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Date: Tue, 22 Dec 2009 21:06:17 +0200
From: Felix Radensky <felix@embedded-sol.com>
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To: Wolfgang Grandegger <wg@grandegger.com>
Subject: Re: I2C bus clock on MPC85XX systems
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Hi, Wolfgang

Wolfgang Grandegger wrote:
> Felix Radensky wrote:
>   
>> Hi, Wolfgang
>>
>> Wolfgang Grandegger wrote:
>>     
>>> Felix Radensky wrote:
>>>  
>>>       
>>>> Hi,
>>>>
>>>> Almost all MPC85XX based systems have the compatible=:"fsl-i2c" in
>>>> respective
>>>> i2c device tree nodes. This causes FSL i2c driver to use the following
>>>> "backward
>>>> compatible" values: FSR=0x31 DFSR=0x10. This is regardless of CCB clock
>>>> frequency and i2c clock prescaler.
>>>>
>>>> On my custom MPC8536 based board with 432MHz CCB clock this results in
>>>> 65KHz i2c clock frequency (checked with scope). U-Boot correctly
>>>> configures
>>>> the clock to 400KHz.
>>>>
>>>> I've fixed the problem by modifying device tree to use different
>>>> compatible value,
>>>> similar to what socrates board does. Is this the right approach ?
>>>>     
>>>>         
>>> Are you aware of the properties described in
>>> "Documentation/powerpc/dts-bindings/fsl/i2c.txt":
>>>
>>> http://lxr.linux.no/#linux+v2.6.32/Documentation/powerpc/dts-bindings/fsl/i2c.txt
>>>
>>>
>>> Wolfgang.
>>>
>>>   
>>>       
>> Sure, I'm aware of these properties. I've used
>>
>> compatible = "fsl,mpc8543-i2c", "fsl-i2c";
>> clock-frequency = <400000>;
>>
>> for my custom board.
>>
>> I think, however, that device trees for FSL reference designs should use
>> them as well, to avoid setting i2c
>> clock to some strange values. I may be wrong, but I think most custom
>>     
>
> The FSR=0x31 DFSR=0x10 are not strange values. They have been used in
> mainline kernels (< 2.6.30) for a long time.
>
>   
>> board developers borrow from
>> reference device trees, so having a sane starting point would help.
>>     
>
> The trees provided by Freescale are usually based on older kernel
> version. Borrow from such trees is OK, but the project developers should
> use a recent kernel version for development.
>   

I was talking about modern vanilla kernels for Freescale reference 
boards. See, for example
http://lxr.linux.no/#linux+v2.6.32/arch/powerpc/boot/dts/mpc8536ds.dts

Felix.