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 a04a6c7..04300be 100644
--- a/drivers/usb/host/ehci-hcd.c
+++ b/drivers/usb/host/ehci-hcd.c
@@ -234,6 +234,16 @@
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 @@
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 @@
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 @@
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 @@
(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;
}