linux-kernel - Re: [PATCH v6 4/4] i2c, i2c

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Date:	Fri, 28 Feb 2014 12:14:22 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Mauro Carvalho Chehab <m.chehab@...sung.com>
Cc:	Jean Delvare <khali@...ux-fr.org>,
	"linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Wolfram Sang <wsa@...-dreams.de>,
	Rui Wang <ruiv.wang@...il.com>,
	"Luck, Tony" <tony.luck@...el.com>,
	Guenter Roeck <linux@...ck-us.net>
Subject: Re: [PATCH v6 4/4] i2c, i2c_imc: Add DIMM bus code

On Feb 28, 2014 11:15 AM, "Mauro Carvalho Chehab" <m.chehab@...sung.com> wrote:
>
> Em Wed, 19 Feb 2014 17:39:07 -0800
> Andy Lutomirski <luto@...capital.net> escreveu:
>
> > On Wed, Feb 19, 2014 at 11:03 AM, Luck, Tony <tony.luck@...el.com> wrote:
> > >> (I'm c/c Tony here, as he also shared the same concern that I had on a
> > >> previous feedback about using I2C to talk with the DIMM).
> > >
> > > Correct - I've heard the same issues that reads on I2C can be misinterpreted
> > > as writes ... and oops, you have a brick.
> >
> > Is this true on DDR3 DIMMs, i.e. anything that's compatible with
> > LGA2011?  If you plug a DIMM into an LGA2011 board's memory slot,
> > then, one way or another, it's very likely that there will be TSOD
> > traffic, if for no other purpose than to determine that there is no
> > TSOD present.  TSOD traffic consists of reads and writes, both with
> > and without register numbers.  (Sorry, I can never remember the smbus
> > terminology here -- the relevant transactions are two-byte reads and
> > two-byte writes, both with a command specified and without one.  One
> > of the bits in the iMC SMBUS registers tells the controller which kind
> > of read to use to probe the thermometer.)
>
> An update on that: I double-checked with a DIMM manufacturer.
> I was told that some DIMM models have write protect circuits but
> others don't.
>
> So, yeah, accessing it could eventually brick the DIMM, depending
> on the DIMM model, if such driver won't block I2C write ops or
> if the BIOS is also trying to access the bus at the same time.

I'm not sure I buy the argument about blocking writes -- as far as I
know, i2c-i801 on desktop platforms has exactly the same issue.  Or
maybe I'm misunderstanding you.

I'd be okay with adding code to block writes to SPD addresses, but I'm
not sure I see the point.  If root wants to brick a DIMM, I'm sure
root can find a way.

On the other hand, racing with BIOS is a real problem.  I'd like to
know how Intel plans to handle this.  I suspect that POLL_EN is the
right bit to use, but the docs are vague, and there could be strange
BIOSes out there.

(Grr.  Everything would be saner if SMM didn't exist.)

--Andy
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