Re: [PATCH v7 38/50] powerpc/powernv: Exclude root bus in pnv_pci_reset_secondary_bus()

Gavin Shan [off-list ref] writes:

When pnv_pci_reset_secondary_bus() is called to issue reset on
the indicated secondary bus, the bus can't be root bus. So we
needn't consider root bus in the function.

It took me a while to convince myself that this is correct. For the
record, this is why it's correct:

pnv_pci_reset_secondary_bus fills the reset_secondary_bus callback in
the pci_controller_ops structure, and isn't used elsewhere.

In arch/powerpc/kernel/pci.c, that callback is called (if it exists) in 
pcibios_reset_secondary_bus(). It's not called anywhere else.

The PPC pcibios_reset_secondary_bus overrides the weak version in
drivers/pci/pci.c. It's called from the same file by
pci_reset_bridge_secondary_device() (and nowhere else).

pci_reset_bridge_secondary_device() is nicely documented:

/**
 * pci_reset_bridge_secondary_bus - Reset the secondary bus on a PCI bridge.
 * @dev: Bridge device
 *
 * Use the bridge control register to assert reset on the secondary bus.
 * Devices on the secondary bus are left in power-on state.
 */

Therefore, by the definiton of pci_reset_bridge_secondary_bus,
pnv_pci_reset_secondary_bus() can only be called with a bridge
device. As such, a bridge reset only is appropriate. If this breaks
anything, the caller is broken.

It might be worth including a condensed version of this in the commit
message.

Reviewed-by: Daniel Axtens <redacted>

Regards,
Daniel Axtens