On Thu, Aug 29, 2024 at 11:32:47AM +0200, Lukas Wunner wrote: > On Wed, Aug 28, 2024 at 05:15:24PM -0400, Esther Shimanovich wrote: > > On Sun, Aug 25, 2024 at 10:49???AM Lukas Wunner <lukas@xxxxxxxxx> wrote: > > > On Fri, Aug 23, 2024 at 04:53:16PM +0000, Esther Shimanovich wrote: > > > > --- a/drivers/pci/probe.c > > > > +++ b/drivers/pci/probe.c > > > > +static bool pcie_has_usb4_host_interface(struct pci_dev *pdev) > > > > +{ > > > > + struct fwnode_handle *fwnode; > > > > + > > > > + /* > > > > + * For USB4, the tunneled PCIe root or downstream ports are marked > > > > + * with the "usb4-host-interface" ACPI property, so we look for > > > > + * that first. This should cover most cases. > > > > + */ > > > > + fwnode = fwnode_find_reference(dev_fwnode(&pdev->dev), > > > > + "usb4-host-interface", 0); > > > > > > This is all ACPI only, so it should either be #ifdef'ed to CONFIG_ACPI > > > or moved to drivers/pci/pci-acpi.c. > > > > > > Alternatively, it could be moved to arch/x86/pci/ because ACPI can also > > > be enabled on arm64 or riscv but the issue seems to only affect x86. > > > > Thanks for the feedback! Adding an #ifdef to CONFIG_ACPI seems more > > straightforward, but I do like the idea of not having unnecessary code > > run on non-x86 systems. > > > > I'd appreciate some guidance here. How would I move a portion of a > > function into a completely different location in the kernel src? > > Could you show me an example? > > One way to do this would be to move pcie_is_tunneled(), > pcie_has_usb4_host_interface() and pcie_switch_directly_under() > to arch/x86/pci/acpi.c. > > Rename pcie_is_tunneled() to arch_pci_dev_is_removable() and remove > the "static" declaration specifier from that function. > > Add a function declaration for arch_pci_dev_is_removable() to > include/linux/pci.h. > > Add a __weak arch_pci_dev_is_removable() function which just returns > false in drivers/pci/probe.c right above pci_set_removable(). > > And that's it. > > See pcibios_device_add() for an example. > > That's one way to do it. It ensures that the code is only compiled > on x86 and only if CONFIG_ACPI=y. Basically the linker picks the > arch_pci_dev_is_removable() in arch/x86/pci/acpi.c, or the empty > __weak function of the same name on !x86 or if CONFIG_ACPI=n. > > An alternative approach would involve using an empty static inline. > I think the difference is that an empty static inline is optimized > away by the compiler, whereas the empty __weak function is not > optimized away by the compiler, but may be optimized away by the > linker if CONFIG_LTO=y. > > For the static inline it's basically the same but you omit the > __weak arch_pci_dev_is_removable() in drivers/pci/probe.c and > instead constrain the function declaration in include/linux/pci.h to: > #if defined(CONFIG_X86) && defined(CONFIG_ACPI) > ...and the #else branch would contain the empty static inline > which just returns false. > > See pci_mmcfg_early_init() for an example. > > Maybe the empty static inline is better because then the entire > "if (arch_pci_dev_is_removable(...))" clause can be optimized away > without reliance on CONFIG_LTO=y. Was there ever any followup on this? Do we need any? This uses fwnode_find_reference("usb4-host-interface"), and while "usb4-host-interface" is only defined for ACPI systems (as far as I know), the fwnode_find_reference() interface itself is not ACPI-specific. So maybe this is OK as-is, and it will just never find that property on non-ACPI systems? Bjorn