Message-ID: <43ABB76B.8000301@ru.mvista.com>
Date: Fri, 23 Dec 2005 11:38:03 +0300
From: Vitaly Wool <vwool@ru.mvista.com>
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To: David Brownell <david-b@pacbell.net>
CC: linux-kernel@vger.kernel.org, spi-devel-general@sourceforge.net
Subject: Re: [PATCH 2.6-git] SPI: add set_clock() to bitbang
References: <20051222180449.4335a8e6.vwool@ru.mvista.com> <200512221637.07895.david-b@pacbell.net> <43ABA27C.6020309@ru.mvista.com> <200512230028.03682.david-b@pacbell.net>
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David Brownell wrote:

>>>How exactly that's done is system-specific.  Many controllers
>>>just have a register per chipselect, listing stuff like SPI mode,
>>>clock divisor, and word size.  So switching to that chipselect
>>>kicks those in automatically ... devices ignore the clock unless
>>>they've been selected.
>>>      
>>>
>>Hmm, usually clocks are configured for the bus not device.
>>    
>>
>
>Not a chance.  The clock is activated to talk to a given device;
>and there's no requirement that all devices on the bus use the
>same clock rate.  (If one chipselect gives access to a linked series
>of devices, clearly they'll all need to be clocked alike.  But
>that's not a bus, it's just a compound device ... like a big shift
>register.)
>
>I did my homework when putting that API together, and looked at
>quite a few SPI controllers.  **Not one** of them forces all
>their chipselets to use the same clock rate.
>  
>
I admit that thw word 'usually' is incorrect here, but still we have two 
Philips ARM boards where the SPI clock is configured _only_ on the bus, 
by setting the bus clock divisor on per-message basis. I was also 
keeping in mind PXA, so it wasn't just bare words...

Vitaly
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