ALSA: rawmidi: Make tied_device=0 as default / unknown

In the original change, rawmidi_info.tied_device showed -1 for the
unknown or untied device.  But this would require the user-space to
check the protocol version and judge the value conditionally, which
is rather error-prone.

Instead, set the tied_device = 0 to be default as unknown, and
indicate the real device with the offset 1, for achieving more
backward compatibility.

Suggested-by: Jaroslav Kysela <perex@perex.cz>
Link: https://patch.msgid.link/20250114104711.19197-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/rawmidi.h

@@ -191,4 +191,13 @@ long snd_rawmidi_kernel_read(struct snd_rawmidi_substream *substream,
 long snd_rawmidi_kernel_write(struct snd_rawmidi_substream *substream,
 			      const unsigned char *buf, long count);
 
+/* set up the tied devices */
+static inline void snd_rawmidi_tie_devices(struct snd_rawmidi *r1,
+					   struct snd_rawmidi *r2)
+{
+	/* tied_device field keeps the device+1 (so that 0 being unknown) */
+	r1->tied_device = r2->device + 1;
+	r2->tied_device = r1->device + 1;
+}
+
 #endif /* __SOUND_RAWMIDI_H */

include/uapi/sound/asound.h

@@ -730,7 +730,7 @@ enum {
 #define SNDRV_RAWMIDI_INFO_UMP			0x00000008
 #define SNDRV_RAWMIDI_INFO_STREAM_INACTIVE	0x00000010
 
-#define SNDRV_RAWMIDI_DEVICE_UNKNOWN		-1
+#define SNDRV_RAWMIDI_DEVICE_UNKNOWN		0
 
 struct snd_rawmidi_info {
 	unsigned int device;		/* RO/WR (control): device number */

sound/core/rawmidi.c

@@ -1837,7 +1837,6 @@ int snd_rawmidi_init(struct snd_rawmidi *rmidi,
 	INIT_LIST_HEAD(&rmidi->streams[SNDRV_RAWMIDI_STREAM_INPUT].substreams);
 	INIT_LIST_HEAD(&rmidi->streams[SNDRV_RAWMIDI_STREAM_OUTPUT].substreams);
 	rmidi->info_flags = info_flags;
-	rmidi->tied_device = SNDRV_RAWMIDI_DEVICE_UNKNOWN;
 
 	if (id != NULL)
 		strscpy(rmidi->id, id, sizeof(rmidi->id));

sound/core/ump.c

@@ -1382,8 +1382,7 @@ int snd_ump_attach_legacy_rawmidi(struct snd_ump_endpoint *ump,
 	ump_legacy_set_rawmidi_name(ump);
 	update_legacy_names(ump);
 
-	rmidi->tied_device = ump->core.device;
-	ump->core.tied_device = rmidi->device;
+	snd_rawmidi_tie_devices(rmidi, &ump->core);
 
 	ump_dbg(ump, "Created a legacy rawmidi #%d (%s)\n", device, id);
 	return 0;