usb: ehci-pci: Clarify and cleanup the EHCI controller detection

The detection function of the EHCI PCI controller was really cryptic,
add a beefy comment and clean the portion of the code up a bit. No
change in the logic of the code.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>

diff --git a/drivers/usb/host/ehci-pci.c b/drivers/usb/host/ehci-pci.c
index 7a1ffe5e28bbc1fd6bf9dc806649e49a30d1ec2c..991b19998b7e407a214f698939284577d61cb4da 100644 (file)
--- a/drivers/usb/host/ehci-pci.c
+++ b/drivers/usb/host/ehci-pci.c
@@ -54,9 +54,31 @@ static pci_dev_t ehci_find_class(int index)
                                        bdf += PCI_BDF(0, 0, 1)) {
                                pci_read_config_dword(bdf, PCI_CLASS_REVISION,
                                                      &class);
-                               if ((class >> 8 == PCI_CLASS_SERIAL_USB_EHCI)
-                                               && !index--)
-                                       return bdf;
+                               class >>= 8;
+                               /*
+                                * Here be dragons! In case we have multiple
+                                * PCI EHCI controllers, this function will
+                                * be called multiple times as well. This
+                                * function will scan the PCI busses, always
+                                * starting from bus 0, device 0, function 0,
+                                * until it finds an USB controller. The USB
+                                * stack gives us an 'index' of a controller
+                                * that is currently being registered, which
+                                * is a number, starting from 0 and growing
+                                * in ascending order as controllers are added.
+                                * To avoid probing the same controller in tne
+                                * subsequent runs of this function, we will
+                                * skip 'index - 1' detected controllers and
+                                * report the index'th controller.
+                                */
+                               if (class != PCI_CLASS_SERIAL_USB_EHCI)
+                                       continue;
+                               if (index) {
+                                       index--;
+                                       continue;
+                               }
+                               /* Return index'th controller. */
+                               return bdf;
                        }
                }
        }