PCI bus node location

[David Gibson]

On Mon, Nov 09, 2009 at 07:36:57PM -0700, Grant Likely wrote:
> On Mon, Nov 9, 2009 at 12:20 PM, Rafal Jaworowski <raj at semihalf.com> wrote:
> > Hi,
> > I have a couple of questions regarding host/PCI bridges nodes location:
> >
> > - What is the reason most of the DTS definitions have the host/PCI bridges
> > hanging off the root node, even though they are most often really part of
> > the soc?
> > - Is this is some OF heritage (I couldn't find anything explicit about it in
> > the original PCI bindings docs)?
> > - Is this convention enforced in FDT, or could PCI bus nodes be children of
> > the soc node as well?
> 
> It was a solution to an engineering problem.  The PCI control
> registers are indeed within the IMMR region, and when we first started
> doing PowerPC FDT board ports, the PCI node was a child of the SoC
> node.  However, since PCI is also bridge with its own address space
> translation, having it live in the SoC node causes difficulties.
> Specifically, all of the entries in the PCI node ranges property would
> need similar counterparts in the SoC node ranges property; a scheme
> that doesn't reflect well the actual behaviour of the IMMR region.
> 
> Two alternate solutions were proposed.  One was to split the PCI node
> into a PCI bridge node which describes the translations, and a PCI
> control node which describes how to access the PCI bridge registers.
> Some sort of linkage (probably a phandle) would be needed to relate
> the two.  The second was to simply move the PCI node out to be a
> parent of the root.  The second option was the one chosen because it
> was the path of least resistance.  It may not be the most 'correct'
> solution, but it has worked out quite well in practice.  There is
> nothing in the PCI or FDT infrastructure code that enforces this
> convention.  In fact, if the appropriate ranges properties were added
> back to the IMMR node, then the PCI node could become a child of the
> IMMR node without any code changes (but it still wouldn't be a 100%
> 'correct' description of the hardware).

Right.

Under the new scheme, the "soc" node is really a historical misnomer -
it represents just the things within the IMMR, not everything on the
SoC.  A number of chips also have the localbus controller as a
separate node, likewise within the SoC but not within the IMMR, so not
a child of the soc node.

Note also that 4xx chips, unlike the Freescale ones do have the PCI
host bridge under the plb node (which represents the main bus on the
SoC).

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
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