Re: [PATCH v5 2/4] arm64: dts: APM X-Gene PCIe device tree nodes

[Liviu Dudau <Liviu.Dudau@arm.com>]

On Thu, Apr 17, 2014 at 02:24:34AM +0100, Jason Gunthorpe wrote:
> On Thu, Apr 17, 2014 at 01:20:42AM +0100, Liviu Dudau wrote:
> 
> > > No spec says you can put config space into the ranges at all, nobody
> > > should be doing that today, obviously some cases were missed during
> > > review..
> > 
> > ePAPR documents allows that when ss == 00.
> 
> Which do you mean? The 'PCI Bus Binding' spec has fairly specific
> language on how ranges should be used and interpreted, and it
> precludes doing anything meaningful with config space (like requiring
> b,d,f and r to be zeroed when doing compares against ranges, requiring
> the ranges to represent the bridge windows, etc).
> 
> There is certainly room to invent something (like ECAM mapping) but
> nothing is specified in that document.

On more carefull reading of the Power_ePAPR_APPROVED_v1.0.pdf document
that I have I agree, there is no meaningful way of describing one's
config ranges.

> 
> The ePAPR document I have doesn't talk about PCI..
> 
> If you've found a document that defines how it works then that changes
> things.. ;)
> 
> > > The comment about ECAM was intended as a general guidance on what
> > > config space in ranges could/should be used for.
> > > 
> > > Right now config space shouldn't propagate out side any driver, so you
> > > can probably just filter it in your generic code, and make it very hard
> > > and obviously wrong for a driver to parse ranges for config space, so
> > > we don't get more usages.
> > 
> > OK, this goes slightly against your email from 26th March:
> > 
> > "When we talked about this earlier on the DT bindings list the
> > consensus seemed to be that configuration MMIO ranges should only be
> > used if the underlying memory was exactly ECAM, and was not to be used
> > for random configuration related register blocks.
> > 
> > The rational being that generic code, upon seeing that ranges entry,
> > could just go ahead and assume ECAM mapping."
> > 
> > What I'm saying is that the only code that will see this ranges entry will
> > be the parsing code as if we try to create a resource out of the range
> > and add it to the host bridge structure (not driver) we will confuse the
> > rest of the pci_host_bridge API. So we cannot do any ECAM accesses (yet?).
> 
> Sorry if this seems unclear, what you quoted was from a specification
> standpoint - someday defining config space ranges to be the ECAM
> window makes the most sense. This is from the direction of precluding
> drivers from using it for random purposes.
> 
> From a Linux standpoint, there is simply no infrastructure for generic
> config access outside the driver, so config space must remain
> contained in the driver, and shouldn't leak into the host bridge or
> other core structures.
> 
> I think the shared code you are working on should simply ignore config
> ss ranges entirely, they have no defined meaning..

Agree. Less things to code for is always better!

Best regards,
Liviu

> 
> Regards,
> Jason
> 

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