ohci: set effectively usb frame rate to 1kHz

USB frame rate is slightly lower than 1kHz: ie. ~950Hz.
Thus usb-audio device is not able to perform a simple audio playback
without underruns on audio backend.
eg. "-device pci-ohci,id=ohci -device usb-audio,bus=ohci.0" vs PulseAudio
backend. more than 50 underruns are observed per second.

Update ohci_sof_time computation, using QEMU_CLOCK_VIRTUAL in
ohci_usb_start(), and increment by usb_frame_time in ohci_sof()
makes USB frame rate close to 1kHz.
This way, no audio underrun are observed during audio playback.

Signed-off-by: Miguel GAIO <mgaio35@gmail.com>
Message-Id: <20180927151936.3647-1-mgaio35@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/usb/hcd-ohci.c

@@ -1253,12 +1253,12 @@ static int ohci_service_ed_list(OHCIState *ohci, uint32_t head, int completion)
 /* set a timer for EOF */
 static void ohci_eof_timer(OHCIState *ohci)
 {
-    ohci->sof_time = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
     timer_mod(ohci->eof_timer, ohci->sof_time + usb_frame_time);
 }
 /* Set a timer for EOF and generate a SOF event */
 static void ohci_sof(OHCIState *ohci)
 {
+    ohci->sof_time += usb_frame_time;
     ohci_eof_timer(ohci);
     ohci_set_interrupt(ohci, OHCI_INTR_SF);
 }
@@ -1362,6 +1362,7 @@ static int ohci_bus_start(OHCIState *ohci)
      * can meet some race conditions
      */
 
+    ohci->sof_time = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
     ohci_eof_timer(ohci);
 
     return 1;
@@ -1476,6 +1477,9 @@ static uint32_t ohci_get_frame_remaining(OHCIState *ohci)
      * set already.
      */
     tks = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) - ohci->sof_time;
+    if (tks < 0) {
+        tks = 0;
+    }
 
     /* avoid muldiv if possible */
     if (tks >= usb_frame_time)