Last major commit and updates for RELENG_7:

• Rework the entire pcm_channel structure:
  • Remove rarely used link placeholder, instead, make each pcm_channel as head/link of each own/each other. Unlock - Lock sequence due to sleep malloc has been reduced.
  • Implement "busy" queue which will contain list of busy/active channels. This greatly reduce locking contention for example while servicing interrupt for hardware with many channels or when virtual channels reach its 256 peak channels.

• So I heard you like v chan ... O RLY? Welcome to Virtual Record Channels (vrec, rec vchans, vchans for recording, Rec-Chan, you decide), the ultimate solutions for your nagging O_RDWR full-duplex wannabe (note: flash plugins) monopolizing single record channel causing EBUSY. Vrec works exactly like Vchans (or, should I rename it to "Vplay" :) , except that it operates on the opposite direction (recording). Up to 256 vrecs (like vchans) are possible.
  
  Notes:
  • Relocate dev.pcm.%d.{vchans,vchanformat,vchanrate} to each of its respective node/direction: dev.pcm.%d.play.* for "play" (cdev = dsp%d.vp%d) dev.pcm.%d.rec.* for "record" (cdev = dsp%d.vr%d)
  • Don't expect that it will magically give you ability to split "recording source" (eg: 1 channel for cdrom, 1 channel for mic, etc). Just admit that you only have a *single* recording source / channel. Please bug your hardware vendor instead :)

• Bump maxautovchans from 4 to 16. For a full-fledged multimedia desktop/workstation with too many soundservers installed (esound, artsd, jackd, pulse/polypaudio, ding-dong pling plong mudkip fuh fuh, etc), 4 seems inadequate. There will be no memory penalty here, since virtual channels are allocate only by demand.

• Nuke/Rework the entire statically created cdev entries. Everything is clonable through snd own clone manager which designed to withstand many kind of abusive devfs droids such as:
  • while : ; do /bin/test -e /dev/dsp ; done
  • jot 16777216 0 | while read x ; do ls /dev/dsp0.$x ; done
  • hundreds (could be thousands) concurrent threads/process opening

"/dev/dsp" (previously, this might result EBUSY even with just
3 contesting threads/procs).

o Reusable clone objects (instead of creating new one like there's no
  tomorrow) after certain expiration deadline. The clone allocator will
  decide whether to reuse, share, or creating new clone.
o Automatic garbage collector.

• Dynamic unit magic allocator. Maximum attached soundcards can be tuned using tunable "hw.snd.maxunit" (Default to 512). Minimum is 16, and maximum is 2048.

• ..other fixes, mostly related to concurrency issues.

joel@ will do the manpage updates on sound(4).

Have fun.