uhci: Retry to fill the queue while waiting for td completion

If the guest is using multiple transfers to try and keep the usb bus busy /
used at maximum efficiency, currently we would see / do the following:

1) submit transfer 1 to the device
2) submit transfer 2 to the device
3) report transfer 1 completion to guest
4) report transfer 2 completion to guest
5) submit transfer 1 to the device
6) report transfer 1 completion to guest
7) submit transfer 2 to the device
8) report transfer 2 completion to guest
etc.

So after the initial submission we would effectively only have 1 transfer
in flight, rather then 2. This is caused by us not checking the queue for
addition of new transfers by the guest (ie the resubmission of a recently
finished transfer), while waiting for a pending transfer to complete.
This patch does add a check for this, changing the sequence to:

1) submit transfer 1 to the device
2) submit transfer 2 to the device
3) report transfer 1 completion to guest
4) submit transfer 1 to the device
5) report transfer 2 completion to guest
6) submit transfer 2 to the device
etc.

Thus keeping 2 transfers in flight (most of the time, and always 1),
as intended by the guest.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/usb/hcd-uhci.c

@@ -110,7 +110,7 @@ struct UHCIQueue {
     UHCIState *uhci;
     USBEndpoint *ep;
     QTAILQ_ENTRY(UHCIQueue) next;
-    QTAILQ_HEAD(, UHCIAsync) asyncs;
+    QTAILQ_HEAD(asyncs_head, UHCIAsync) asyncs;
     int8_t    valid;
 };
 
@@ -843,15 +843,25 @@ static int uhci_handle_td(UHCIState *s, UHCIQueue *q, uint32_t qh_addr,
     }
 
     if (async) {
-        if (!async->done)
-            return TD_RESULT_ASYNC_CONT;
         if (queuing) {
             /* we are busy filling the queue, we are not prepared
                to consume completed packages then, just leave them
                in async state */
             return TD_RESULT_ASYNC_CONT;
         }
+        if (!async->done) {
+            UHCI_TD last_td;
+            UHCIAsync *last = QTAILQ_LAST(&async->queue->asyncs, asyncs_head);
+            /*
+             * While we are waiting for the current td to complete, the guest
+             * may have added more tds to the queue. Note we re-read the td
+             * rather then caching it, as we want to see guest made changes!
+             */
+            uhci_read_td(s, &last_td, last->td_addr);
+            uhci_queue_fill(async->queue, &last_td);
 
+            return TD_RESULT_ASYNC_CONT;
+        }
         uhci_async_unlink(async);
         goto done;
     }