[alsa-devel] [PATCH 06/25] ALSA: firewire-lib: add throttle for MIDI data rate

[Takashi Sakamoto]

Typically, the target devices have internal buffer to adjust output of
received MIDI messages for MIDI serial bus, while the capacity of the
buffer is limited. IEEE 1394 transactions can transfer more MIDI messages
than MIDI serial bus can. This can cause buffer over flow in device side.

This commit adds throttle to limit MIDI data rate by counting intervals
by jiffies between two MIDI messages. Usual MIDI messages consists of two
or three bytes. This requires 1.302 to 1.953 mili-seconds interval between
these messages. Using jiffies for this purpose is not perfect idea.
Further work is needed for better implementation.

Signed-off-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>
---
 sound/firewire/lib.c | 18 +++++++++++++++++-
 sound/firewire/lib.h |  1 +
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/sound/firewire/lib.c b/sound/firewire/lib.c
index e309b9b..452728c 100644
--- a/sound/firewire/lib.c
+++ b/sound/firewire/lib.c
@@ -76,6 +76,9 @@ static void async_midi_port_callback(struct fw_card *card, int rcode,
 
 	if (rcode == RCODE_COMPLETE && substream != NULL)
 		snd_rawmidi_transmit_ack(substream, port->consume_bytes);
+	else if (!rcode_is_permanent_error(rcode))
+		/* To start next transaction immediately for recovery. */
+		port->next_tick = 0;
 
 	port->idling = true;
 
@@ -99,6 +102,12 @@ static void midi_port_tasklet(unsigned long data)
 	if (substream == NULL || snd_rawmidi_transmit_empty(substream))
 		return;
 
+	/* Do it in next chance. */
+	if (time_is_after_jiffies(port->next_tick)) {
+		tasklet_schedule(&port->tasklet);
+		return;
+	}
+
 	/*
 	 * Fill the buffer. The callee must use snd_rawmidi_transmit_peek().
 	 * Later, snd_rawmidi_transmit_ack() may be called.
@@ -107,8 +116,10 @@ static void midi_port_tasklet(unsigned long data)
 	port->consume_bytes = port->packetize(substream, port->buf);
 	if (port->consume_bytes <= 0) {
 		/* Do it in next chance, immediately. */
-		if (port->consume_bytes == 0)
+		if (port->consume_bytes == 0) {
+			port->next_tick = 0;
 			tasklet_schedule(&port->tasklet);
+		}
 		return;
 	}
 
@@ -118,6 +129,10 @@ static void midi_port_tasklet(unsigned long data)
 	else
 		type = TCODE_WRITE_BLOCK_REQUEST;
 
+	/* Set interval to next transaction. */
+	port->next_tick =
+		jiffies_64 + msecs_to_jiffies(port->consume_bytes * 8 / 31250);
+
 	/* Start this transaction. */
 	port->idling = false;
 	generation = port->parent->generation;
@@ -153,6 +168,7 @@ int snd_fw_async_midi_port_init(struct snd_fw_async_midi_port *port,
 	port->addr = addr;
 	port->packetize = packetize;
 	port->idling = true;
+	port->next_tick = 0;
 
 	tasklet_init(&port->tasklet, midi_port_tasklet, (unsigned long)port);
 
diff --git a/sound/firewire/lib.h b/sound/firewire/lib.h
index 9d76f5c..e4b00e2 100644
--- a/sound/firewire/lib.h
+++ b/sound/firewire/lib.h
@@ -26,6 +26,7 @@ struct snd_fw_async_midi_port {
 	struct fw_device *parent;
 	struct tasklet_struct tasklet;
 	bool idling;
+	unsigned long next_tick;
 
 	__u64 addr;
 	struct fw_transaction transaction;
-- 
2.1.4