[+cc Takashi, Andy, Colin, Myron for potential distro impact] [Beginning of thread: https://lore.kernel.org/linux-pci/20181102181055.130531-1-brian.woods@xxxxxxx/] On Sat, Nov 03, 2018 at 12:29:48AM +0100, Borislav Petkov wrote: > On Fri, Nov 02, 2018 at 02:59:25PM -0500, Bjorn Helgaas wrote: > > This isn't my code, and I'm not really objecting to these changes, but > > from where I sit, the fact that you need this sort of vendor-specific > > topology discovery is a little bit ugly and seems like something of a > > maintenance issue. I think this is the most important part, and I should have elaborated on it instead of getting into the driver structure details below. It is a major goal of ACPI and PCI that an old kernel should work unchanged on a new platform unless it needs to use new functionality introduced in the new platform. amd_nb.c prevents us from achieving that goal. These patches don't add new functionality; they merely describe minor topographical differences in new hardware. We usually try to do that in a more generic way, e.g., via an ACPI method, so the new platform can update the ACPI method and use an old, already-qualified, already-shipped kernel. I'm not strenuously objecting to these because this isn't a *huge* deal, but I suspect it is a source of friction for distros that don't want to update and requalify their software for every new platform. > > You could argue that this is sort of an "AMD CPU > > driver", which is entitled to be device-specific, and that does make > > some sense. > > It is a bunch of glue code which enumerates the PCI devices a CPU > has and other in-kernel users can use that instead of doing the > discovery/enumeration themselves. > > > But device-specific code is typically packaged as a driver that uses > > driver registration interfaces like acpi_bus_register_driver(), > > pci_register_driver(), etc. That gives you a consistent structure > > and, more importantly, a framework for dealing with hotplug. It > > doesn't look like amd_nb.c would deal well with hot-add of CPUs. > > If you mean physical hotadd, then that's a non-issue as, AFAIK, AMD > doesn't support that. > > Now, TBH I've never tried soft-offlining the cores of a node and then > check whether using the PCI devices of that node would work. > > Now, I don't mind this getting converted to a proper PCI driver as long > as it is not a module as it has to be present at all times. Other than > that, I'm a happy camper. amd_nb.c uses pci_get_device(), which is incompatible with hotplug and subverts the usual driver/device ownership model. We could pursue this part of the conversation, but I think it's more fruitful to approach this from the "new machine, old kernel" angle above.
