Sound

Purpose

sound/ is the ALSA (Advanced Linux Sound Architecture) subsystem: PCM playback/capture, MIDI, mixers, control surfaces, sequencer, plus the ASoC (ALSA System on Chip) framework for embedded audio.

Directory layout

sound/
├── core/            # Card, PCM, control, timer, sequencer, RawMIDI, jack
├── usb/             # USB audio class drivers
├── pci/             # PCI sound cards: hda/, ymfpci/, emu10k1/, ice1712/, …
├── pcmcia/, isa/, parisc/, sparc/, mips/, ppc/, sh/   # Bus/legacy
├── soc/             # ASoC: SoC + codec + machine drivers
├── hda/             # HDA (Intel HDA) shared code
├── synth/, drivers/ # Synth chips, miscellaneous
├── firewire/        # FireWire audio
├── x86/             # x86-only audio glue (HDMI sleep state, etc.)
├── aoa/             # PowerMac AOA framework
├── virtio/          # Virtio sound device
├── sh/, sparc/      # Per-arch sound bits
├── oss/             # OSS compatibility layer (limited)
└── soc/             # ASoC, the largest single sub-area today

Key abstractions

Symbol	File	Purpose
struct snd_card	include/sound/core.h	A logical sound card.
struct snd_pcm, snd_pcm_substream	include/sound/pcm.h	PCM stream — ring buffer between user and hw.
struct snd_kcontrol	include/sound/control.h	A control element (volume, mux, switch).
struct snd_rawmidi, snd_seq_*	include/sound/rawmidi.h, seq*.h	RawMIDI and sequencer.
struct snd_soc_card, snd_soc_dai, snd_soc_codec, snd_soc_dai_link	include/sound/soc.h	ASoC building blocks.
struct hda_codec	include/sound/hda_codec.h	HDA codec descriptor.

How it works

graph LR
    APP[User-space app via /dev/snd/*] --> ALSA[core/ ALSA core]
    ALSA --> CARD[snd_card]
    CARD --> PCM[snd_pcm]
    CARD --> CTL[snd_kcontrol]
    CARD --> RAW[snd_rawmidi]
    PCM -->|hw_ops| DRV1["pci/hda/ or usb/ or soc/"]
    DRV1 --> HW[Hardware]
    SOC["soc/ ASoC machine"] --> CODEC["soc/codecs/"]
    SOC --> CPU_DAI["soc/<vendor>/ CPU DAI"]

A driver registers a snd_card, populates it with PCMs and controls, and provides snd_pcm_ops (open, hw_params, prepare, trigger, pointer). User space talks to /dev/snd/pcmCxDxp (playback) / pcmCxDxc (capture) and /dev/snd/controlCx (mixer).

ASoC composes a "machine" from a CPU DAI (the SoC audio peripheral) plus one or more codec DAIs (chips that drive speakers/mics). The machine driver describes the wiring; the framework runs the streaming.

Integration points

• drivers/: the underlying buses (PCI, USB, I2C, SPI, platform).
• fs/: the snd-* char devices via the ALSA core.
• mm/: PCM buffers may be allocated via DMA mapping.
• kernel/: ftrace events for ASoC.

Key source files

File	Purpose
sound/core/sound.c	snd_card lifecycle.
sound/core/pcm_native.c	The big PCM state machine.
sound/core/control.c	Mixer controls.
sound/soc/soc-core.c	ASoC core.
sound/pci/hda/hda_codec.c	HDA codec layer.

Entry points for modification

• New PCI/USB sound card: add a driver under sound/pci/<vendor>/ or sound/usb/. Most just register snd_card and PCMs.
• New ASoC codec: sound/soc/codecs/<chip>.c — implement snd_soc_codec_driver and DAI ops.
• New ASoC platform: sound/soc/<vendor>/<soc>/ for the I2S/TDM/PDM peripheral.

Related pages

• Drivers — bus drivers used by sound.
• Memory management — DMA mapping for PCM buffers.