linux-kernel - Re: [PATCH] rtc-ds1307: True SMBus compatibility

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Message-ID: <20090107142426.4be04d4d@hyperion.delvare>
Date:	Wed, 7 Jan 2009 14:24:26 +0100
From:	Jean Delvare <khali@...ux-fr.org>
To:	Ed Swierk <eswierk@...stanetworks.com>
Cc:	linux-i2c@...r.kernel.org, David Brownell <david-b@...bell.net>,
	Alessandro Zummo <a.zummo@...ertech.it>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	BARRE Sebastien <sbarre@...lcc.com>
Subject: Re: [PATCH] rtc-ds1307: True SMBus compatibility

Hi Ed,

On Mon, 05 Jan 2009 09:41:01 -0800, Ed Swierk wrote:
> [Resent to correct linux-i2c address]
> 
> Following up to Sebastien Barre's recent patch, here I go one step
> further, replacing i2c block transfers with SMBus byte transfers.

Please, no. Adding a compatibility quirk may be acceptable, but forcing
everyone to use it instead of the more efficient original code is not
fair.

> Sticking to pure SMBus makes the driver work on our board, which has an
> nVidia SMBus controller driving a DS1339; nforce2 controllers
> unfortunately don't support any flavor of i2c block transfer.

Are you certain the nForce2 controllers can't do it? The i2c-nforce2
driver doesn't implement it, but this doesn't mean the hardware can't
do it. I don't have any datasheet for these chips, but I know their
SMBus implementation is very similar to those of the AMD 8111, and
i2c-amd8111 has support for I2C block reads and writes. I think it
would be worth giving it a try, by copying the i2c-amd8111
implementation into the i2c-nforce2 driver and seeing if it happens to
just work. If it works, that would be more elegant than your proposed
hack to the rtc-ds1307 driver.

> Reading or writing registers repeatedly until they stick is rather
> nauseating but I'm aiming for correctness if not elegance.  I would
> appreciate any suggestions for improvement.

See above.

-- 
Jean Delvare
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