linux-kernel - Re: [PATCH 09/14] i2c: Add Device Tree support to the Nomadik I2C driver

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Message-ID: <4FDB1FFD.7040204@linaro.org>
Date:	Fri, 15 Jun 2012 12:43:57 +0100
From:	Lee Jones <lee.jones@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	linus.walleij@...ricsson.com, arnd@...db.de,
	Linus Walleij <linus.walleij@...aro.org>,
	linux-kernel@...r.kernel.org, grant.likely@...retlab.ca,
	linux-i2c@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 09/14] i2c: Add Device Tree support to the Nomadik I2C
 driver

On 15/06/12 11:32, Russell King - ARM Linux wrote:
> On Thu, Jun 14, 2012 at 07:57:32PM +0100, Lee Jones wrote:
>> On 14/06/12 19:36, Mark Brown wrote:
>>> On Wed, Jun 13, 2012 at 01:28:17PM +0100, Lee Jones wrote:
>>>
>>>> Device Tree. However, we have just as much control by keeping them
>>>> in separate structs in the C file and selecting the right one using
>>>> the compatible sting.
>>>
>>> You're not understanding Linus' point.  The compatible string isn't
>>> useful here because properties like the maximum clock rate of the bus
>>> depend on the board design, not the silicon.  The controller may be
>>> perfectly happy to run at a given rate but other devices on the bus or
>>> the electrical engineering of the PCB itself may restrict this further.
>>
>> And you're not understanding mine. ;)
>>
>> You can have multiple compatible strings for a single driver. Which one
>> you reference from the Device Tree will dictate which group of settings
>> are used, including variation of clock rates.
>
> However, if you list these settings separately, rather than selecting them
> based on a compatible string, it means when other board designs come along,
> you don't have to modify the kernel code to provide yet another compatible
> to settings translation in the code - you can keep all that information
> entirely within DT with no kernel code mods.
>
> I thought that was partly the point of DT...

Right. See my most recent emails on the subject. :)

-- 
Lee Jones
Linaro ST-Ericsson Landing Team Lead
M: +44 77 88 633 515
Linaro.org │ Open source software for ARM SoCs
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