Audio

Audio playback supports AC97 and High Definition Audio controllers (src/drivers/sound/). sound_init() initializes both, prefers HDA, and allocates a 2 MiB streaming ring; all operations dispatch to the active card. Playback is polled — neither driver uses interrupts — and sound_poll() is pumped from the kernel idle loop, which is what keeps both whole-file and streaming playback fed.

Common Model

• Stream configuration: sample rate, channels, bits.
• File playback by extension (SYS_SOUND_PLAY): .wav (PCM-only parser, RIFF chunk scan), .mp3 (parser stub — not implemented), otherwise raw PCM at 22050 Hz stereo.
• Buffer descriptor lists: 32 entries of 32 KB each in both drivers. Legacy whole-file playback copies up to one ring of the source up front; the poll function refills consumed entries from the source buffer as the hardware plays them.
• Volume: sound_set_volume(0-100) programs the card's master volume (NAM master volume on AC97, output widget gain on HDA).

Streaming Playback

Userspace decodes audio and pushes PCM instead of buffering whole files (src/drivers/sound/sound.cpp):

Mechanics:

• The dispatcher ring is 2 MiB. DMA starts once a full card ring (1 MiB = 32 x 32 KiB) is queued, so playback never begins with silence; STREAM_START_THRESHOLD gates this. A stream end with less queued flushes immediately. A latched pause defers the start until resume.
• The drivers run in stream_mode: BDL refills pull from the ring through a refill callback instead of a fixed source buffer. An exhausted ring pads silence and the card stops when the last real byte is consumed; the dispatcher restarts the card when enough new data is queued (underrun recovery) or closes the stream after a drained STREAM_END. Restarts also honor a pause latched while the card was stopped.
• The stream records the group that opened it (leader pid). When that group fully exits, the kernel stops the stream and wakes blocked writers — a dead owner cannot leave the card clocking audio (serial marker: sound: stopping the stream an exited process left open).
• An ended stream auto-closes only when its ring is fully drained: a pause during the final drain also stops the card transiently, and that must not close the stream while data is still queued.
• AC97 DMAs stereo pairs, so mono streams are upmixed to stereo in the dispatcher; HDA passes the source channel count through.
• Writers block on a wait queue while the ring is full and are woken by the refill path, so playback never depends on holding a whole decoded file in memory. Blocking writers rely on the idle-loop pump advancing the card.

AC97

src/drivers/sound/ac97/ac97.cpp (PCI class 0x04 / subclass 0x01):

• NAM (BAR0) and NABM (BAR1) I/O registers; cold reset through the NABM global control.
• Variable sample rate detection via the extended capabilities register.
• PCM-out stream: BDL base, last-valid index, start/stop/reset controls. Volume via the NAM master volume register (inverted attenuation).
• Playback only.

HDA

src/drivers/sound/hda/hda.cpp (PCI class 0x04 / subclass 0x03), MMIO BAR0:

• Controller reset, STATESTS codec scan, CORB/RIRB command rings (256 entries each) for codec verbs.
• Known limitation: the CORB/RIRB exchange times out against QEMU's intel-hda (no codec response ever lands in the RIRB, so hda_init gives up at failed to find codec); the QEMU run targets therefore attach an AC97 card. Verified working in QEMU: AC97; HDA is untested against real hardware.
• Codec/widget tree discovery: AFG nodes, pin/mixer/output widgets, EAPD, amplifier gain setup.
• Playback and recording streams (output stream chosen after input streams); per-node channel volume.

Userspace Path