[PATCH 2/3] ALSA: 6fire: add a playback channel map

["Piotr Gliźniewicz" <p.glizniewicz@gmail.com>]

The driver registers no "Playback Channel Map" control, so a program
that needs the speaker position of each playback channel has to assume
an order; speaker-test, for example, assumes FL FR RL RR FC LFE for 6
channels. Channel n of the PCM goes to Line Out n, and the manual of
the card assigns Line Out 1 to 6 to front left and right, centre and
subwoofer, and rear left and right.

Add a playback channel map in the order of the manual,
FL FR FC LFE RL RR. A stream with 2 to 5 channels gets the first
positions of the list. A 1-channel stream gets MONO, as in every
1-channel entry of a channel map in sound/; the card plays it on Line
Out 1 only. The map has an entry for every channel count from 1 to 6,
because reading the control fails with -EINVAL for a count without an
entry.

Tested on a DMX 6Fire USB with kernel 7.2.10: during playback with 1,
3, 5 and 6 channels the control reads MONO, FL FR FC, FL FR FC LFE RL
and FL FR FC LFE RL RR, and the "Front Center" test of
speaker-test -c 6 played on Line Out 3.

Assisted-by: LLM
Signed-off-by: Piotr Gliźniewicz <p.glizniewicz@gmail.com>
---
 sound/usb/6fire/pcm.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/sound/usb/6fire/pcm.c b/sound/usb/6fire/pcm.c
index 0285d79ac..d99395229 100644
--- a/sound/usb/6fire/pcm.c
+++ b/sound/usb/6fire/pcm.c
@@ -612,6 +612,28 @@ static void usb6fire_pcm_free(struct pcm_runtime *rt)
 	kfree(rt);
 }
 
+/*
+ * playback channel map: channel n goes to analog output n, which the
+ * manual of the card assigns to FL FR FC LFE RL RR for n = 1..6; a
+ * 1-channel stream is MONO, as in snd_pcm_std_chmaps, and plays on
+ * analog output 1 only
+ */
+static const struct snd_pcm_chmap_elem usb6fire_pcm_chmaps[] = {
+	{ .channels = 1, .map = { SNDRV_CHMAP_MONO } },
+	{ .channels = 2, .map = { SNDRV_CHMAP_FL, SNDRV_CHMAP_FR } },
+	{ .channels = 3, .map = { SNDRV_CHMAP_FL, SNDRV_CHMAP_FR,
+				  SNDRV_CHMAP_FC } },
+	{ .channels = 4, .map = { SNDRV_CHMAP_FL, SNDRV_CHMAP_FR,
+				  SNDRV_CHMAP_FC, SNDRV_CHMAP_LFE } },
+	{ .channels = 5, .map = { SNDRV_CHMAP_FL, SNDRV_CHMAP_FR,
+				  SNDRV_CHMAP_FC, SNDRV_CHMAP_LFE,
+				  SNDRV_CHMAP_RL } },
+	{ .channels = 6, .map = { SNDRV_CHMAP_FL, SNDRV_CHMAP_FR,
+				  SNDRV_CHMAP_FC, SNDRV_CHMAP_LFE,
+				  SNDRV_CHMAP_RL, SNDRV_CHMAP_RR } },
+	{ }
+};
+
 int usb6fire_pcm_init(struct sfire_chip *chip)
 {
 	int i;
@@ -665,6 +687,14 @@ int usb6fire_pcm_init(struct sfire_chip *chip)
 	rt->instance = pcm;
 
 	chip->pcm = rt;
+
+	/* on error, rt is freed with the card */
+	ret = snd_pcm_add_chmap_ctls(pcm, SNDRV_PCM_STREAM_PLAYBACK,
+			usb6fire_pcm_chmaps, OUT_N_CHANNELS, 0, NULL);
+	if (ret < 0) {
+		dev_err(&chip->dev->dev, "cannot add channel map.\n");
+		return ret;
+	}
 	return 0;
 
  error:
-- 
2.56.0