsigned long i915_chipset_val(struct drm_i915_private
*dev_priv)
int i;
diff1 = now - dev_priv->last_time1;
+ /*
+ * 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.
+ */
+ if (diff1 == 0)
+ return (dev_priv->last_chipset_val);
count1 = I915_READ(DMIEC);
count2 = I915_READ(DDREC);
@@ -1622,7 +1627,8 @@ unsigned long i915_chipset_val(struct drm_i915_private
*dev_priv)
dev_priv->last_count1 = total_count;
dev_priv->last_time1 = now;
- return ret;
+ dev_priv->last_chipset_val = ret;
+ return (ret);
}
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;
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