sound: Fix VCHANs' starting and ending points in feeder chain

[christos]

Summary

sndstat(4) falsely reports "hardware" as the starting point of
recording, and ending point of playback VCHANs. Recording VCHANs get
their input from the physical recording channel, and playback VCHANs
send their input to the physical playback channel.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days

Test Plan

pcm0:play:dsp0.p0[pcm0:virtual_play:dsp0.vp0]: spd 8000, fmt 0x00100008, flags 0x10000000, 0x00000000
interrupts 0, underruns 0, feed 0, ready 0 [b:0/0/0|bs:0/0/0]
channel flags=0x10000000<VIRTUAL>
{userland} -> feeder_root(0x00000000) -> {pcm0:play:dsp0.p0}
...
pcm0:record:dsp0.r0[pcm0:virtual_record:dsp0.vr0]: spd 8000, fmt 0x00100008, flags 0x10000000, 0x00000000
interrupts 0, overruns 0, feed 0, hfree 0, sfree 0 [b:0/0/0|bs:0/0/0]
channel flags=0x10000000<VIRTUAL>
{pcm0:record:dsp0.r0} -> feeder_root(0x00000000) -> {userland}

[dev_submerge.ch]

Good one, that was always a source of confusion.
What about the recording vchans? Would it be possible to print the primary recording channel as source there?

[christos]

In D46177#1052544, @dev_submerge.ch wrote:

Good one, that was always a source of confusion.
What about the recording vchans? Would it be possible to print the primary recording channel as source there?

Yes. I had the patch written already to be submitted today, but I think it's better to just merge them into a single patch. I'll update the diff shortly.

[christos]

Fix recording VCHANs as well.

[dev_submerge.ch]

Nice!
Just a nitpick on the commit message, I think "primary" channel would be more understandable than "physical" here.