linux-kernel - Re: [spi-devel-general] [PATCH 3/4] spi: Add OF binding support for SPI busses

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Message-Id: <200805221926.24112.david-b@pacbell.net>
Date:	Thu, 22 May 2008 19:26:23 -0700
From:	David Brownell <david-b@...bell.net>
To:	spi-devel-general@...ts.sourceforge.net
Cc:	"Grant Likely" <grant.likely@...retlab.ca>,
	"Segher Boessenkool" <segher@...nel.crashing.org>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
	fabrizio.garetto@...il.com,
	Guennadi Liakhovetski <g.liakhovetski@....de>
Subject: Re: [spi-devel-general] [PATCH 3/4] spi: Add OF binding support for SPI busses

On Wednesday 21 May 2008, Grant Likely wrote:
> > spi-controller {
> >        #address-cells = 2;
> >        #size-cells = 0;
> >        some-device@0,f000 { reg = < 0 f000 >; } // CS 0, SPI address f000
> >        some-device@1,f000 { reg = < 1 f000 >; } // CS 1, SPI address f000
> >        some-device@1,ff00 { reg = < 1 ff00 >; } // CS 1, SPI address ff00
> > }
> 
> For SPI the CS # *is* the address.  :-)
> 
> Unlike I2C, SPI doesn't impose any protocol on the data.  It is all
> anonymous data out, anonymous data in, a clock and a chip select.

Very true ... but then there are SPI chips which embed addressing.

I have in mind the mcp23s08 (and mcp23s17) GPIO expanders, which
support up to four chips wired in parallel on a given chipselect.
The devices are distinguished by how two address pins are wired;
and two bits in the command byte must match them.  (I think they
just recycled an I2C design into the SPI world.)

- Dave
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