sound: make OSS device number claiming optional and schedule its removal

If any OSS support is enabled, regardless of built-in or module,
sound_core claims full OSS major number (that is, the old 0-255
region) to trap open attempts and request sound modules using custom
module aliases.  This feature is redundant as chrdev already has such
mechanism.  This preemptive claiming prevents alternative OSS
implementation.

The custom module aliases are scheduled to be removed and the previous
patch made soundcore emit the standard chrdev aliases too to help
transition.

This patch schedule the feature for removal in a year and makes it
optional so that developers and distros can try new things in the
meantime without rebuilding the kernel.  The pre-claiming can be
turned off by using SOUND_OSS_CORE_PRECLAIM and/or kernel parameter
soundcore.preclaim_oss.

As this allows sound minors to be individually grabbed by other users,
this patch updates sound_insert_unit() such that if registering
individual device region fails, it tries the next available slot.

For details on removal plan, please read the entry added by this patch
in feature-removal-schedule.txt .

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

1 files changed, 28 insertions, 0 deletions

diff --git a/sound/Kconfig b/sound/Kconfig
index 1eceb85..439e15c 100644
--- a/sound/Kconfig
+++ b/sound/Kconfig
@@ -32,6 +32,34 @@ config SOUND_OSS_CORE
 	bool
 	default n
 
+config SOUND_OSS_CORE_PRECLAIM
+	bool "Preclaim OSS device numbers"
+	depends on SOUND_OSS_CORE
+	default y
+	help
+	  With this option enabled, the kernel will claim all OSS device
+	  numbers if any OSS support (native or emulation) is enabled
+	  whether the respective module is loaded or not and try to load the
+	  appropriate module using sound-slot/service-* and char-major-*
+	  module aliases when one of the device numbers is opened.  With
+	  this option disabled, kernel will only claim actually in-use
+	  device numbers and opening a missing device will generate only the
+	  standard char-major-* aliases.
+
+	  The only visible difference is use of additional module aliases
+	  and whether OSS sound devices appear multiple times in
+	  /proc/devices.  sound-slot/service-* module aliases are scheduled
+	  to be removed (ie. PRECLAIM won't be available) and this option is
+	  to make the transition easier.  This option can be overridden
+	  during boot using the kernel parameter soundcore.preclaim_oss.
+
+	  Disabling this allows alternative OSS implementations.
+
+	  Please read Documentation/feature-removal-schedule.txt for
+	  details.
+
+	  If unusre, say Y.
+
 source "sound/oss/dmasound/Kconfig"
 
 if !M68K