ALSA: firewire-lib: compute the value of second field in cycle count for IR context

In callback function of isochronous context, modules can queue packets to
indicated isochronous cycles. Although the cycle to queue a packet is
deterministic by calculation, this module doesn't implement the calculation
because it's useless for processing.

In future, the cycle count is going to be printed with the other parameters
for debugging. This commit is the preparation. The cycle count is computed
by cycle unit, and correctly arranged to corresponding packets. The
calculated count is used in later commit.

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

sound/firewire/amdtp-stream.c

@@ -566,6 +566,13 @@ static inline u32 increment_cycle_count(u32 cycle, unsigned int addend)
 	return cycle;
 }
 
+static inline u32 decrement_cycle_count(u32 cycle, unsigned int subtrahend)
+{
+	if (cycle < subtrahend)
+		cycle += 8 * CYCLES_PER_SECOND;
+	return cycle - subtrahend;
+}
+
 static void out_stream_callback(struct fw_iso_context *context, u32 tstamp,
 				size_t header_length, void *header,
 				void *private_data)
@@ -607,6 +614,7 @@ static void in_stream_callback(struct fw_iso_context *context, u32 tstamp,
 	unsigned int payload_quadlets, max_payload_quadlets;
 	unsigned int data_blocks;
 	__be32 *buffer, *headers = header;
+	u32 cycle;
 
 	if (s->packet_index < 0)
 		return;
@@ -614,10 +622,16 @@ static void in_stream_callback(struct fw_iso_context *context, u32 tstamp,
 	/* The number of packets in buffer */
 	packets = header_length / IN_PACKET_HEADER_SIZE;
 
+	cycle = compute_cycle_count(tstamp);
+
+	/* Align to actual cycle count for the last packet. */
+	cycle = decrement_cycle_count(cycle, packets);
+
 	/* For buffer-over-run prevention. */
 	max_payload_quadlets = amdtp_stream_get_max_payload(s) / 4;
 
 	for (p = 0; p < packets; p++) {
+		cycle = increment_cycle_count(cycle, 1);
 		buffer = s->buffer.packets[s->packet_index].buffer;
 
 		/* The number of quadlets in this packet */