linux-kernel - Re: [PATCH] eeprom: at24: Add support for large EEPROMs connected to SMBus adapters

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Date:	Thu, 5 Feb 2015 00:35:16 +0100
From:	Wolfram Sang <wsa@...-dreams.de>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] eeprom: at24: Add support for large EEPROMs connected to
 SMBus adapters

On Wed, Feb 04, 2015 at 11:08:19AM -0800, Guenter Roeck wrote:
> On Wed, Feb 04, 2015 at 06:47:23PM +0100, Wolfram Sang wrote:
> > 
> > On Wed, Feb 04, 2015 at 08:23:37AM -0800, Guenter Roeck wrote:
> > > Large EEPROMS (24c32 and larger) require a two-byte data address
> > > instead of just a single byte. Implement support for such EEPROMs
> > > with SMBus commands.
> > > 
> > > Support has limitations (reads are not multi-master safe) and is slow,
> > > but it works. Practical use is for a system with 24c32 connected to
> > > Intel 82801I (ICH9).
> > 
> > Can't you simply use i2c-dev to access the EEPROM? In multi-master
> > environments, things can really go wrong, so I wouldn't like to add
> > something dangerous by default. Maybe with a module parameter named
> > "allow-multimaster-unsafe-access-to-large-eeproms-with-smbus" which is
> > default off. But I'd really prefer the i2c-dev solution. Hooking a 16bit
> > EEPROM to SMBus is daring, after all. SMBus is multi-master, too.
> > 
> Hi Wolfram,
> 
> At the same time multi-master access is quite rare.

It should still work in those rare cases. A setup of an SoC, an EC
(those like to be masters, as well), and an EEPROM is not very far off.

> Also, many of the kernel's i2c drivers are not multi-master-access
> clean. In many cases that isn't even possible due to the chip
> architecture (a good example are chips with multiple 'pages', where
> the page address is set with one i2c command and the actual access
> occurs with subsequent commands).

Huh, using i2c_transfer to ensure 'repeated start' between messages
should do the trick, no? Drivers doing this via SMBus calls are broken
and should be fixed.

And what do you think of my suggestion enabling your mode with a module
parameter?


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