ALSA: firewire-tascam: queue events for change of control surface

Units of TASCAM FireWire series transfer image of states of the unit in
tx isochronous packets. Demultiplexing of the states from the packets
is done in software interrupt context regardless of any process context.
In a view of userspace applications, it needs to have notification
mechanism to catch change of the states.

This commit implements a queue to store events for the notification. The
image of states includes fluctuating data such as level of gain/volume
for physical input/output and position of knobs. Therefore the events
are queued corresponding to some control features only.

Furthermore, the queued events are planned to be consumed by userspace
applications via ALSA hwdep interface. This commit suppresses event
queueing when no applications open the hwdep interface.

However, the queue is maintained in an optimistic scenario, thus without
any care against overrrun. This is reasonable because target events are
useless just to handle PCM frames. It starts queueing when an usespace
application opens hwdep interface, thus it's expected to read the queued
events steadily.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/firewire/tascam/amdtp-tascam.c

@@ -121,13 +121,45 @@ static void read_status_messages(struct amdtp_stream *s,
 				 __be32 *buffer, unsigned int data_blocks)
 {
 	struct snd_tscm *tscm = container_of(s, struct snd_tscm, tx_stream);
+	bool used = READ_ONCE(tscm->hwdep->used);
 	int i;
 
 	for (i = 0; i < data_blocks; i++) {
 		unsigned int index;
+		__be32 before;
+		__be32 after;
 
 		index = be32_to_cpu(buffer[0]) % SNDRV_FIREWIRE_TASCAM_STATE_COUNT;
-		tscm->state[index] = buffer[s->data_block_quadlets - 1];
+		before = tscm->state[index];
+		after = buffer[s->data_block_quadlets - 1];
+
+		if (used && index > 4 && index < 16) {
+			__be32 mask;
+
+			if (index == 5)
+				mask = cpu_to_be32(~0x0000ffff);
+			else if (index == 6)
+				mask = cpu_to_be32(~0x0000ffff);
+			else if (index == 8)
+				mask = cpu_to_be32(~0x000f0f00);
+			else
+				mask = cpu_to_be32(~0x00000000);
+
+			if ((before ^ after) & mask) {
+				struct snd_firewire_tascam_change *entry =
+						&tscm->queue[tscm->push_pos];
+
+				spin_lock_irq(&tscm->lock);
+				entry->index = index;
+				entry->before = before;
+				entry->after = after;
+				if (++tscm->push_pos >= SND_TSCM_QUEUE_COUNT)
+					tscm->push_pos = 0;
+				spin_unlock_irq(&tscm->lock);
+			}
+		}
+
+		tscm->state[index] = after;
 		buffer += s->data_block_quadlets;
 	}
 }

sound/firewire/tascam/tascam-hwdep.c

@@ -195,5 +195,7 @@ int snd_tscm_create_hwdep_device(struct snd_tscm *tscm)
 	hwdep->private_data = tscm;
 	hwdep->exclusive = true;
 
+	tscm->hwdep = hwdep;
+
 	return err;
 }

sound/firewire/tascam/tascam.h

@@ -62,6 +62,8 @@ struct snd_fw_async_midi_port {
 	int consume_bytes;
 };
 
+#define SND_TSCM_QUEUE_COUNT	16
+
 struct snd_tscm {
 	struct snd_card *card;
 	struct fw_unit *unit;
@@ -92,6 +94,10 @@ struct snd_tscm {
 
 	// A cache of status information in tx isoc packets.
 	__be32 state[SNDRV_FIREWIRE_TASCAM_STATE_COUNT];
+	struct snd_hwdep *hwdep;
+	struct snd_firewire_tascam_change queue[SND_TSCM_QUEUE_COUNT];
+	unsigned int pull_pos;
+	unsigned int push_pos;
 };
 
 #define TSCM_ADDR_BASE			0xffff00000000ull