Per CXL spec r3.1, sec 8.1.5.2, the Secondary Bus Reset (SBR) bit in the Bridge Control register of a CXL port has no effect unless the "Unmask SBR" bit is set. Return -ENOTTY if we attempt a bus reset on a device below a CXL Port where "Unmask SBR" is 0. Otherwise, the bus reset would appear to have succeeded even though setting the bridge SBR bit had no effect. Link: https://lore.kernel.org/linux-cxl/20240220203956.GA1502351@bhelgaas/ Link: https://lore.kernel.org/r/20240502165851.1948523-4-dave.jiang@intel.com Signed-off-by: Dave Jiang <dave.jiang@intel.com> [bhelgaas: simplify commit log and comments] Signed-off-by: Bjorn Helgaas <bhelgaas@google.com> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com> Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com> Reviewed-by: Dan Williams <dan.j.williams@intel.com>