[PATCH RFC v2] ALSA: seq: Mark OSS sequencer emulation as deprecated

[Takashi Iwai <tiwai@suse.de>]

OSS sequencer API emulation has been implemented as a client of ALSA
sequencer, decades ago, at the time where OSS was still use.  But the
API itself became very niche, and almost noone using it for a real
purpose any longer -- only exceptions are AI and fuzzers, who have
been causing significant maintenance burden for issues they hit &
found.

Meanwhile, the most features of OSS sequencer API emulation can be
achieved in user-space, either by ALSA-OSS layer (aoss) or oss-seq-emu
program:
  https://github.com/alsa-project/alsa-oss
  https://github.com/tiwai/oss-seq-emu
where the former is LD_PRELOAD wrapper and the latter is a CUSE-based
daemon.  So we can drop the stuff from the kernel sooner or later.

Now, let's mark the OSS sequencer emulation as deprecated and give a
warning when loaded, so that we can drop it later actually.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
v1->v2: put the warning for snd-seq-oss at the right place

 sound/core/seq/Kconfig       | 7 +++++--
 sound/core/seq/oss/seq_oss.c | 3 +++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/sound/core/seq/Kconfig b/sound/core/seq/Kconfig
index e4f58cb985d4..0d606159103e 100644
--- a/sound/core/seq/Kconfig
+++ b/sound/core/seq/Kconfig
@@ -27,14 +27,17 @@ config SND_SEQ_DUMMY
 	  will be called snd-seq-dummy.
 
 config SND_SEQUENCER_OSS
-	tristate "OSS Sequencer API"
+	tristate "OSS Sequencer API (deprecated)"
 	depends on SND_OSSEMUL
 	select SND_SEQ_MIDI_EVENT
 	help
 	  Say Y here to enable OSS sequencer emulation (both
 	  /dev/sequencer and /dev/music interfaces).
 
-	  Many programs still use the OSS API, so say Y.
+	  The OSS sequencer API is almost nowhere used, hence this option will
+	  be dropped soon.
+	  Use user-space wrapper (alsa-oss) or CUSE daemon (oss-seq-emu)
+	  instead of the kernel-side emulation.
 
 	  To compile this driver as a module, choose M here: the module
 	  will be called snd-seq-oss.
diff --git a/sound/core/seq/oss/seq_oss.c b/sound/core/seq/oss/seq_oss.c
index 2835576040ed..64a7e62029cc 100644
--- a/sound/core/seq/oss/seq_oss.c
+++ b/sound/core/seq/oss/seq_oss.c
@@ -67,6 +67,9 @@ static int __init alsa_seq_oss_init(void)
 {
 	int rc;
 
+	pr_warn("seq-oss: kernel OSS sequencer emulation is deprecated\n");
+	pr_warn("seq-oss: Use user-space solution (alsa-oss or oss-seq-emu) instead.\n");
+
 	rc = register_device();
 	if (rc < 0)
 		goto error;
-- 
2.55.0