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The ability to emulate a host bridge is useful not only for hardware PCI controllers like CONFIG_VMD, or virtual PCI controllers like CONFIG_PCI_HYPERV, but also for test and development scenarios like CONFIG_SAMPLES_DEVSEC [1]. One stumbling block for defining CONFIG_SAMPLES_DEVSEC, a sample implementation of a platform TSM for PCI Device Security, is the need to accommodate PCI_DOMAINS_GENERIC architectures alongside x86 [2]. In support of supplementing the existing CONFIG_PCI_BRIDGE_EMUL infrastructure for host bridges: * Introduce pci_bus_find_emul_domain_nr() as a common way to find a free PCI domain number whether that is to reuse the existing dynamic allocation code in the !ACPI case, or to assign an unused domain above the last ACPI segment. * Convert pci-hyperv to the new allocator so that the PCI core can unconditionally assume that bridge->domain_nr != PCI_DOMAIN_NR_NOT_SET is the dynamically allocated case. A follow on patch can also convert vmd to the new scheme. Currently vmd is limited to CONFIG_PCI_DOMAINS_GENERIC=n (x86) so, unlike pci-hyperv, it does not immediately conflict with this new pci_bus_find_emul_domain_nr() mechanism. Link: http://lore.kernel.org/174107249038.1288555.12362100502109498455.stgit@dwillia2-xfh.jf.intel.com [1] Reported-by: Suzuki K Poulose <suzuki.poulose@arm.com> Closes: http://lore.kernel.org/20250311144601.145736-3-suzuki.poulose@arm.com [2] Signed-off-by: Dan Williams <dan.j.williams@intel.com> Signed-off-by: Bjorn Helgaas <bhelgaas@google.com> Tested-by: Suzuki K Poulose <suzuki.poulose@arm.com> Tested-by: Michael Kelley <mhklinux@outlook.com> Reviewed-by: Michael Kelley <mhklinux@outlook.com> Cc: Lorenzo Pieralisi <lpieralisi@kernel.org> Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org> Cc: Rob Herring <robh@kernel.org> Cc: K. Y. Srinivasan <kys@microsoft.com> Cc: Haiyang Zhang <haiyangz@microsoft.com> Cc: Wei Liu <wei.liu@kernel.org> Cc: Dexuan Cui <decui@microsoft.com> Link: https://patch.msgid.link/20251024224622.1470555-2-dan.j.williams@intel.com