linux-kernel - Re: [RFC/PATCH 0/3] Add devicetree scanning for randomness

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Message-ID: <20140218181927.GE29304@obsidianresearch.com>
Date:	Tue, 18 Feb 2014 11:19:27 -0700
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Grant Likely <grant.likely@...aro.org>
Cc:	Jason Cooper <jason@...edaemon.net>, Arnd Bergmann <arnd@...db.de>,
	keescook@...omium.org, devicetree@...r.kernel.org,
	Laura Abbott <lauraa@...eaurora.org>,
	linux-kernel@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
	Kumar Gala <galak@...eaurora.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC/PATCH 0/3] Add devicetree scanning for randomness

On Mon, Feb 17, 2014 at 03:54:19PM +0000, Grant Likely wrote:

> I applied a patch that did exactly that (109b623629), and then reverted
> it (b920ecc82) shortly thereafter because add_device_randomness() is
> a rather slow function and FDTs can get large. I'd like to see someone
> do a reasonable analysis on the cost of using an FDT for randomness
> before I reapply a patch doing something similar. An awful lot of the
> FDT data is not very random, but there are certainly portions of it that
> are appropriate for the random pool.

I read through the original thread from Tim Bird and FWIW I agree with
the assessment that passing the FDT through MD5 first is a good
approach.

Thinking into the future, I'd expect to see similar variable data in
DT on servers as we see in DMI, including:
  - Vendor serial number for the HW, manufacturing date, model number,
    and HW UUID
  - Serial numbers and vendor part numbers for DIMMS
  - MAC addresses for all the ethernet
  - OEM specific data

At worst a 'choosen/linux,no-dt-random = 1' value in the DT to disable
it would solve the problem for those in embedded that care about
microseconds during booting.

Regards,
Jason
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