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SubjectRe: [PATCH 2.6-git] SPI: add set_clock() to bitbang
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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