@@ -554,7 +554,7 @@ static void snd_usbmidi_output_midiman_p
/*
* Converts MIDI commands to USB MIDI packets.
*/
-static void snd_usbmidi_transmit_byte(struct usbmidi_out_port *port,
+static int snd_usbmidi_transmit_byte(struct usbmidi_out_port *port,
uint8_t b, struct urb *urb)
{
uint8_t p0 = port->cable;
@@ -563,6 +563,7 @@ static void snd_usbmidi_transmit_byte(st
if (b >= 0xf8) {
output_packet(urb, p0 | 0x0f, b, 0, 0);
+ return 1;
} else if (b >= 0xf0) {
switch (b) {
case 0xf0:
@@ -585,20 +586,20 @@ static void snd_usbmidi_transmit_byte(st
case 0xf6:
output_packet(urb, p0 | 0x05, 0xf6, 0, 0);
port->state = STATE_UNKNOWN;
- break;
+ return 1;
case 0xf7:
switch (port->state) {
case STATE_SYSEX_0:
output_packet(urb, p0 | 0x05, 0xf7, 0, 0);
- break;
+ return 1;
case STATE_SYSEX_1:
output_packet(urb, p0 | 0x06, port->data[0],
0xf7, 0);
- break;
+ return 1;
case STATE_SYSEX_2:
output_packet(urb, p0 | 0x07, port->data[0],
port->data[1], 0xf7);
- break;
+ return 1;
}
port->state = STATE_UNKNOWN;
break;
@@ -619,7 +620,7 @@ static void snd_usbmidi_transmit_byte(st
port->state = STATE_UNKNOWN;
}
output_packet(urb, p0, port->data[0], b, 0);
- break;
+ return 1;
case STATE_2PARAM_1:
port->data[1] = b;
port->state = STATE_2PARAM_2;
@@ -633,7 +634,7 @@ static void snd_usbmidi_transmit_byte(st
port->state = STATE_UNKNOWN;
}
output_packet(urb, p0, port->data[0], port->data[1], b);
- break;
+ return 1;
case STATE_SYSEX_0:
port->data[0] = b;
port->state = STATE_SYSEX_1;
@@ -646,29 +647,49 @@ static void snd_usbmidi_transmit_byte(st
output_packet(urb, p0 | 0x04, port->data[0],
port->data[1], b);
port->state = STATE_SYSEX_0;
- break;
+ return 1;
}
}
+ return 0;
}
static void snd_usbmidi_standard_output(struct snd_usb_midi_out_endpoint *ep,
struct urb *urb)
{
int p;
+ int start = 0;
+ int total = 0x10;
+ int max = ep->max_transfer - 4;
+
+ if (max < 0)
+ return;
+
+ while (1) {
+ int first = -1;
+ int final = -1;
- /* FIXME: lower-numbered ports can starve higher-numbered ports */
- for (p = 0; p < 0x10; ++p) {
- struct usbmidi_out_port *port = &ep->ports[p];
- if (!port->active)
- continue;
- while (urb->transfer_buffer_length + 3 < ep->max_transfer) {
+ for (p = start; p < total; ++p) {
+ struct usbmidi_out_port *port = &ep->ports[p];
uint8_t b;
+ if (!port->active)
+ continue;
+repeat:
if (snd_rawmidi_transmit(port->substream, &b, 1) != 1) {
port->active = 0;
- break;
+ continue;
}
- snd_usbmidi_transmit_byte(port, b, urb);
+ if (!snd_usbmidi_transmit_byte(port, b, urb))
+ goto repeat;
+ if (urb->transfer_buffer_length > max)
+ return;
+ if (first == -1)
+ first = p;
+ final = p;
}
+ if (final == -1)
+ return;
+ start = first;
+ total = final + 1;
}
}
In case of a multiport USB MIDI interface the lower numbered ports starve higher numbered ports when more than one port is busy transmitting data. The starvation as of the current code is complete and can be for an arbitrarily long time. Control from userspace is not possible without at least halving the theoretically possible device throughput. This is especially bad as there are now 16x16 interface products available. An unpredicable and arbitrarily long latency is not acceptable. The loss of half of the throughput is not acceptable, either. Fair scheduling of all busy ports is required. The patch below balances the actual USB output between all busy ports. It is done in such a way that the single port use performance before applying the patch is identical to the one after applying the patch. To get there, the snd_usbmidi_transmit_byte helper had to be converted to return output notification to allow for the 'repeat' shortcut. The patch tries to avoid O(2) load increase by scaling the balancing loop to the ports that are actually busy as far as this is possible. For the patch to properly work the wMaxPacketSize of the device must be large enough to allow for at least one MIDI event per port in a URB. If this constraint is not met, which is quite improbable, starvation will occur again. Though this could be fixed the likelyhood of such a device is so low that the additional overhead introduced for all other devices is not worth it. Current multi port MIDI interfaces do typically have 2^n output ports and 2^x as wMaxPacketSize where x>n. For a four port USB MIDI interface with a wMaxPacketSize of 64 the maximum latency for any port is changed by the patch from indefinite to 107.52ms. The patch affects the output of all class compliant USB MIDI interfaces. Users will typically experience either no change or increased response, depending on their use case. Signed-off-by: Andreas Steinmetz <ast@domdv.de>