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From: John Baldwin <jhb@freebsd.org>
To: Mariusz Gromada <mariusz.gromada@gmail.com>
Date: Tue, 25 Sep 2012 17:13:00 -0400
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Cc: Ben Laurie <benl@freebsd.org>, freebsd-security@freebsd.org,
	RW <rwmaillists@googlemail.com>,
	Jonathan Anderson <jonathan.anderson@cl.cam.ac.uk>,
	Pawel Jakub Dawidek <pjd@freebsd.org>
Subject: Re: Collecting entropy from device_attach() times.
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On Tuesday, September 25, 2012 4:05:34 pm Mariusz Gromada wrote:
> Our task is to check if there are any autocorrelations in the X(w,t) 
> process, which means checking if there are any dependencies between 
> random variables X(w,t1) and X(w,t2) where t1 < t2.

Just to state an obvious fact (not sure how that impacts your analysis 
though):  There are, of course, many dependencies among device attach routines 
since your total time for the attach routine for a bus is going to include all 
of the time it takes for attach to run on all of the child devices.  That is, 
pci0's attach time includes the attach time of all of it's descendant devices, 
and a given leaf node's attach time will be accounted for in the attach time 
of all of its parent nodes up to the root.  For example:

nexus0
  acpi0
    pcib0
      pci0
        ehci0
          usbus0
            uhub0
              uhub3
                uhub4
                  ukbd0

In this portion of my desktop's device tree, all of the devices listed will 
include the time of ukbd0's attach in their respective attach times.

-- 
John Baldwin