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

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Date:	Thu, 20 Feb 2014 16:42:09 +0000
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Mauro Carvalho Chehab <m.chehab@...sung.com>,
	Wolfram Sang <wsa@...-dreams.de>,
	"linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
	Jean Delvare <khali@...ux-fr.org>,
	Guenter Roeck <linux@...ck-us.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rui Wang <ruiv.wang@...il.com>
Subject: RE: [PATCH v6 4/4] i2c, i2c_imc: Add DIMM bus code

> NV-DIMM control registers are exposed via i2c, presumably because
> trying to access them through the memory pins would be a giant mess.
> So, one way or another, something needs to be able to initiate
> transactions to access those registers.  BIOS will do some initial
> setup, but the OS will need to poke at these registers, too.  (The
> actual docs are covered by NDA.  I suspect that this will change if
> the manufacturers ever want these things to be widely used, though,
> since these things really want a full-featured kernel driver so that
> things like pmfs will work cleanly.)
>
> As a secondary benefit, having access to the TSOD and SPD is nice,
> albeit far from critical.
>
> AFAICT Intel actively working on NV-DIMM-related things, so maybe
> Intel will contribute an engineer who help :)

Yes - we have people looking at pmfs and NV-DIMMs.  I don't know
the internal details ... to keep these accesses safe may require letting
the platform BIOS code perform them (via some ACPI thingy) ... messy
and slow - but probably workable if these registers are only required
for some maintenance/configuration usage patterns.  Not so good if
they are in the high frequency read/write path (but I2C in the critical
path sounds like a recipe for failure).

-Tony
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