USB: Document the QH and qTD antics in EHCI-HCD

The construction of QH and qTD lists in ehci_submit_async() call is cryptic
business, add at least a bit of comments so if someone is reading it, he can at
least reference the intel ehci manual (ehci-r10.pdf).

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>

diff --git a/drivers/usb/host/ehci-hcd.c b/drivers/usb/host/ehci-hcd.c
index a04a6c71d1dd7215d6aa4c2d492a85d135417dd7..04300be1100acb0c7bf23d4d744d0f13d5ed16b3 100644 (file)
--- a/drivers/usb/host/ehci-hcd.c
+++ b/drivers/usb/host/ehci-hcd.c
@@ -234,6 +234,16 @@ ehci_submit_async(struct usb_device *dev, unsigned long pipe, void *buffer,
 
        toggle = usb_gettoggle(dev, usb_pipeendpoint(pipe), usb_pipeout(pipe));
 
+       /*
+        * Setup QH (3.6 in ehci-r10.pdf)
+        *
+        *   qh_link ................. 03-00 H
+        *   qh_endpt1 ............... 07-04 H
+        *   qh_endpt2 ............... 0B-08 H
+        * - qh_curtd
+        *   qh_overlay.qt_next ...... 13-10 H
+        * - qh_overlay.qt_altnext
+        */
        qh.qh_link = cpu_to_hc32((uint32_t)&qh_list | QH_LINK_TYPE_QH);
        c = (usb_pipespeed(pipe) != USB_SPEED_HIGH &&
             usb_pipeendpoint(pipe) == 0) ? 1 : 0;
@@ -255,6 +265,15 @@ ehci_submit_async(struct usb_device *dev, unsigned long pipe, void *buffer,
        tdp = &qh.qh_overlay.qt_next;
 
        if (req != NULL) {
+               /*
+                * Setup request qTD (3.5 in ehci-r10.pdf)
+                *
+                *   qt_next ................ 03-00 H
+                *   qt_altnext ............. 07-04 H
+                *   qt_token ............... 0B-08 H
+                *
+                *   [ buffer, buffer_hi ] loaded with "req".
+                */
                qtd[qtd_counter].qt_next = cpu_to_hc32(QT_NEXT_TERMINATE);
                qtd[qtd_counter].qt_altnext = cpu_to_hc32(QT_NEXT_TERMINATE);
                token = (0 << 31) |
@@ -265,12 +284,22 @@ ehci_submit_async(struct usb_device *dev, unsigned long pipe, void *buffer,
                        debug("unable construct SETUP td\n");
                        goto fail;
                }
+               /* Update previous qTD! */
                *tdp = cpu_to_hc32((uint32_t)&qtd[qtd_counter]);
                tdp = &qtd[qtd_counter++].qt_next;
                toggle = 1;
        }
 
        if (length > 0 || req == NULL) {
+               /*
+                * Setup request qTD (3.5 in ehci-r10.pdf)
+                *
+                *   qt_next ................ 03-00 H
+                *   qt_altnext ............. 07-04 H
+                *   qt_token ............... 0B-08 H
+                *
+                *   [ buffer, buffer_hi ] loaded with "buffer".
+                */
                qtd[qtd_counter].qt_next = cpu_to_hc32(QT_NEXT_TERMINATE);
                qtd[qtd_counter].qt_altnext = cpu_to_hc32(QT_NEXT_TERMINATE);
                token = (toggle << 31) |
@@ -284,11 +313,19 @@ ehci_submit_async(struct usb_device *dev, unsigned long pipe, void *buffer,
                        debug("unable construct DATA td\n");
                        goto fail;
                }
+               /* Update previous qTD! */
                *tdp = cpu_to_hc32((uint32_t)&qtd[qtd_counter]);
                tdp = &qtd[qtd_counter++].qt_next;
        }
 
        if (req != NULL) {
+               /*
+                * Setup request qTD (3.5 in ehci-r10.pdf)
+                *
+                *   qt_next ................ 03-00 H
+                *   qt_altnext ............. 07-04 H
+                *   qt_token ............... 0B-08 H
+                */
                qtd[qtd_counter].qt_next = cpu_to_hc32(QT_NEXT_TERMINATE);
                qtd[qtd_counter].qt_altnext = cpu_to_hc32(QT_NEXT_TERMINATE);
                token = (toggle << 31) |
@@ -298,6 +335,7 @@ ehci_submit_async(struct usb_device *dev, unsigned long pipe, void *buffer,
                    (3 << 10) |
                    ((usb_pipein(pipe) ? 0 : 1) << 8) | (0x80 << 0);
                qtd[qtd_counter].qt_token = cpu_to_hc32(token);
+               /* Update previous qTD! */
                *tdp = cpu_to_hc32((uint32_t)&qtd[qtd_counter]);
                tdp = &qtd[qtd_counter++].qt_next;
        }