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>

2 files changed, 98 insertions, 18 deletions

diff --git a/sound/Kconfig b/sound/Kconfig
index 1eceb85287c5..439e15c8faa3 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
diff --git a/sound/sound_core.c b/sound/sound_core.c
index 786067c49b01..bb4b88e606bb 100644
--- a/sound/sound_core.c
+++ b/sound/sound_core.c
@@ -128,6 +128,46 @@ extern int msnd_pinnacle_init(void);
 #endif
 
 /*
+ * By default, OSS sound_core claims full legacy minor range (0-255)
+ * of SOUND_MAJOR to trap open attempts to any sound minor and
+ * requests modules using custom sound-slot/service-* module aliases.
+ * The only benefit of doing this is allowing use of custom module
+ * aliases instead of the standard char-major-* ones.  This behavior
+ * prevents alternative OSS implementation and is scheduled to be
+ * removed.
+ *
+ * CONFIG_SOUND_OSS_CORE_PRECLAIM and soundcore.preclaim_oss kernel
+ * parameter are added to allow distros and developers to try and
+ * switch to alternative implementations without needing to rebuild
+ * the kernel in the meantime.  If preclaim_oss is non-zero, the
+ * kernel will behave the same as before.  All SOUND_MAJOR minors are
+ * preclaimed and the custom module aliases along with standard chrdev
+ * ones are emitted if a missing device is opened.  If preclaim_oss is
+ * zero, sound_core only grabs what's actually in use and for missing
+ * devices only the standard chrdev aliases are requested.
+ *
+ * All these clutters are scheduled to be removed along with
+ * sound-slot/service-* module aliases.  Please take a look at
+ * feature-removal-schedule.txt for details.
+ */
+#ifdef CONFIG_SOUND_OSS_CORE_PRECLAIM
+static int preclaim_oss = 1;
+#else
+static int preclaim_oss = 0;
+#endif
+
+module_param(preclaim_oss, int, 0444);
+
+static int soundcore_open(struct inode *, struct file *);
+
+static const struct file_operations soundcore_fops =
+{
+        /* We must have an owner or the module locking fails */
+        .owner  = THIS_MODULE,
+        .open   = soundcore_open,
+};
+
+/*
  *      Low level list operator. Scan the ordered list, find a hole and
  *      join into it. Called with the lock asserted
  */
@@ -219,8 +259,9 @@ static int sound_insert_unit(struct sound_unit **list, const struct file_operati
 
         if (!s)
                 return -ENOMEM;
-                
+
         spin_lock(&sound_loader_lock);
+retry:
         r = __sound_insert_unit(s, list, fops, index, low, top);
         spin_unlock(&sound_loader_lock);
         
@@ -231,11 +272,31 @@ static int sound_insert_unit(struct sound_unit **list, const struct file_operati
         else
                 sprintf(s->name, "sound/%s%d", name, r / SOUND_STEP);
 
+        if (!preclaim_oss) {
+                /*
+                 * Something else might have grabbed the minor.  If
+                 * first free slot is requested, rescan with @low set
+                 * to the next unit; otherwise, -EBUSY.
+                 */
+                r = __register_chrdev(SOUND_MAJOR, s->unit_minor, 1, s->name,
+                                      &soundcore_fops);
+                if (r < 0) {
+                        spin_lock(&sound_loader_lock);
+                        __sound_remove_unit(list, s->unit_minor);
+                        if (index < 0) {
+                                low = s->unit_minor + SOUND_STEP;
+                                goto retry;
+                        }
+                        spin_unlock(&sound_loader_lock);
+                        return -EBUSY;
+                }
+        }
+
         device_create(sound_class, dev, MKDEV(SOUND_MAJOR, s->unit_minor),
                       NULL, s->name+6);
-        return r;
+        return s->unit_minor;
 
- fail:
+fail:
         kfree(s);
         return r;
 }
@@ -254,6 +315,9 @@ static void sound_remove_unit(struct sound_unit **list, int unit)
         p = __sound_remove_unit(list, unit);
         spin_unlock(&sound_loader_lock);
         if (p) {
+                if (!preclaim_oss)
+                        __unregister_chrdev(SOUND_MAJOR, p->unit_minor, 1,
+                                            p->name);
                 device_destroy(sound_class, MKDEV(SOUND_MAJOR, p->unit_minor));
                 kfree(p);
         }
@@ -491,19 +555,6 @@ void unregister_sound_dsp(int unit)
 
 EXPORT_SYMBOL(unregister_sound_dsp);
 
-/*
- *      Now our file operations
- */
-
-static int soundcore_open(struct inode *, struct file *);
-
-static const struct file_operations soundcore_fops=
-{
-        /* We must have an owner or the module locking fails */
-        .owner  = THIS_MODULE,
-        .open   = soundcore_open,
-};
-
 static struct sound_unit *__look_for_unit(int chain, int unit)
 {
         struct sound_unit *s;
@@ -539,7 +590,7 @@ static int soundcore_open(struct inode *inode, struct file *file)
         s = __look_for_unit(chain, unit);
         if (s)
                 new_fops = fops_get(s->unit_fops);
-        if (!new_fops) {
+        if (preclaim_oss && !new_fops) {
                 spin_unlock(&sound_loader_lock);
 
                 /*
@@ -605,7 +656,8 @@ static void cleanup_oss_soundcore(void)
 
 static int __init init_oss_soundcore(void)
 {
-        if (register_chrdev(SOUND_MAJOR, "sound", &soundcore_fops)==-1) {
+        if (preclaim_oss &&
+            register_chrdev(SOUND_MAJOR, "sound", &soundcore_fops) == -1) {
                 printk(KERN_ERR "soundcore: sound device already in use.\n");
                 return -EBUSY;
         }