Linux 内核 ASoC 框架，在概念上将嵌入式音频系统拆分为多个可复用的组件驱动程序，包括 Codec 类驱动程序、平台类驱动程序和机器类驱动程序。在实现上，机器类驱动程序用 struct snd_soc_card 和 struct snd_soc_dai_link 结构描述，属于平台类驱动程序的 DMA 引擎驱动程序由 struct snd_soc_component_driver 结构描述，codec 类驱动程序和 I2S 等驱动程序，由 struct snd_soc_component_driver、struct snd_soc_dai_driver 和 struct snd_soc_dai_ops 等结构描述。除平台类驱动程序外的各种驱动程序都通过 component 抽象组织在一起，即这些驱动程序都作为 struct snd_soc_component_driver 注册给 Linux 内核 ASoC 框架，Linux 内核 ASoC 框架为它们各自创建 struct snd_soc_component 结构对象，并保存在 sound/soc/soc-core.c 文件中定义的全局链表 component_list 中。

一个 DMA 驱动程序的示例为 soc/pxa/pxa2xx-pcm.c：

static const struct snd_soc_component_driver pxa2xx_soc_platform = {
    .pcm_construct  = pxa2xx_soc_pcm_new,
    .pcm_destruct   = pxa2xx_soc_pcm_free,
    .open       = pxa2xx_soc_pcm_open,
    .close      = pxa2xx_soc_pcm_close,
    .hw_params  = pxa2xx_soc_pcm_hw_params,
    .hw_free    = pxa2xx_soc_pcm_hw_free,
    .prepare    = pxa2xx_soc_pcm_prepare,
    .trigger    = pxa2xx_soc_pcm_trigger,
    .pointer    = pxa2xx_soc_pcm_pointer,
    .mmap       = pxa2xx_soc_pcm_mmap,
};

static int pxa2xx_soc_platform_probe(struct platform_device *pdev)
{
    return devm_snd_soc_register_component(&pdev->dev, &pxa2xx_soc_platform,
                           NULL, 0);
}

static struct platform_driver pxa_pcm_driver = {
    .driver = {
        .name = "pxa-pcm-audio",
    },

    .probe = pxa2xx_soc_platform_probe,
};

module_platform_driver(pxa_pcm_driver);

DMA 引擎驱动程序和 I2S 或 Codec 驱动程序一样，通过 devm_snd_soc_register_component() 函数以 struct snd_soc_component_driver 的形式注册给 Linux 内核 ASoC 框架，但比较特别的地方在于，它的 dai driver 参数为空。

机器类驱动程序定义的 struct snd_soc_dai_link 结构对象通过 struct snd_soc_dai_link_component 描述它引用的其它类型的驱动程序，如机器类驱动程序 sound/soc/pxa/e800_wm9712.c 有如下的代码片段：

SND_SOC_DAILINK_DEFS(ac97,
    DAILINK_COMP_ARRAY(COMP_CPU("pxa2xx-ac97")),
    DAILINK_COMP_ARRAY(COMP_CODEC("wm9712-codec", "wm9712-hifi")),
    DAILINK_COMP_ARRAY(COMP_PLATFORM("pxa-pcm-audio")));

SND_SOC_DAILINK_DEFS(ac97_aux,
    DAILINK_COMP_ARRAY(COMP_CPU("pxa2xx-ac97-aux")),
    DAILINK_COMP_ARRAY(COMP_CODEC("wm9712-codec", "wm9712-aux")),
    DAILINK_COMP_ARRAY(COMP_PLATFORM("pxa-pcm-audio")));

static struct snd_soc_dai_link e800_dai[] = {
    {
        .name = "AC97",
        .stream_name = "AC97 HiFi",
        SND_SOC_DAILINK_REG(ac97),
    },
    {
        .name = "AC97 Aux",
        .stream_name = "AC97 Aux",
        SND_SOC_DAILINK_REG(ac97_aux),
    },
};

static struct snd_soc_card e800 = {
    .name = "Toshiba e800",
    .owner = THIS_MODULE,
    .dai_link = e800_dai,
    .num_links = ARRAY_SIZE(e800_dai),

    .dapm_widgets = e800_dapm_widgets,
    .num_dapm_widgets = ARRAY_SIZE(e800_dapm_widgets),
    .dapm_routes = audio_map,
    .num_dapm_routes = ARRAY_SIZE(audio_map),
};

SND_SOC_DAILINK_DEFS() 宏用于为 struct snd_soc_dai_link 方便地定义引用的 cpus、codecs 和 platforms 等 struct snd_soc_dai_link_component 数组，其中用于定义 platforms 的 DAILINK_COMP_ARRAY(COMP_PLATFORM("pxa-pcm-audio")) 引用了上面我们看到的 DMA 引擎驱动。

Linux 内核 ASoC 框架提供了一个通用的 DMA 引擎驱动程序，位于文件 sound/soc/soc-generic-dmaengine-pcm 中。这个驱动程序本身不会主动向 Linux 内核 ASoC 框架注册自己，需要使用 DMA 引擎在设备和内存之间传数据的驱动程序要在 probe 时注册它，如 sound/soc/rockchip/rockchip_pcm.c：

static const struct snd_pcm_hardware snd_rockchip_hardware = {
    .info           = SNDRV_PCM_INFO_MMAP |
                  SNDRV_PCM_INFO_MMAP_VALID |
                  SNDRV_PCM_INFO_PAUSE |
                  SNDRV_PCM_INFO_RESUME |
                  SNDRV_PCM_INFO_INTERLEAVED,
    .period_bytes_min   = 32,
    .period_bytes_max   = 8192,
    .periods_min        = 1,
    .periods_max        = 52,
    .buffer_bytes_max   = 64 * 1024,
    .fifo_size      = 32,
};

static const struct snd_dmaengine_pcm_config rk_dmaengine_pcm_config = {
    .pcm_hardware = &snd_rockchip_hardware,
    .prepare_slave_config = snd_dmaengine_pcm_prepare_slave_config,
    .prealloc_buffer_size = 32 * 1024,
};

int rockchip_pcm_platform_register(struct device *dev)
{
    return devm_snd_dmaengine_pcm_register(dev, &rk_dmaengine_pcm_config,
        SND_DMAENGINE_PCM_FLAG_COMPAT);
}
EXPORT_SYMBOL_GPL(rockchip_pcm_platform_register);

这里的 rockchip_pcm_platform_register() 函数在 I2S 驱动程序的 probe 操作 (位于 sound/soc/rockchip/rockchip_i2s.c) 中调用：

static int rockchip_i2s_probe(struct platform_device *pdev)
{
 . . . . . .

    ret = devm_snd_soc_register_component(&pdev->dev,
                          &rockchip_i2s_component,
                          soc_dai, 1);

    if (ret) {
        dev_err(&pdev->dev, "Could not register DAI\n");
        goto err_suspend;
    }

    ret = rockchip_pcm_platform_register(&pdev->dev);
    if (ret) {
        dev_err(&pdev->dev, "Could not register PCM\n");
        goto err_suspend;
    }

    return 0;
 . . . . . .
    return ret;
}

rockchip_pcm_platform_register() 函数调用 devm_snd_dmaengine_pcm_register() 函数注册通用 DMA 引擎驱动程序。devm_snd_dmaengine_pcm_register() 函数定义 (位于 sound/soc/soc-devres.c) 如下：

static void devm_dmaengine_pcm_release(struct device *dev, void *res)
{
    snd_dmaengine_pcm_unregister(*(struct device **)res);
}

/**
 * devm_snd_dmaengine_pcm_register - resource managed dmaengine PCM registration
 * @dev: The parent device for the PCM device
 * @config: Platform specific PCM configuration
 * @flags: Platform specific quirks
 *
 * Register a dmaengine based PCM device with automatic unregistration when the
 * device is unregistered.
 */
int devm_snd_dmaengine_pcm_register(struct device *dev,
    const struct snd_dmaengine_pcm_config *config, unsigned int flags)
{
    struct device **ptr;
    int ret;

    ptr = devres_alloc(devm_dmaengine_pcm_release, sizeof(*ptr), GFP_KERNEL);
    if (!ptr)
        return -ENOMEM;

    ret = snd_dmaengine_pcm_register(dev, config, flags);
    if (ret == 0) {
        *ptr = dev;
        devres_add(dev, ptr);
    } else {
        devres_free(ptr);
    }

    return ret;
}
EXPORT_SYMBOL_GPL(devm_snd_dmaengine_pcm_register);

#endif

devm_snd_dmaengine_pcm_register() 函数和之前看到的devm_snd_soc_register_card() 与 devm_snd_soc_register_component() 函数一样，只是它封装的是 snd_dmaengine_pcm_register() 函数。snd_dmaengine_pcm_register() 函数定义 (位于 sound/soc/soc-generic-dmaengine-pcm.c) 如下：

static const struct snd_soc_component_driver dmaengine_pcm_component = {
    .name       = SND_DMAENGINE_PCM_DRV_NAME,
    .probe_order    = SND_SOC_COMP_ORDER_LATE,
    .open       = dmaengine_pcm_open,
    .close      = dmaengine_pcm_close,
    .hw_params  = dmaengine_pcm_hw_params,
    .trigger    = dmaengine_pcm_trigger,
    .pointer    = dmaengine_pcm_pointer,
    .pcm_construct  = dmaengine_pcm_new,
};

static const struct snd_soc_component_driver dmaengine_pcm_component_process = {
    .name       = SND_DMAENGINE_PCM_DRV_NAME,
    .probe_order    = SND_SOC_COMP_ORDER_LATE,
    .open       = dmaengine_pcm_open,
    .close      = dmaengine_pcm_close,
    .hw_params  = dmaengine_pcm_hw_params,
    .trigger    = dmaengine_pcm_trigger,
    .pointer    = dmaengine_pcm_pointer,
    .copy_user  = dmaengine_copy_user,
    .pcm_construct  = dmaengine_pcm_new,
};

static const char * const dmaengine_pcm_dma_channel_names[] = {
    [SNDRV_PCM_STREAM_PLAYBACK] = "tx",
    [SNDRV_PCM_STREAM_CAPTURE] = "rx",
};

static int dmaengine_pcm_request_chan_of(struct dmaengine_pcm *pcm,
    struct device *dev, const struct snd_dmaengine_pcm_config *config)
{
    unsigned int i;
    const char *name;
    struct dma_chan *chan;

    if ((pcm->flags & SND_DMAENGINE_PCM_FLAG_NO_DT) || (!dev->of_node &&
        !(config && config->dma_dev && config->dma_dev->of_node)))
        return 0;

    if (config && config->dma_dev) {
        /*
         * If this warning is seen, it probably means that your Linux
         * device structure does not match your HW device structure.
         * It would be best to refactor the Linux device structure to
         * correctly match the HW structure.
         */
        dev_warn(dev, "DMA channels sourced from device %s",
             dev_name(config->dma_dev));
        dev = config->dma_dev;
    }

    for_each_pcm_streams(i) {
        if (pcm->flags & SND_DMAENGINE_PCM_FLAG_HALF_DUPLEX)
            name = "rx-tx";
        else
            name = dmaengine_pcm_dma_channel_names[i];
        if (config && config->chan_names[i])
            name = config->chan_names[i];
        chan = dma_request_chan(dev, name);
        if (IS_ERR(chan)) {
            /*
             * Only report probe deferral errors, channels
             * might not be present for devices that
             * support only TX or only RX.
             */
            if (PTR_ERR(chan) == -EPROBE_DEFER)
                return -EPROBE_DEFER;
            pcm->chan[i] = NULL;
        } else {
            pcm->chan[i] = chan;
        }
        if (pcm->flags & SND_DMAENGINE_PCM_FLAG_HALF_DUPLEX)
            break;
    }

    if (pcm->flags & SND_DMAENGINE_PCM_FLAG_HALF_DUPLEX)
        pcm->chan[1] = pcm->chan[0];

    return 0;
}

static void dmaengine_pcm_release_chan(struct dmaengine_pcm *pcm)
{
    unsigned int i;

    for_each_pcm_streams(i) {
        if (!pcm->chan[i])
            continue;
        dma_release_channel(pcm->chan[i]);
        if (pcm->flags & SND_DMAENGINE_PCM_FLAG_HALF_DUPLEX)
            break;
    }
}

/**
 * snd_dmaengine_pcm_register - Register a dmaengine based PCM device
 * @dev: The parent device for the PCM device
 * @config: Platform specific PCM configuration
 * @flags: Platform specific quirks
 */
int snd_dmaengine_pcm_register(struct device *dev,
    const struct snd_dmaengine_pcm_config *config, unsigned int flags)
{
    const struct snd_soc_component_driver *driver;
    struct dmaengine_pcm *pcm;
    int ret;

    pcm = kzalloc(sizeof(*pcm), GFP_KERNEL);
    if (!pcm)
        return -ENOMEM;

#ifdef CONFIG_DEBUG_FS
    pcm->component.debugfs_prefix = "dma";
#endif
    pcm->config = config;
    pcm->flags = flags;

    ret = dmaengine_pcm_request_chan_of(pcm, dev, config);
    if (ret)
        goto err_free_dma;

    if (config && config->process)
        driver = &dmaengine_pcm_component_process;
    else
        driver = &dmaengine_pcm_component;

    ret = snd_soc_component_initialize(&pcm->component, driver, dev);
    if (ret)
        goto err_free_dma;

    ret = snd_soc_add_component(&pcm->component, NULL, 0);
    if (ret)
        goto err_free_dma;

    return 0;

err_free_dma:
    dmaengine_pcm_release_chan(pcm);
    kfree(pcm);
    return ret;
}
EXPORT_SYMBOL_GPL(snd_dmaengine_pcm_register);

/**
 * snd_dmaengine_pcm_unregister - Removes a dmaengine based PCM device
 * @dev: Parent device the PCM was register with
 *
 * Removes a dmaengine based PCM device previously registered with
 * snd_dmaengine_pcm_register.
 */
void snd_dmaengine_pcm_unregister(struct device *dev)
{
    struct snd_soc_component *component;
    struct dmaengine_pcm *pcm;

    component = snd_soc_lookup_component(dev, SND_DMAENGINE_PCM_DRV_NAME);
    if (!component)
        return;

    pcm = soc_component_to_pcm(component);

    snd_soc_unregister_component_by_driver(dev, component->driver);
    dmaengine_pcm_release_chan(pcm);
    kfree(pcm);
}
EXPORT_SYMBOL_GPL(snd_dmaengine_pcm_unregister);

MODULE_LICENSE("GPL");

snd_dmaengine_pcm_register() 函数的执行过程如下：

1. 动态分配一个 struct dmaengine_pcm 结构对象，并初始化其 config 和 flags 等字段，struct dmaengine_pcm 结构定义 (位于 include/sound/dmaengine_pcm.h) 如下：

   struct dmaengine_pcm {
   struct dma_chan *chan[SNDRV_PCM_STREAM_LAST + 1];
   const struct snd_dmaengine_pcm_config *config;
   struct snd_soc_component component;
   unsigned int flags;
   };

这个结构有一个 struct snd_soc_component 结构成员；

1. 为数据的发送和接收申请 DMA 通道，这个需要在设备树的设备节点定义中，指定发送和接收引用的 DMA 通道，像下面这样：

   i2s0_8ch: i2s@fe470000 {

   . . . . . .
   dmas = <&dmac0 0>, <&dmac0 1>;
   dma-names = "tx", "rx";
   . . . . . .
   };

2. 根据传入的 config 参数，选择 struct snd_soc_component_driver，dmaengine_pcm_component_process 和 dmaengine_pcm_component 仅有的区别是，前者多定义了一个 copy_user 操作；

3. 初始化并添加 struct snd_soc_component 结构对象，有一个函数 (位于 include/sound/dmaengine_pcm.h) 可以通过 struct snd_soc_component 结构对象获得 struct dmaengine_pcm 结构对象：

   static inline struct dmaengine_pcm *soc_component_to_pcm(struct snd_soc_component *p)
   {
   return container_of(p, struct dmaengine_pcm, component);
   }

通用 DMA 引擎驱动程序支持的操作由 struct snd_soc_component_driver 定义。在全局 component 链表中，相应的 struct snd_soc_component 由注册它的 dev 标识。

使用了通用 DMA 引擎驱动程序的 ASoC 机器驱动程序，示例 (位于 sound/soc/rockchip/rockchip_rt5645.c) 如下：

SND_SOC_DAILINK_DEFS(pcm,
    DAILINK_COMP_ARRAY(COMP_EMPTY()),
    DAILINK_COMP_ARRAY(COMP_CODEC(NULL, "rt5645-aif1")),
    DAILINK_COMP_ARRAY(COMP_EMPTY()));

static struct snd_soc_dai_link rk_dailink = {
    .name = "rt5645",
    .stream_name = "rt5645 PCM",
    .init = rk_init,
    .ops = &rk_aif1_ops,
    /* set rt5645 as slave */
    .dai_fmt = SND_SOC_DAIFMT_I2S | SND_SOC_DAIFMT_NB_NF |
        SND_SOC_DAIFMT_CBS_CFS,
    SND_SOC_DAILINK_REG(pcm),
};

static struct snd_soc_card snd_soc_card_rk = {
    .name = "I2S-RT5650",
    .owner = THIS_MODULE,
    .dai_link = &rk_dailink,
    .num_links = 1,
    .dapm_widgets = rk_dapm_widgets,
    .num_dapm_widgets = ARRAY_SIZE(rk_dapm_widgets),
    .dapm_routes = rk_audio_map,
    .num_dapm_routes = ARRAY_SIZE(rk_audio_map),
    .controls = rk_mc_controls,
    .num_controls = ARRAY_SIZE(rk_mc_controls),
};
 . . . . . .
static int snd_rk_mc_probe(struct platform_device *pdev)
{
 . . . . . .
    rk_dailink.cpus->of_node = of_parse_phandle(np,
            "rockchip,i2s-controller", 0);
    if (!rk_dailink.cpus->of_node) {
        dev_err(&pdev->dev,
            "Property 'rockchip,i2s-controller' missing or invalid\n");
        ret = -EINVAL;
        goto put_codec_of_node;
    }

    rk_dailink.platforms->of_node = rk_dailink.cpus->of_node;
 . . . . . .
}

这里为 dai link 定义的 cpus dai 数组和 platforms 数组中都只有一个空元素，但在 probe 操作中，根据设备树中设备节点的定义，查找了对应的 of_node。尽管 cpus dai 和 platforms 引用了相同的 of_node，但在 snd_soc_add_pcm_runtime() 函数中，为 CPU DAI 添加 struct snd_soc_component 的过程是，先查找对应的 struct snd_soc_dai，再从 struct snd_soc_dai 获得 struct snd_soc_component，这也就意味着，为 CPU DAI 查找 struct snd_soc_component 时，不会找到没有 struct snd_soc_dai 的通用 DMA 引擎的 struct snd_soc_component。为 platforms 添加 struct snd_soc_component 的过程，则是直接查找所有匹配的 struct snd_soc_component 并添加。

在为 PCM 创建 struct snd_soc_pcm_runtime 时，即在机器类驱动程序的 probe 操作中，如果为 snd_soc_dai_link 做了类似于这里的处理，使 CPU DAI 和 platform 指向相同的 of_node，且对应于 of_node 的设备驱动程序注册了通用 DMA 引擎驱动程序，通用 DMA 引擎的 struct snd_soc_component 将会包含在它的 component 链表中。

通用 DMA 引擎驱动程序的 struct snd_soc_component_driver 还有一个特别的地方是，指定了 probe_order 为 SND_SOC_COMP_ORDER_LATE，这使得它的 probe 和 init 操作，相对而言，执行的更晚，同时它在 snd_soc_card 中的位置也更靠后一点。如在 soc_probe_link_components() 函数 (位于 sound/soc/soc-core.c) 中：

static int soc_probe_component(struct snd_soc_card *card,
                   struct snd_soc_component *component)
{
    struct snd_soc_dapm_context *dapm =
        snd_soc_component_get_dapm(component);
    struct snd_soc_dai *dai;
    int probed = 0;
    int ret;

    if (!strcmp(component->name, "snd-soc-dummy"))
        return 0;

    if (component->card) {
        if (component->card != card) {
            dev_err(component->dev,
                "Trying to bind component to card \"%s\" but is already bound to card \"%s\"\n",
                card->name, component->card->name);
            return -ENODEV;
        }
        return 0;
    }

    ret = snd_soc_component_module_get_when_probe(component);
    if (ret < 0)
        return ret;

    component->card = card;
    soc_set_name_prefix(card, component);

    soc_init_component_debugfs(component);

    snd_soc_dapm_init(dapm, card, component);

    ret = snd_soc_dapm_new_controls(dapm,
                    component->driver->dapm_widgets,
                    component->driver->num_dapm_widgets);

    if (ret != 0) {
        dev_err(component->dev,
            "Failed to create new controls %d\n", ret);
        goto err_probe;
    }

    for_each_component_dais(component, dai) {
        ret = snd_soc_dapm_new_dai_widgets(dapm, dai);
        if (ret != 0) {
            dev_err(component->dev,
                "Failed to create DAI widgets %d\n", ret);
            goto err_probe;
        }
    }

    ret = snd_soc_component_probe(component);
    if (ret < 0) {
        dev_err(component->dev,
            "ASoC: failed to probe component %d\n", ret);
        goto err_probe;
    }
    WARN(dapm->idle_bias_off &&
         dapm->bias_level != SND_SOC_BIAS_OFF,
         "codec %s can not start from non-off bias with idle_bias_off==1\n",
         component->name);
    probed = 1;

    /*
     * machine specific init
     * see
     *  snd_soc_component_set_aux()
     */
    ret = snd_soc_component_init(component);
    if (ret < 0)
        goto err_probe;

    ret = snd_soc_add_component_controls(component,
                         component->driver->controls,
                         component->driver->num_controls);
    if (ret < 0)
        goto err_probe;

    ret = snd_soc_dapm_add_routes(dapm,
                      component->driver->dapm_routes,
                      component->driver->num_dapm_routes);
    if (ret < 0) {
        if (card->disable_route_checks) {
            dev_info(card->dev,
                 "%s: disable_route_checks set, ignoring errors on add_routes\n",
                 __func__);
        } else {
            dev_err(card->dev,
                "%s: snd_soc_dapm_add_routes failed: %d\n",
                __func__, ret);
            goto err_probe;
        }
    }

    /* see for_each_card_components */
    list_add(&component->card_list, &card->component_dev_list);

err_probe:
    if (ret < 0)
        soc_remove_component(component, probed);

    return ret;
}
 . . . . . .
static int soc_probe_link_components(struct snd_soc_card *card)
{
    struct snd_soc_component *component;
    struct snd_soc_pcm_runtime *rtd;
    int i, ret, order;

    for_each_comp_order(order) {
        for_each_card_rtds(card, rtd) {
            for_each_rtd_components(rtd, i, component) {
                if (component->driver->probe_order != order)
                    continue;

                ret = soc_probe_component(card, component);
                if (ret < 0)
                    return ret;
            }
        }
    }

    return 0;
}

这里用到的 for_each_comp_order() 宏定义 (位于 include/sound/soc-component.h) 如下：

#define SND_SOC_COMP_ORDER_FIRST    -2
#define SND_SOC_COMP_ORDER_EARLY    -1
#define SND_SOC_COMP_ORDER_NORMAL     0
#define SND_SOC_COMP_ORDER_LATE         1
#define SND_SOC_COMP_ORDER_LAST         2

#define for_each_comp_order(order)        \
    for (order  = SND_SOC_COMP_ORDER_FIRST; \
         order <= SND_SOC_COMP_ORDER_LAST;  \
         order++)

通用 DMA 引擎驱动程序提供的操作如下：

static const struct snd_soc_component_driver dmaengine_pcm_component = {
    .name       = SND_DMAENGINE_PCM_DRV_NAME,
    .probe_order    = SND_SOC_COMP_ORDER_LATE,
    .open       = dmaengine_pcm_open,
    .close      = dmaengine_pcm_close,
    .hw_params  = dmaengine_pcm_hw_params,
    .trigger    = dmaengine_pcm_trigger,
    .pointer    = dmaengine_pcm_pointer,
    .pcm_construct  = dmaengine_pcm_new,
};

static const struct snd_soc_component_driver dmaengine_pcm_component_process = {
    .name       = SND_DMAENGINE_PCM_DRV_NAME,
    .probe_order    = SND_SOC_COMP_ORDER_LATE,
    .open       = dmaengine_pcm_open,
    .close      = dmaengine_pcm_close,
    .hw_params  = dmaengine_pcm_hw_params,
    .trigger    = dmaengine_pcm_trigger,
    .pointer    = dmaengine_pcm_pointer,
    .copy_user  = dmaengine_copy_user,
    .pcm_construct  = dmaengine_pcm_new,
};

这些操作中，最早被调用的是 pcm_construct 操作，也就是 dmaengine_pcm_new() 函数。在机器类驱动程序中，调用 devm_snd_soc_register_card() 函数注册声卡，在这个函数中有如下调用过程，snd_soc_register_card() -> snd_soc_bind_card() -> soc_init_pcm_runtime() -> soc_new_pcm() -> snd_soc_pcm_component_new()，snd_soc_pcm_component_new() 函数执行各个 struct snd_soc_component_driver 的 pcm_construct 操作。snd_soc_pcm_component_new() 函数定义 (位于 sound/soc/soc-component.c) 如下：

int snd_soc_pcm_component_new(struct snd_soc_pcm_runtime *rtd)
{
    struct snd_soc_component *component;
    int ret;
    int i;

    for_each_rtd_components(rtd, i, component) {
        if (component->driver->pcm_construct) {
            ret = component->driver->pcm_construct(component, rtd);
            if (ret < 0)
                return soc_component_ret(component, ret);
        }
    }

    return 0;
}

回到通用 DMA 引擎驱动程序的 pcm_construct 操作 dmaengine_pcm_new() 函数，这个函数定义 (位于 sound/soc/soc-generic-dmaengine-pcm.c) 如下：

static int dmaengine_pcm_new(struct snd_soc_component *component,
                 struct snd_soc_pcm_runtime *rtd)
{
    struct dmaengine_pcm *pcm = soc_component_to_pcm(component);
    const struct snd_dmaengine_pcm_config *config = pcm->config;
    struct device *dev = component->dev;
    struct snd_pcm_substream *substream;
    size_t prealloc_buffer_size;
    size_t max_buffer_size;
    unsigned int i;

    if (config && config->prealloc_buffer_size) {
        prealloc_buffer_size = config->prealloc_buffer_size;
        max_buffer_size = config->pcm_hardware->buffer_bytes_max;
    } else {
        prealloc_buffer_size = 512 * 1024;
        max_buffer_size = SIZE_MAX;
    }

    for_each_pcm_streams(i) {
        substream = rtd->pcm->streams[i].substream;
        if (!substream)
            continue;

        if (!pcm->chan[i] && config && config->chan_names[i])
            pcm->chan[i] = dma_request_slave_channel(dev,
                config->chan_names[i]);

        if (!pcm->chan[i] && (pcm->flags & SND_DMAENGINE_PCM_FLAG_COMPAT)) {
            pcm->chan[i] = dmaengine_pcm_compat_request_channel(
                component, rtd, substream);
        }

        if (!pcm->chan[i]) {
            dev_err(component->dev,
                "Missing dma channel for stream: %d\n", i);
            return -EINVAL;
        }

        snd_pcm_set_managed_buffer(substream,
                SNDRV_DMA_TYPE_DEV_IRAM,
                dmaengine_dma_dev(pcm, substream),
                prealloc_buffer_size,
                max_buffer_size);

        if (!dmaengine_pcm_can_report_residue(dev, pcm->chan[i]))
            pcm->flags |= SND_DMAENGINE_PCM_FLAG_NO_RESIDUE;

        if (rtd->pcm->streams[i].pcm->name[0] == '\0') {
            strscpy_pad(rtd->pcm->streams[i].pcm->name,
                    rtd->pcm->streams[i].pcm->id,
                    sizeof(rtd->pcm->streams[i].pcm->name));
        }
    }

    return 0;
}

这个函数分别为播放和录制申请 DMA 通道，并分配 DMA buffer 用于用户空间应用程序、内核 ALSA/ASoc 框架和硬件设备之间的数据交换。snd_pcm_set_managed_buffer() 函数分配 DMA 缓冲区。snd_pcm_set_managed_buffer() 函数定义 (位于 sound/core/pcm_memory.c) 如下：

static void preallocate_pages(struct snd_pcm_substream *substream,
                  int type, struct device *data,
                  size_t size, size_t max, bool managed)
{
    if (snd_BUG_ON(substream->dma_buffer.dev.type))
        return;

    substream->dma_buffer.dev.type = type;
    substream->dma_buffer.dev.dev = data;

    if (size > 0 && preallocate_dma && substream->number < maximum_substreams)
        preallocate_pcm_pages(substream, size);

    if (substream->dma_buffer.bytes > 0)
        substream->buffer_bytes_max = substream->dma_buffer.bytes;
    substream->dma_max = max;
    if (max > 0)
        preallocate_info_init(substream);
    if (managed)
        substream->managed_buffer_alloc = 1;
}
 . . . . . .
void snd_pcm_set_managed_buffer(struct snd_pcm_substream *substream, int type,
                struct device *data, size_t size, size_t max)
{
    preallocate_pages(substream, type, data, size, max, true);
}
EXPORT_SYMBOL(snd_pcm_set_managed_buffer);

open 操作 dmaengine_pcm_open() 函数完成数据传输之前的准备工作，这个函数定义如下：

static int
dmaengine_pcm_set_runtime_hwparams(struct snd_soc_component *component,
                   struct snd_pcm_substream *substream)
{
    struct snd_soc_pcm_runtime *rtd = asoc_substream_to_rtd(substream);
    struct dmaengine_pcm *pcm = soc_component_to_pcm(component);
    struct device *dma_dev = dmaengine_dma_dev(pcm, substream);
    struct dma_chan *chan = pcm->chan[substream->stream];
    struct snd_dmaengine_dai_dma_data *dma_data;
    struct snd_pcm_hardware hw;

    if (rtd->num_cpus > 1) {
        dev_err(rtd->dev,
            "%s doesn't support Multi CPU yet\n", __func__);
        return -EINVAL;
    }

    if (pcm->config && pcm->config->pcm_hardware)
        return snd_soc_set_runtime_hwparams(substream,
                pcm->config->pcm_hardware);

    dma_data = snd_soc_dai_get_dma_data(asoc_rtd_to_cpu(rtd, 0), substream);

    memset(&hw, 0, sizeof(hw));
    hw.info = SNDRV_PCM_INFO_MMAP | SNDRV_PCM_INFO_MMAP_VALID |
            SNDRV_PCM_INFO_INTERLEAVED;
    hw.periods_min = 2;
    hw.periods_max = UINT_MAX;
    hw.period_bytes_min = 256;
    hw.period_bytes_max = dma_get_max_seg_size(dma_dev);
    hw.buffer_bytes_max = SIZE_MAX;
    hw.fifo_size = dma_data->fifo_size;

    if (pcm->flags & SND_DMAENGINE_PCM_FLAG_NO_RESIDUE)
        hw.info |= SNDRV_PCM_INFO_BATCH;

    /**
     * FIXME: Remove the return value check to align with the code
     * before adding snd_dmaengine_pcm_refine_runtime_hwparams
     * function.
     */
    snd_dmaengine_pcm_refine_runtime_hwparams(substream,
                          dma_data,
                          &hw,
                          chan);

    return snd_soc_set_runtime_hwparams(substream, &hw);
}

static int dmaengine_pcm_open(struct snd_soc_component *component,
                  struct snd_pcm_substream *substream)
{
    struct dmaengine_pcm *pcm = soc_component_to_pcm(component);
    struct dma_chan *chan = pcm->chan[substream->stream];
    int ret;

    ret = dmaengine_pcm_set_runtime_hwparams(component, substream);
    if (ret)
        return ret;

    return snd_dmaengine_pcm_open(substream, chan);
}

这里调用 snd_soc_dai_get_dma_data() 函数获得类型为 struct snd_dmaengine_dai_dma_data 的 DMA 数据。这是通用 DMA 引擎驱动程序和 I2S 等 DAI 驱动程序的一个约定，即需要在 DAI 驱动程序的 probe 操作中设置 DMA 数据，类似于下面这样：

struct i2s_dev {
    struct device *dev;
 . . . . . .
    struct snd_dmaengine_dai_dma_data capture_dma_data;
    struct snd_dmaengine_dai_dma_data playback_dma_data;
 . . . . . .
}
 . . . . . .
static int i2s_dai_probe(struct snd_soc_dai *dai)
{
    struct i2s_dev *i2s = snd_soc_dai_get_drvdata(dai);

    snd_soc_dai_init_dma_data(dai,
        i2s->has_playback ? &i2s->playback_dma_data : NULL,
        i2s->has_capture  ? &i2s->capture_dma_data  : NULL);

    return 0;
}
 . . . . . .
static int i2s_init_dai(struct i2s_dev *i2s, struct resource *res,
                                 struct snd_soc_dai_driver **dp)
{
 . . . . . .
        i2s->playback_dma_data.addr = res->start + I2S_TXFIFODATA;
        i2s->playback_dma_data.addr_width = DMA_SLAVE_BUSWIDTH_4_BYTES;
        i2s->playback_dma_data.maxburst = 8;

 . . . . . .
        i2s->capture_dma_data.addr = res->start + I2S_RXFIFODATA;
        i2s->capture_dma_data.addr_width = DMA_SLAVE_BUSWIDTH_4_BYTES;
        i2s->capture_dma_data.maxburst = 8;
 . . . . . .
}

dmaengine_pcm_open() 函数主要是为流设置了运行时硬件参数。

对于其它操作，暂时不做太多说明。

Done.