linux-kernel - Re: [PATCH] of: Support a PCI device that is compatible with 'simple-bus'

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Message-ID: <20121214215814.GA14149@obsidianresearch.com>
Date:	Fri, 14 Dec 2012 14:58:14 -0700
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	linux-kernel@...r.kernel.org, devicetree-discuss@...ts.ozlabs.org,
	Rob Herring <rob.herring@...xeda.com>
Subject: Re: [PATCH] of: Support a PCI device that is compatible with
 'simple-bus'

On Fri, Dec 14, 2012 at 08:26:29PM +0000, Grant Likely wrote:

> > > If the soc_devices are getting triggered on that and they shouldn't be,
> > > then we need a mechanism in the soc_bridge node to kick out of that
> > > behavoir for its children.
> > 
> > Is this what you were thinking?
> 
> Not really. I see what you're trying to do, but doing it this way forces
> all children of PCI nodes to use the PCI addressing space. Others have
> had simple children of PCI devices and didn't use the PCI address layout
> at all. Those users would break with this approach.

Yes, that's right.

If you drop 'device_type=pci' from the PCI device (keep it on the host
bridge), then you can setup a ranges down to a smaller width and
things seem to work OK. That must be what other users are doing.

However, you can't stay at address-cells=3 for the children. That
doesn't work.

So, if you have separate PCI regions, like MMIO and prefetch it looks
like this works OK:

 pci_device@0 {
   ranges =
        // MMIO region, BAR 0
       <0x20000000 0x00000000  0x02000000 0x00000000 0x00000000  0x0 0x8000000
        // Prefetch region, BAR 1
        0x40000000 0x00000000  0x42000000 0x00000000 0x00000000  0x0 0x8000000>;

   #size-cells = <1>;
   #address-cells = <2>;
   sub {
     // MMIO region at BAR 0 offset 0x2000
     reg = <0x20000000 0x00002000 0x1000>;
   }
   sub2 {
     // Prefetch region at BAR 1 offset 0x4000
     reg = <0x40000000 0x00004000 0x1000>;
   }
 }

Which is weird, but OK..

This is good enough for my application.. fixing up address-cells=3 to
work generally seems pretty complicated at first blush?

> However, if you want to pass a unity mapping from the PCI device to the
> a child of it, it should be sufficient to use an empty 'ranges;'
> property in the PCI device node instead of listing out the ranges that
> you want to translate.

It isn't a unity mapping - the children see address 0 as being the
start of a BAR. The DTS has three levels of translation:

- platform device child - 0 is the start of a BAR in the pci device
- pci device - 0 is the start of the host bridge memory window for the
  BAR's type
- pci controller - 0 is the start of physical memory

Thanks,
Jason
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