sound: Allocate vchans on-demand

Refactor pcm_chnalloc() and merge with parts of vchan_setnew() (now
removed) and dsp_open()’s channel creation into a new dsp_chn_alloc()
function. The function is responsible for either using a free HW channel
(if vchans are disabled), or allocating a new vchan.

Clean up allocated vchans associated with a given dsp_cdevpriv on
dsp_close() instead of leaving them unused.

hw.snd.vchans_enable (previously hw.snd.maxautovchans) and
dev.pcm.X.{play|rec}.vchans now work as tunables to only enable/disable
vchans, as opposed to setting their number and/or (de-)allocating
vchans. Since these sysctls do not trigger any (de-)allocations anymore,
their effect is instantaneous, whereas before we could have frozen the
machine (when trying to allocate new vchans) when setting
dev.pcm.X.{play|rec}.vchans to a very large value.

Create a new "primary" channel sublist so that we do not waste time
looping through all channels in dsp_chn_alloc(), since we are only
looking for a parent channel to either use, or add a new vchan to. This
guarantees a steady traversal speed, as the parent channels are most
likely going to be just a handful (2). What was currently in place was a
loop through the whole channel list, which meant that the traversal
would take longer the more channels were added to that list.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Reviewed by:	dev_submerge.ch
Differential Revision:	https://reviews.freebsd.org/D47917

0 files changed, 0 insertions, 0 deletions