[RFC] Rework of i2c-mpc.c - Freescale i2c driver

Grant Likely grant.likely at secretlab.ca
Tue Nov 6 10:03:09 EST 2007

On 11/5/07, Scott Wood <scottwood at freescale.com> wrote:
> Matt Sealey wrote:
> > Scott Wood wrote:
> >> Jon Smirl wrote:
> >
> >>>>> cell-index = <1>;
> >>>> What is cell-index for?
> >>> I was using it to control the bus number, is that the wrong
> >>> attribute?
> >>
> >> It shouldn't be specified at all -- the hardware has no concept of
> >> a device number.
> >
> > Well, all i2c devices have a chip id you can probe for,
> I meant a controller device number (a.k.a. bus number), which (outside
> of documentation) is purely a Linux invention, and which is what
> cell-index was being used for above.
> > as for buses I think cell-index is a holdover from the way the PSC
> > code is organised on the MPC5200 for example - if you have multiple
> > buses which use the same registers, for example. It's redundant on
> > the PSC's for programming because they all use different register
> > offsets but if you move to other devices like the GPTs, then it is
> > then useful for debugging (it is far more interesting to say GPT1
> > than GPT @ offset to match the)

Actually, it is not intended for this.  cell-index is not intended to
be able to say GPT1 instead of GPT at xxx (while that may be possible, it
is not the intent).  Nor is it a holdover from the PSC code design.
It is designed to describe the internal structure of the SoC.  The
5200 has 6 PSCs, and there are some chip registers which are shared
between all the PSCs (for clocking, IO mode, etc).  It is not
sufficient to simply plop down a device tree node for each PSC because
it doesn't give enough information about which bits to use in the
shared regs.  (For example, the port_config register)

> > and in general for tweaking OTHER
> > parts of the chip (for instance the CDM - very relevant!) which use
> > single registers to control entire swathes of units.
> Right, that's what cell-index is for.  This is different. :-)

Yes, this is correct.


Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.
grant.likely at secretlab.ca
(403) 399-0195

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