L1 guide: sound drivers

The surface

drivers/snd/driver_snd.h (28 declarations; the numbered 26):

driver_snd_register(name, ops, ctx)
driver_snd_play(dev, samples, count)
driver_snd_stop(dev)
driver_snd_set_rate(dev, hz)
driver_snd_set_volume(dev, pct)
driver_snd_get_caps(dev, out)

Plus driver_snd_probe_all, driver_snd_make_tone, the per-hw init entries (hda/ac97/sb16/es1370/virtio) and the lookup/listing helpers.

Bring-up notes per family

• HDA: codecs + streams, the widget graph.
• AC97: the mixer registers, the AC-link timing.
• SB16: DSP commands + DMA16.
• ES1370: the mixed playback channels.
• virtio-snd: the virtqueue protocol (and -device virtio-sound in QEMU).

play (a tone), sound (status), the *_test commands, and virtio_snd_test/sample_rate_test are your gates. ISO endpoints zero-pad their tails (no OOB reads - the audio audit fixed that).