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SubjectRe: [PATCH 05/14] lib: Add I/O map cache implementation
On Thu, Jan 10, 2013 at 09:17:19AM +0000, Arnd Bergmann wrote:
> On Thursday 10 January 2013, Thierry Reding wrote:
> > On Wed, Jan 09, 2013 at 04:17:58PM -0700, Jason Gunthorpe wrote:
> > > On Wed, Jan 09, 2013 at 04:12:31PM -0700, Stephen Warren wrote:
> > > You could decrease the size of the mapping to only span the bus
> > > numbers that are configured for use via DT.
> >
> > That won't work, unfortunately. The mapping is such that the bus number
> > is not encoded in the uppermost bits, the extended register number is.
> > So the only thing that we could do is decrease the size of the extended
> > register space for *all* devices.
> But you could still a method to map 16 separate areas per bus, each 65536
> bytes long, which results in 1MB per bus. That is probably ok, since
> very few systems have more than a handful of buses in practice.
> In theory, doing static mappings on a per-page base would let you
> do 16 devices at a time, but it's probably worth doing at this fine
> granularity.

I don't understand how this would help. The encoding is like this:

[27:24] extended register number
[23:16] bus number
[15:11] device number
[10: 8] function number
[ 7: 0] register number

So it doesn't matter whether I use separate areas per bus or not. As
soon as the whole extended configuration space needs to be accessed a
whopping 28 bits (256 MiB) are required.

What you propose would work if only regular configuration space is
supported. I'm not sure if that's an option.

> > > Are there any concerns about these config registers being accessed
> > > from a context where a new mapping can't be made? Interrupt? Machine
> > > Check? PCI-E Advanced Error Reporting?
> >
> > I haven't checked but I would expect configuration space accesses to not
> > happen in interrupt context. Usually they are limited to enumeration and
> > driver probe.
> Actually, AER probably needs this, and I believe some broken devices
> need to mask interrupts using the PCI command word in the config space,
> it it can happen.

Ugh... that would kill any such dynamic mapping approach. Perhaps if we
could mark a device as requiring a static mapping we could pin that
cache entry. But that doesn't sound very encouraging.

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