On Wed, May 31, 2023 at 01:04:36AM +0300, Vladimir Oltean wrote: > On Tue, May 30, 2023 at 04:58:55PM -0500, Bjorn Helgaas wrote: > > Can you write this description in terms of PCI topology? The > > nitty-gritty SERDES details are not relevant at this level, except to > > say that Function 0 is present in some cases but not others, and when > > it is not present, *other* functions may be present. > > No. It is to say that within the device, all PCIe functions (including 0) > are always available and have the same number, but depending on SERDES > configuration, their PCIe presence might be practically useful or not. > So that's how function 0 may end having status = "disabled" in the > device tree. > > > Sigh. Per spec (PCIe r6.0, sec 7.5.1.1.9), software is not permitted > > to probe for Functions other than 0 unless "explicitly indicated by > > another mechanism, such as an ARI or SR-IOV Capability." > > > > Does it "work" to probe when the spec prohibits it? Probably. Does > > it lead to some breakage elsewhere eventually? Quite possibly. They > > didn't put "software must not probe" in the spec just to make > > enumeration faster. > > > > So I'm a little grumpy about further complicating this already messy > > path just to accommodate a new non-compliant SoC. Everybody pays the > > price of understanding all this stuff, and it doesn't seem in balance. > > > > Can you take advantage of some existing mechanism like > > PCI_SCAN_ALL_PCIE_DEVS or hypervisor_isolated_pci_functions() (which > > could be renamed and made more general)? > > Not responding yet to the rest of the email since it's not clear to me > that you've understood function 0 is absolutely present and responds > to all config space accesses - it's just disabled in the device tree > because the user doesn't have something useful to do with it. Ah, you're right, sorry I missed that. Dispensing with the SERDES details would make this more obvious. Not sure why this needs to change the pci_scan_slot() path, since Function 0 is present and enumerable even though it's not useful in some cases. Seems like something in pci_set_of_node() or a quirk could do whatever you need to do. Bjorn
