usb: xhci: add comments explaining specific interrupt behaviour

HCD does not allocate or request interrupt for the xhci driver, but HCD
does free and sync xhci interrupts in some cases. Add comment detailing
in which cases HCD will free/sync xhci interrupts.

Signed-off-by: Niklas Neronin <niklas.neronin@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20240905143300.1959279-9-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Epicurius

drivers/usb/host/xhci-pci.c

@@ -93,6 +93,10 @@ static const struct xhci_driver_overrides xhci_pci_overrides __initconst = {
 	.update_hub_device = xhci_pci_update_hub_device,
 };
 
+/*
+ * Primary Legacy and MSI IRQ are synced in suspend_common().
+ * All MSI-X IRQs and secondary MSI IRQs should be synced here.
+ */
 static void xhci_msix_sync_irqs(struct xhci_hcd *xhci)
 {
 	struct usb_hcd *hcd = xhci_to_hcd(xhci);
@@ -105,13 +109,12 @@ static void xhci_msix_sync_irqs(struct xhci_hcd *xhci)
 	}
 }
 
-/* Free any IRQs and disable MSI-X */
+/* Legacy IRQ is freed by usb_remove_hcd() or usb_hcd_pci_shutdown() */
 static void xhci_cleanup_msix(struct xhci_hcd *xhci)
 {
 	struct usb_hcd *hcd = xhci_to_hcd(xhci);
 	struct pci_dev *pdev = to_pci_dev(hcd->self.controller);
 
-	/* return if using legacy interrupt */
 	if (hcd->irq > 0)
 		return;