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. Thierry
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