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Commit 25ca917c authored by Clemens Ladisch's avatar Clemens Ladisch Committed by Takashi Iwai
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ALSA: firewire-lib: limit the MIDI data rate 



Do no send MIDI bytes at the full rate at which FireWire packets happen
to be sent, but restrict them to the actual rate of a real MIDI port.
This is required by the specification, and prevents data loss when the
device's buffer overruns.

Signed-off-by: default avatarClemens Ladisch <clemens@ladisch.de>
Reviewed-by: default avatarTakashi Sakamoto <o-takashi@sakamocchi.jp>
Tested-by: default avatarTakashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: default avatarTakashi Iwai <tiwai@suse.de>
parent 5c697e5b

sound/firewire/amdtp.c

+55 −6
Original line number Diff line number Diff line
@@ -21,6 +21,12 @@ 
#define CYCLES_PER_SECOND	8000

#define TICKS_PER_SECOND	(TICKS_PER_CYCLE * CYCLES_PER_SECOND)



/*

 * Nominally 3125 bytes/second, but the MIDI port's clock might be

 * 1% too slow, and the bus clock 100 ppm too fast.

 */

#define MIDI_BYTES_PER_SECOND	3093



/*

 * Several devices look only at the first eight data blocks.

 * In any case, this is more than enough for the MIDI data rate.
@@ -226,6 +232,14 @@ void amdtp_stream_set_parameters(struct amdtp_stream *s, 
	for (i = 0; i < pcm_channels; i++)

		s->pcm_positions[i] = i;

	s->midi_position = s->pcm_channels;



	/*

	 * We do not know the actual MIDI FIFO size of most devices.  Just

	 * assume two bytes, i.e., one byte can be received over the bus while

	 * the previous one is transmitted over MIDI.

	 * (The value here is adjusted for midi_ratelimit_per_packet().)

	 */

	s->midi_fifo_limit = rate - MIDI_BYTES_PER_SECOND * s->syt_interval + 1;

}

EXPORT_SYMBOL(amdtp_stream_set_parameters);
@@ -467,6 +481,36 @@ static void amdtp_fill_pcm_silence(struct amdtp_stream *s, 
	}

}



/*

 * To avoid sending MIDI bytes at too high a rate, assume that the receiving

 * device has a FIFO, and track how much it is filled.  This values increases

 * by one whenever we send one byte in a packet, but the FIFO empties at

 * a constant rate independent of our packet rate.  One packet has syt_interval

 * samples, so the number of bytes that empty out of the FIFO, per packet(!),

 * is MIDI_BYTES_PER_SECOND * syt_interval / sample_rate.  To avoid storing

 * fractional values, the values in midi_fifo_used[] are measured in bytes

 * multiplied by the sample rate.

 */

static bool midi_ratelimit_per_packet(struct amdtp_stream *s, unsigned int port)

{

	int used;



	used = s->midi_fifo_used[port];

	if (used == 0) /* common shortcut */

		return true;



	used -= MIDI_BYTES_PER_SECOND * s->syt_interval;

	used = max(used, 0);

	s->midi_fifo_used[port] = used;



	return used < s->midi_fifo_limit;

}



static void midi_rate_use_one_byte(struct amdtp_stream *s, unsigned int port)

{

	s->midi_fifo_used[port] += amdtp_rate_table[s->sfc];

}



static void amdtp_fill_midi(struct amdtp_stream *s,

			    __be32 *buffer, unsigned int frames)

{
@@ -474,16 +518,21 @@ static void amdtp_fill_midi(struct amdtp_stream *s, 
	u8 *b;



	for (f = 0; f < frames; f++) {

		buffer[s->midi_position] = 0;

		b = (u8 *)&buffer[s->midi_position];



		port = (s->data_block_counter + f) % 8;

		if ((f >= MAX_MIDI_RX_BLOCKS) ||

		    (s->midi[port] == NULL) ||

		    (snd_rawmidi_transmit(s->midi[port], b + 1, 1) <= 0))

			b[0] = 0x80;

		else

		if (f < MAX_MIDI_RX_BLOCKS &&

		    midi_ratelimit_per_packet(s, port) &&

		    s->midi[port] != NULL &&

		    snd_rawmidi_transmit(s->midi[port], &b[1], 1) == 1) {

			midi_rate_use_one_byte(s, port);

			b[0] = 0x81;

		} else {

			b[0] = 0x80;

			b[1] = 0;

		}

		b[2] = 0;

		b[3] = 0;



		buffer += s->data_block_quadlets;

	}

sound/firewire/amdtp.h

+2 −0
Original line number Diff line number Diff line
@@ -148,6 +148,8 @@ struct amdtp_stream { 
	bool double_pcm_frames;



	struct snd_rawmidi_substream *midi[AMDTP_MAX_CHANNELS_FOR_MIDI * 8];

	int midi_fifo_limit;

	int midi_fifo_used[AMDTP_MAX_CHANNELS_FOR_MIDI * 8];



	/* quirk: fixed interval of dbc between previos/current packets. */

	unsigned int tx_dbc_interval;