> From: Bjorn Helgaas [mailto:bhelgaas@xxxxxxxxxx] > On Wed, May 27, 2015 at 08:58:38PM +0000, Tang, Jason (ES) wrote: > > ... > > Q1: Doesn't PCI Hotplug already solve this problem? > > A1: From what I can tell, the existing PCI Hotplug infrastructure only > > works with PCI endpoints; it has no way to handle hot adding a > > bridge, and then later on hot adding an endpoint. > > __pci_bus_size_bridges() uses pci_hotplug_mem_size (default 2MB, > overridable with "pci=hpmemsize=") to reserve space for things that may be > hot-added below a bridge that supports hotplug. The thing that's added may > be a bridge, and that bridge might support hotplug, and it should work at > hot-add a device below that bridge. Although memory could be reserved for a hot added bridge, I see no way to reserve bus numbers for the bridge. > > Static enumeration can also be used in cases where the memory > > behind a bridge needs to be changed due to faulty hardware. > > Can you elaborate on this a bit? Are you saying you have a piece of > defective hardware, and you can avoid the defect by assigning different > addresses to it? Correct, in cases where some faulty hardware requests less memory than it really needs. > I would expect to see a dev->is_hotplug_bridge test in the bridge > enumeration path (maybe somewhere in pci_scan_bridge()) so we could > reserve some bus numbers for future hot adds. But I don't, so maybe > this is just a bug. The two places where I see 'is_hotplug_bridge' set are in quirk_hotplug_bridge() and in set_pcie_hotplug_bridge(). Both of these check that the bridge is a particular type (a PLX 6254, or if the device has the PCI_>QP_SLTCAP_HPC capability). What I propose is a more general purpose that works for any PCI bridge. I too could not find anywhere within upstream code that reserves bus numbers. I suppose the next revision of these patches could: 1. Let user mark a bridge as hotpluggable, even if it does not have the capability. 2. If that bridge is hotpluggable, then allow user to reserve bus numbers. > There's a lot of this stuff that works fairly well as long as the BIOS has > preconfigured things. If your BIOS doesn't do that, I could certainly > believe it wouldn't work as well. I would prefer to make Linux work better > and depend less on BIOS if we can. Yes, these patches will override BIOS settings. At least on my system, there is no way to force the BIOS to statically enumerate devices. -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
