linux-kernel - Re: [PATCH 03/11] x86/dtb: Add a device tree for CE4100

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Date:	Mon, 29 Nov 2010 11:32:14 -1000
From:	Mitch Bradley <wmb@...mworks.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
CC:	Scott Wood <scottwood@...escale.com>, sodaville@...utronix.de,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	x86@...nel.org, devicetree-discuss@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/11] x86/dtb: Add a device tree for CE4100

On 11/29/2010 10:44 AM, Benjamin Herrenschmidt wrote:
> On Mon, 2010-11-29 at 10:32 -1000, Mitch Bradley wrote:
>> The usual layout is that the PCI bus is a direct child of
>> the root node, and the ISA bus is a child of the PCI bus.
>
> Right, tho we have been relaxing that on SoC for some time now, at least
> on powerpc, since the PCI bus itself tend to hang off one of the SoC
> internal busses (as a sibling of other busses) and that those tend to be
> represented in the tree, so we make PCI be a child of that SoC bus.


That seems fine to me.  It's not important that PCI be directly attached 
to the root bus.  I was mostly concerned about the relative positions of 
ISA and PCI.

>
> This is also useful in the case where you have multiple SoCs (some are
> capable of SMP interconnects) in which case you really have multiple
> separate PCI busses and it's clearer to have each of them be the child
> of its own SoC node.
>
>> That reflects the "Northbridge + Southbridge" wiring that
>> was common at the time that PCI was first introduced.
>> It's usually the case that faster and wider buses are closer
>> to the root, with speed and address width decreasing as you
>> go away from the root.
>>
>> The fact that PCI configuration accesses are done via I/O
>> port 0x3fc doesn't make it a child of the ISA bus, because
>> I/O space is inherent in the x86 CPU architecture and thus
>> can be considered to be part of the root address space.
>>
>> In the systems that I have worked with, the ISA bridge is a
>> first-class PCI device with a PCI config header, so it fits
>> naturally underneath the PCI bus.
>
> This is actually the case of most systems, tho those Atoms SoC are a bit
> weird as, afaik, they don't really have PCI... they just simulate some
> kind of PCI config space for on-chip devices ,at least that's my
> understanding.
>
> Sebastian, do you have a block diagram of the SoC ? Following the actual
> bus hierarchy of the chip might be the best approach.
>
> Cheers,
> Ben.
>
>> Here are the properties for PCI and ISA on the OLPC XO-1.5
>> platform (Via C7 x86 CPU with Via VX855 IO chip):
>>
>>
>> ok dev /pci
>> ok .properties
>> interrupt-map            00000800 00000000 00000000 00000001 ff86bf34 0000000a 00000000
>>                           00006000 00000000 00000000 00000001 ff86bf34 0000000a 00000000
>>                           00008000 00000000 00000000 00000001 ff86bf34 0000000a 00000000
>>                           00008100 00000000 00000000 00000002 ff86bf34 00000009 00000000
>>                           00008200 00000000 00000000 00000003 ff86bf34 0000000b 00000000
>>                           00008400 00000000 00000000 00000004 ff86bf34 0000000a 00000000
>>                           0000a000 00000000 00000000 00000001 ff86bf34 00000009 00000000
>> interrupt-map-mask       0000ff00 00000000 00000000 00000007
>> #interrupt-cells         00000001
>> slot-names               00000000
>> slave-only               00000000
>> clock-frequency          01fca055
>> bus-range                00000000 00000000
>> #size-cells              00000002
>> #address-cells           00000003
>> device_type              pci
>> name                     pci
>>
>> ok dev /pci/isa
>> ok .properties
>> devsel-speed             00000001
>> class-code               00060100
>> subsystem-vendor-id      0000152d
>> subsystem-id             00000833
>> max-latency              00000000
>> min-grant                00000000
>> revision-id              00000000
>> device-id                00008409
>> vendor-id                00001106
>> interrupt-parent         ff86bf34
>> #interrupt-cells         00000002
>> ranges                   00000000 00000000 02000000 00000000 00000000 01000000
>>                           00000001 00000000 01000000 00000000 00000000 00010000
>> clock-frequency          007ea5e0
>> reg                      00008800 00000000 00000000 00000000 00000000
>> #size-cells              00000001
>> #address-cells           00000002
>> device_type              isa
>> name                     isa
>>
>> Note that the PCI node has no reg property.  On a system with multiple independent PCI buses at the top level, it would be necessary to distinguish them with reg properties reflecting their different addresses in the root address space.  PC-style architectures typically (always?) have a single top-level PCI domain, so I've never never needed to do that in x86 land.  It used to be pretty common on PPC "big iron".
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>
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