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); #endifdevm_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：
- 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；
- 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"; . . . . . . };-
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； -
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++)Linux 内核 ASoC 通用 DMA 引擎驱动程序的操作
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 intdmaengine_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.
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