Chapter 53: Sound: ALSA and ASoC

What: the kernel’s audio framework, ALSA (Advanced Linux Sound Architecture) and ASoC (ALSA System on Chip, the embedded refinement). ASoC splits an audio chain into three drivers. The CPU-DAI is the SoC’s I²S/SAI controller. The codec is the analog chip (WM8960, SGTL5000). The machine driver wires them together for one specific board. By the end you can aplay test.wav over a WM8960 on the Point Atom ALPHA. ASoC: ALSA System-on-Chip, the embedded audio layer that connects CPU audio ports, codecs, and board wiring. ALSA: Linux’s kernel and user-space audio stack.

Why: audio is one of the tightest real-time loops in any embedded system. 48000 samples/second × 2 channels × 16 bits = a 96 KB/s data stream that must never glitch. ASoC’s three-way split is the kernel’s solution to not re-writing a complete audio driver for every new SoC + codec combination, you reuse the CPU-DAI and codec drivers, only writing a small machine driver per board.

Focus: the three drivers cooperate via snd_soc_dai_link. The machine driver declares “CPU-DAI X drives codec Y over format Z at clock W.” Once you understand this binding, ASoC drivers become readable.

Tooling. This chapter uses alsa-utils (alsamixer, aplay, arecord, amixer).

  • Ubuntu-base (target): apt install alsa-utils libasound2-dev

  • Buildroot: BR2_PACKAGE_ALSA_UTILS=y BR2_PACKAGE_ALSA_LIB=y Buildroot: a configuration-driven build system that produces a complete root filesystem and related images.

  • Full per-tool reference: Userspace tooling appendix. MCU bridge: Think of the rootfs as the firmware image’s file-backed runtime environment. On an MCU you link everything into flash. On Linux, programs and config live in this mounted tree. rootfs: root filesystem, the directory tree mounted at / that contains /bin, /etc, /dev, and libraries.

53.1 Three drivers cooperating

   ┌───────────────────────────────────────────────────┐
   │           ALSA user-space (aplay, PulseAudio)      │
   └───────────────────────────────────────────────────┘
                              │ /dev/snd/pcmC0D0p
                              ▼
   ┌───────────────────────────────────────────────────┐
   │              ALSA core (snd_pcm)                   │
   └───────────────────────────────────────────────────┘
                              │
                              ▼
   ┌───────────────────────────────────────────────────┐
   │   ASoC core (snd_soc_*)                             │
   └───────────────────────────────────────────────────┘
        │                    │                    │
        ▼                    ▼                    ▼
   ┌─────────┐         ┌─────────┐          ┌─────────┐
   │ Machine │   ←→    │   CPU   │   ←I²S→  │  Codec  │
   │ driver  │         │   DAI   │          │ (WM8960)│
   │ (board) │         │ (SAI3)  │          │         │
   └─────────┘         └─────────┘          └─────────┘
                          │                     │
                          │ MMIO + DMA          │ I²C control + I²S audio
                          ▼                     ▼
                      i.MX SoC               codec chip

  • CPU-DAI: the SoC’s audio peripheral driver. For i.MX6ULL: SAI (Synchronous Audio Interface). The driver is drivers/sound/soc/fsl/fsl_sai.c. Speaks I²S to the codec.

  • Codec: the codec chip’s driver, e.g., sound/soc/codecs/wm8960.c. Talks I²C for control (volume, mute, sample rate). Receives/sends I²S audio data.

  • Machine: the board-specific glue. E.g., sound/soc/fsl/imx-wm8960.c. Declares the DAI link and any board-specific routing (which speaker/headphone outputs are wired, which mics are connected).

53.2 Device tree for WM8960 audio

&sai2 {
    pinctrl-names = "default";
    pinctrl-0 = <&pinctrl_sai2>;
    assigned-clocks = <&clks IMX6UL_CLK_SAI2_SEL>,
                      <&clks IMX6UL_CLK_SAI2>;
    assigned-clock-parents = <&clks IMX6UL_CLK_PLL4_AUDIO_DIV>;
    assigned-clock-rates = <0>, <24576000>;
    status = "okay";
};

&i2c2 {
    wm8960: codec@1a {
        compatible = "wlf,wm8960";
        reg = <0x1a>;
        wlf,shared-lrclk;
        clocks = <&clks IMX6UL_CLK_SAI2>;
        clock-names = "mclk";
    };
};

sound {
    compatible = "fsl,imx-audio-wm8960";
    model = "wm8960-audio";
    audio-cpu = <&sai2>;
    audio-codec = <&wm8960>;
    audio-routing =
        "Headphone Jack", "HP_L",
        "Headphone Jack", "HP_R",
        "Ext Spk", "SPK_LP",
        "Ext Spk", "SPK_LN",
        "Ext Spk", "SPK_RP",
        "Ext Spk", "SPK_RN",
        "LINPUT1", "Mic Jack",
        "LINPUT3", "Mic Jack",
        "Mic Jack", "MICB";
    mux-int-port = <2>;
    mux-ext-port = <6>;
    hp-det-gpio = <&gpio5 4 GPIO_ACTIVE_HIGH>;
};

Three nodes:

  • &sai2: the SoC’s SAI2 audio peripheral. Provides the I²S link.

  • &i2c2 / wm8960: the codec on I²C2. The clocks = <&clks IMX6UL_CLK_SAI2> is the master clock supplying the codec.

  • sound: the machine driver binding. compatible = "fsl,imx-audio-wm8960" picks the i.MX-WM8960 machine driver. audio-routing declares which codec outputs are connected to which physical jacks.

53.3 How a sample plays

   aplay test.wav
       │
       │ open(/dev/snd/pcmC0D0p)
       │ ioctl(SNDRV_PCM_IOCTL_HW_PARAMS, format=S16_LE, rate=48000, channels=2)
       │ write() loop ── packed into period-sized chunks ──
       │
       ▼
   ALSA core fills a periodic DMA ring buffer
       │
       │ via dmaengine — cyclic transfer (Ch 51.5)
       ▼
   SDMA continuously transfers from ring buffer → SAI2 TX FIFO
       │
       │ I²S serial clock + frame sync + data
       ▼
   WM8960 sigma-delta DAC ──→ analog audio ──→ amp ──→ speaker

DMA fires an IRQ every period (typically 1024 samples, about 21 ms at 48 kHz). ALSA refills that period from the user-space buffer. The cycle continues. If user-space writes fast enough to avoid an underrun, playback continues without glitches.

MCU bridge: Think of an IRQ like an EXTI/NVIC interrupt path, except Linux splits the hard interrupt from deferred work and must share lines across drivers. MCU bridge: Think of DMA like the MCU DMA controller you used for UART or SPI, but with cache coherency, scatter-gather descriptors, and kernel ownership rules added. IRQ: interrupt request, the signal path that tells the CPU or interrupt controller that hardware needs service. DMA: Direct Memory Access. Hardware moves data to or from memory without the CPU copying each byte.

If userspace can’t keep up: xrun (underrun). ALSA logs it. Sound clicks or pauses. Reasons: CPU too loaded, period too short, user-space process scheduling jitter (Ch 52A’s PREEMPT_RT helps).

PREEMPT_RT: the Linux real-time patch set that makes more kernel paths preemptible and reduces latency.

53.4 Writing a machine driver

The machine driver is the thing you’d write for a new board. The CPU-DAI and codec drivers come from mainline. Sketch:

#include <sound/soc.h>

static const struct snd_soc_dapm_widget my_dapm_widgets[] = {
    SND_SOC_DAPM_HP("Headphone Jack", NULL),
    SND_SOC_DAPM_SPK("Ext Spk", NULL),
    SND_SOC_DAPM_MIC("Mic Jack", NULL),
};

static int my_init(struct snd_soc_pcm_runtime *rtd)
{
    struct snd_soc_card *card = rtd->card;
    /* Optional: clk routing, ALC settings, etc. */
    return 0;
}

SND_SOC_DAILINK_DEFS(hifi,
    DAILINK_COMP_ARRAY(COMP_CPU("imx-sai2")),
    DAILINK_COMP_ARRAY(COMP_CODEC("wm8960.1-001a", "wm8960-hifi")),
    DAILINK_COMP_ARRAY(COMP_PLATFORM("imx-sai2")));

static struct snd_soc_dai_link my_dai_link = {
    .name = "HiFi",
    .stream_name = "HiFi",
    .init = my_init,
    SND_SOC_DAILINK_REG(hifi),
    .dai_fmt = SND_SOC_DAIFMT_I2S
             | SND_SOC_DAIFMT_NB_NF
             | SND_SOC_DAIFMT_CBS_CFS,
};

static struct snd_soc_card my_card = {
    .name = "my-board-audio",
    .owner = THIS_MODULE,
    .dai_link = &my_dai_link,
    .num_links = 1,
    .dapm_widgets = my_dapm_widgets,
    .num_dapm_widgets = ARRAY_SIZE(my_dapm_widgets),
};

static int my_probe(struct platform_device *pdev)
{
    my_card.dev = &pdev->dev;
    return devm_snd_soc_register_card(&pdev->dev, &my_card);
}

The DAI link declares:

  • CPU side: which SAI peripheral.

  • Codec side: which I²C-bound chip + which DAI on that chip (codecs may have multiple DAIs).

  • DAI format: I²S protocol, normal bit clock + frame clock polarity (NB_NF), codec is bit-clock-slave + frame-clock-slave (CBS_CFS, meaning the SoC is the master).

That’s 99% of what a machine driver does. The hard work is in the codec driver and CPU-DAI driver, which are upstream.

53.5 User-space, ALSA tools

[root@pa-mini:~]# aplay -l                          # list playback devices
**** List of PLAYBACK Hardware Devices ****
card 0: wm8960audio [wm8960-audio], device 0: HiFi wm8960-hifi-0 []

[root@pa-mini:~]# arecord -l                        # list capture devices
**** List of CAPTURE Hardware Devices ****
card 0: wm8960audio [wm8960-audio], device 0: HiFi wm8960-hifi-0 []

[root@pa-mini:~]# alsamixer                         # interactive mixer
[root@pa-mini:~]# aplay -D plughw:0,0 -f S16_LE -r 48000 -c 2 test.wav
[root@pa-mini:~]# arecord -D plughw:0,0 -f S16_LE -r 48000 -c 2 -d 5 rec.wav
[root@pa-mini:~]# amixer set 'Headphone' 80%

For higher-level audio (Bluetooth, network), you’d stack PulseAudio or PipeWire on top. For embedded one-app systems, raw ALSA is fine.

53.6 Lab

  1. Get audio working on a board with WM8960. Enable in DT, boot, aplay test.wav, hear sound.

  2. Capture. arecord -d 5 rec.wav. Play back. Confirm round-trip works.

  3. Adjust mixer settings. alsamixer, try Headphone volume, capture-channel selection.

  4. Diagnose an xrun. Use aplay -v test.wav. Reduce period size with --period-size=128. Observe xruns appearing as load increases.

  5. Listen at different rates. aplay -f S24_LE -r 44100 ..., verify codec accepts non-48k rates.

  6. Read the WM8960 driver. sound/soc/codecs/wm8960.c, find the regmap config, the DAPM (Dynamic Audio Power Management) routing, the bias-level callbacks.

MCU bridge: Think of regmap like a typed wrapper around your read_reg() and write_reg() helpers, with caching, locking, and bus differences handled centrally. regmap: a kernel helper that wraps register reads and writes over I2C, SPI, or MMIO. MMIO: memory-mapped I/O, where software accesses peripheral registers through normal load and store instructions.

53.7 Pitfalls

  • Wrong I²S format. Codec expects “left-justified,” driver sends “I²S”: you hear silence or noise. Fix .dai_fmt = SND_SOC_DAIFMT_LEFT_J etc.

  • Master/slave clock mismatch. Both SoC and codec think they’re masters → no clock at all. One must be slave (CBS_CFS or CBM_CFM).

  • MCLK rate wrong. Codec needs e.g., 256× sample rate as master clock. Set assigned-clock-rates in DT for the codec’s MCLK input.

  • DAPM routes incomplete. Headphone outputs muted because DAPM thinks they’re not connected. Trace cat /sys/kernel/debug/asoc/.../dapm/*.

  • Wrong codec address. &i2c2 { wm8960: codec@1a { reg = <0x1a> ... } }, verify with i2cdetect.

  • MASTER clock not enabled. Codec needs MCLK to be running before any I²C command. Tie the codec’s clock to the SAI’s clock-gate so they enable together.

  • No DAPM widgets for jacks. Audio “plays” (DMA happily transfers) but mute amps because DAPM doesn’t know to route. Always declare physical jacks as widgets.

53.8 Going deeper

  • Documentation/sound/: ALSA documentation.

  • Documentation/sound/soc/: ASoC documentation. Start with overview.rst.

  • sound/soc/fsl/imx-wm8960.c: i.MX-WM8960 machine driver (~600 lines). Read this to understand a complete machine driver.

  • sound/soc/codecs/wm8960.c: production codec driver (~1500 lines). Excellent example of regmap + ASoC + DAPM.

  • sound/soc/fsl/fsl_sai.c: i.MX SAI CPU-DAI driver.

  • Documentation/devicetree/bindings/sound/: DT bindings.

Next chapter: Chapter 54: LCD framebuffer and DRM/KMS. Audio done, video next. From “Linux has /dev/fb0” to “Linux has DRM/KMS with mode-setting and Wayland-compatible output.”