[PATCH] riscv: align pcie bus assign logic with ARM64

[sun.wenhan at zte.com.cn]

RISC-V currently relies on the asm-generic default of
pcibios_assign_all_busses() returning 1, so the kernel always
re-assigns all PCI bus numbers and ignores the bus numbers already
set up by the firmware. This has two downsides.

As UEFI 2.8 has brought mature firmware support to RISC-V servers,
the OS can trust and inherit the PCI bus enumeration done by the
BIOS, skipping a redundant re-enumeration at boot and improving the
boot time.

The unconditional re-assignment also causes bus numbering conflicts
during PCI rescan. When a root port that is not the last one in the
hierarchy is removed and the bus is rescanned, the kernel fails
with:

    pci_bus 0000:01: busn_res: can not insert [bus 01-ff] under
    [bus 00-ff] (conflicts with (null) [bus 02])

Define pcibios_assign_all_busses() the same way as ARM64, so that
RISC-V also preserves the bus numbers assigned by the firmware by
default instead of always re-assigning them. This keeps RISC-V
consistent with the other major architectures.

Signed-off-by: sun wenhan <sun.wenhan at zte.com.cn>
---
 arch/riscv/include/asm/pci.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/riscv/include/asm/pci.h b/arch/riscv/include/asm/pci.h
index cc2a184cfc2e..b2f91abebd61 100644
--- a/arch/riscv/include/asm/pci.h
+++ b/arch/riscv/include/asm/pci.h
@@ -15,6 +15,11 @@
 #define PCIBIOS_MIN_IO		4
 #define PCIBIOS_MIN_MEM		16

+/*
+ * Set to 1 if the kernel should re-assign all PCI bus numbers
+ */
+#define pcibios_assign_all_busses() (pci_has_flag(PCI_REASSIGN_ALL_BUS))
+
 #if defined(CONFIG_PCI) && defined(CONFIG_NUMA)
 static inline int pcibus_to_node(struct pci_bus *bus)
 {
-- 
2.27.0