linux-kernel - Re: [PATCH] OMAP: I2C driver for TI OMAP boards #2

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Date:	Thu, 3 Aug 2006 07:30:40 -0700
From:	David Brownell <david-b@...bell.net>
To:	Jean Delvare <khali@...ux-fr.org>
Cc:	Komal Shah <komal_shah802003@...oo.com>, akpm@...l.org,
	gregkh@...e.de, i2c@...sensors.org, imre.deak@...ia.com,
	juha.yrjola@...idboot.com, linux-kernel@...r.kernel.org,
	r-woodruff2@...com, tony@...mide.com
Subject: Re: [PATCH] OMAP: I2C driver for TI OMAP boards #2

On Thursday 03 August 2006 2:19 am, Jean Delvare wrote:
> The i2c core provides a mechanism to bypass the probing when you know
> for sure what device is at a given address. For an embedded system, that
> should work.

Unfortunately the mechanisms I'm aware of require either error-prone
kernel command line parameters, or (not error prone, but inelegant)
board-specific logic in the drivers, before driver registration, to
do equivalent stuff.

> > (There's a separate issue about how the I2C stack doesn't just have a
> > mechanism to just declare "this board has these chips, these addresses",
> > so I2C drivers have needless reliance on probing...)
> 
> This is being (slowly) addressed by Nathan Lutchansky and Mark M.
> Hoffman. The best solution implies converting the i2c subsystem to the
> device driver model - a non-trivial task.

Glad to hear that fixes are in the works.  That's the same conclusion
I reached:  that I2C needed those non-trivial changes.

It may help to see how the SPI core solves that problem.  Unlike I2C,
SPI actually _can't_ probe (except in rare specialized cases), and when
I did the SPI stuff I was thinking about models that could apply easily
to help I2C avoid probing.  (Though not, at this point, code.)

That model of a table of board-specific declarations (with things like
"I2C chip type X at address A, using interrupt I and platform_data P")
should work for I2C too.

- Dave

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