ALSA: fireface: comment cleanup about destination address of async transactions for MIDI messages

In Fireface series, registration of higher 4 bytes of destination
address for asynchronous transaction of MIDI messages is done by
a write transaction to model-specific register.

On the other hand, registration of lower 4 bytes of the address is
selectable from 4 options. A register for this registration includes
the other purpose options such as input attenuation. Thus this
driver expects userspace applications to configure the register.

Actual behaviour for the asynchronous transaction is different
depending on protocols. In former protocol, destination offset
of each transaction is the same as the registered address even if
it is block request. In latter models, destination offset of each
transaction is the offset of previous transaction plus 4 byte
and the transaction is quadlet request.

This commit cleanups comments about the above mechanism.

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

1 files changed, 18 insertions, 0 deletions

diff --git a/sound/firewire/fireface/ff-protocol-latter.c b/sound/firewire/fireface/ff-protocol-latter.c
index 817af4447349..0fbc1950327f 100644
--- a/sound/firewire/fireface/ff-protocol-latter.c
+++ b/sound/firewire/fireface/ff-protocol-latter.c
@@ -266,6 +266,24 @@ static void latter_dump_status(struct snd_ff *ff, struct snd_info_buffer *buffer
 
 // NOTE: transactions are transferred within 0x00-0x7f in allocated range of
 // address. This seems to be for check of discontinuity in receiver side.
+//
+// Like Fireface 400, drivers can select one of 4 options for lower 4 bytes of
+// destination address by bit flags in quadlet register (little endian) at
+// 0x'ffff'0000'0014:
+//
+// bit flags: offset of destination address
+// - 0x00002000: 0x'....'....'0000'0000
+// - 0x00004000: 0x'....'....'0000'0080
+// - 0x00008000: 0x'....'....'0000'0100
+// - 0x00010000: 0x'....'....'0000'0180
+//
+// Drivers can suppress the device to transfer asynchronous transactions by
+// clear these bit flags.
+//
+// Actually, the register is write-only and includes the other settings such as
+// input attenuation. This driver allocates for the first option
+// (0x'....'....'0000'0000) and expects userspace application to configure the
+// register for it.
 static void latter_handle_midi_msg(struct snd_ff *ff, unsigned int offset,
 				   __le32 *buf, size_t length)
 {