usb: optimize acpi companion search for usb port devices

commit ed18c5fa945768a9bec994e786edbbbc7695acf6 upstream.

This optimization significantly reduces xhci driver load time.

In ACPI tables the acpi companion port devices are children of
the hub device. The port devices are identified by their port number
returned by the ACPI _ADR method.
_ADR 0 is reserved for the root hub device.

The current implementation to find a acpi companion port device
loops through all acpi port devices under that parent hub, evaluating
their _ADR method each time a new port device is added.

for a xHC controller with 25 ports under its roothub it
will end up invoking ACPI bytecode 625 times before all ports
are ready, making it really slow.

The _ADR values are already read and cached earler. So instead of
running the bytecode again we can check the cached _ADR value first,
and then fall back to the old way.

As one of the more significant changes, the xhci load time on
Intel kabylake reduced by 70%, (28ms) from
initcall xhci_pci_init+0x0/0x49 returned 0 after 39537 usecs
to
initcall xhci_pci_init+0x0/0x49 returned 0 after 11270 usecs

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

0 files changed, 0 insertions, 0 deletions