1. 前言

限于作者能力水平，本文可能存在谬误，因此而给读者带来的损失，作者不做任何承诺。

2. ASoC 声卡设备硬件参数

ASoC(ALSA System-on-Chip) 声卡驱动框架如下图所示：

整个声卡驱动由 CPU DAI 驱动、CODEC DAI 驱动、Machine 驱动（胶水粘合驱动） 这 3 者拼接构成，所以声卡硬件参数，自然也同时受这 3 者的共同约束。

声卡的硬件参数管理过程包括以下两步：

1. 将 DAI (CPU 侧 + CODEC 侧)、Machine 平台的 声卡硬件参数 添加到声卡
2. 打开 PCM 流时，将 声卡的硬件参数 配置到 PCM 流
3. 应用程序在硬件参数允许的范围内，对 PCM 流参数进行修改调整

2.1 将 DAI、Machine 平台的硬件参数添加到声卡

声卡硬件参数主要包括：通道数，数据格式(S16_LE,S32_LE,...)，采样率，period 大小，FIFO 大小 等。这些声卡硬件参数来自于 DAI 接口驱动、Machine 平台驱动。这里以 CPU 侧 DAI 驱动硬件参数为例，来剖析声卡硬件参数的添加过程。如：

static struct snd_soc_dai_driver sun4i_i2s_dai = {.probe = sun4i_i2s_dai_probe,.capture = {.stream_name = "Capture",.channels_min = 2,.channels_max = 2,.rates = SNDRV_PCM_RATE_8000_192000,.formats = SNDRV_PCM_FMTBIT_S16_LE,},.playback = {.stream_name = "Playback",.channels_min = 2,.channels_max = 2,.rates = SNDRV_PCM_RATE_8000_192000,.formats = SNDRV_PCM_FMTBIT_S16_LE,},.ops = &sun4i_i2s_dai_ops,.symmetric_rates = 1,
};

上面的示例代码定义了 SoC CPU 一侧 DAI 接口 I2S 支持的通道数、数据格式、采样率，然后通过接口 snd_soc_register_component() ，将声卡 DAI 组件注册到系统，即添加到声卡组件对象(struct snd_soc_component)全局列表 component_list ：

devm_snd_soc_register_component(&pdev->dev,&sun4i_i2s_component,&sun4i_i2s_dai, 1)snd_soc_register_component()struct snd_soc_component *component;...component = kzalloc(sizeof(*component), GFP_KERNEL);...ret = snd_soc_component_initialize(component, component_driver, dev);...snd_soc_component_add(component)...list_add(&component->list, &component_list); /* (1) 添加到声卡组件全局列表 @component_list */

然后在注册声卡时，将声卡组件对象(struct snd_soc_component)添加到声卡（声卡组件对象(struct snd_soc_component)作为声卡的一部分），这一过程也将包含在 DAI 驱动内的 DAI 硬件参数添加到了声卡：

snd_soc_register_card()snd_soc_instantiate_card()soc_bind_dai_link()static int soc_bind_dai_link(struct snd_soc_card *card,struct snd_soc_dai_link *dai_link)
{.../* 集成 CPU 一侧 DAI (驱动) 到声卡 */rtd->cpu_dai = snd_soc_find_dai(&cpu_dai_component);snd_soc_rtdcom_add(rtd, rtd->cpu_dai->component);.../* Find CODEC from registered CODECs */codec_dais = rtd->codec_dais;for (i = 0; i < rtd->num_codecs; i++) { /* 集成 CODEC 一侧 DAI (驱动) 到声卡 */codec_dais[i] = snd_soc_find_dai(&codecs[i]); // 查找 DAI...snd_soc_rtdcom_add(rtd, codec_dais[i]->component); // 添加 DAI 到声卡(包括 DAI 携带的硬件参数)}...
}struct snd_soc_dai *snd_soc_find_dai(const struct snd_soc_dai_link_component *dlc)
{.../* Find CPU DAI from registered DAIs*/list_for_each_entry(component, &component_list, list) {// 匹配 DAI...list_for_each_entry(dai, &component->dai_list, list) {...return dai; // 返回匹配的 DAI}}...
}static int snd_soc_rtdcom_add(struct snd_soc_pcm_runtime *rtd,struct snd_soc_component *component)
{...list_add_tail(&new_rtdcom->list, &rtd->component_list);...
}

2.2 打开 PCM 流时将声卡硬件参数配置到 PCM 流

open("/dev/snd/pcmC%dD%dp", ...)...snd_pcm_playback_open() / snd_pcm_capture_open()snd_pcm_open()snd_pcm_open_file()snd_pcm_open_substream()struct snd_pcm_runtime {...struct snd_pcm_hardware hw;struct snd_pcm_hw_constraints hw_constraints;...
};int snd_pcm_open_substream(struct snd_pcm *pcm, int stream,struct file *file,struct snd_pcm_substream **rsubstream)
{...// 将 DAI 等的硬件参数 配置到 PCM 流if ((err = substream->ops->open(substream)) < 0) // ASoC 架构下: soc_pcm_open() goto error;...// 将 参数 转换为 约束参数形式err = snd_pcm_hw_constraints_complete(substream);...
}// 将 DAI 等的硬件参数 配置到 PCM 流
soc_pcm_open() // ASoC 架构下soc_pcm_init_runtime_hw()static void soc_pcm_init_runtime_hw(struct snd_pcm_substream *substream)
{struct snd_pcm_runtime *runtime = substream->runtime;struct snd_pcm_hardware *hw = &runtime->hw;...struct snd_soc_pcm_stream *codec_stream;struct snd_soc_pcm_stream *cpu_stream;...// CPU DAIif (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)cpu_stream = &cpu_dai_drv->playback; // &sun4i_i2s_dai.playbackelsecpu_stream = &cpu_dai_drv->capture; // &sun4i_i2s_dai.capture.../* 配置声卡 DAI 的参数到 PCM 流 */hw->channels_min = max(chan_min, cpu_stream->channels_min);hw->channels_max = min(chan_max, cpu_stream->channels_max);if (hw->formats)hw->formats &= formats & cpu_stream->formats;elsehw->formats = formats & cpu_stream->formats;...
}int snd_pcm_hw_constraints_complete(struct snd_pcm_substream *substream)
{struct snd_pcm_runtime *runtime = substream->runtime;struct snd_pcm_hardware *hw = &runtime->hw;......err = snd_pcm_hw_constraint_mask(runtime, SNDRV_PCM_HW_PARAM_ACCESS, mask);...err = snd_pcm_hw_constraint_mask64(runtime, SNDRV_PCM_HW_PARAM_FORMAT, hw->formats);...err = snd_pcm_hw_constraint_minmax(runtime, SNDRV_PCM_HW_PARAM_CHANNELS,hw->channels_min, hw->channels_max);...err = snd_pcm_hw_constraint_minmax(runtime, SNDRV_PCM_HW_PARAM_PERIODS,hw->periods_min, hw->periods_max);...err = snd_pcm_hw_constraint_minmax(runtime, SNDRV_PCM_HW_PARAM_BUFFER_BYTES,hw->period_bytes_min, hw->buffer_bytes_max);...
}int snd_pcm_hw_constraint_minmax(struct snd_pcm_runtime *runtime, snd_pcm_hw_param_t var,unsigned int min, unsigned int max)
{struct snd_pcm_hw_constraints *constrs = &runtime->hw_constraints;struct snd_interval t;t.min = min;t.max = max;t.openmin = t.openmax = 0;t.integer = 0;return snd_interval_refine(constrs_interval(constrs, var), &t);
}

2.3 应用程序对 PCM 流参数进行修改调整

在 2.2 小节中，在 open("/dev/snd/pcmC%dD%dp", ...) 打开 PCM 流时，设置了 PCM 的默认硬件参数，但这并不一定符合用户的预期，譬如采样率，通道数，数据格式等，这时候用户空间可以通过 ioctl(fd, SNDRV_PCM_IOCTL_HW_PARAMS, &hwparams) 来进行调整：

sys_ioctl()...snd_pcm_ioctl()snd_pcm_common_ioctl()snd_pcm_hw_params_user()snd_pcm_hw_params()static int snd_pcm_hw_params_user(struct snd_pcm_substream *substream,struct snd_pcm_hw_params __user * _params)
{struct snd_pcm_hw_params *params;.../* 复制用户空间参数到内核空间 */params = memdup_user(_params, sizeof(*params));.../* 按用户请求参数 @_params，对当前设置的参数做可能的调整 */err = snd_pcm_hw_params(substream, params);.../* 修改后的参数，从 @_params 返回用户空间 */if (copy_to_user(_params, params, sizeof(*params)))err = -EFAULT;
end:kfree(params);return err;
}static int snd_pcm_hw_params(struct snd_pcm_substream *substream,struct snd_pcm_hw_params *params)
{struct snd_pcm_runtime *runtime;...params->rmask = ~0U;err = snd_pcm_hw_refine(substream, params); /* 对 所有 硬件参数，按用户请求参数 @params，做可能的调整 */.../* 更新 用户参数 @params 到 PCM 流 */runtime->access = params_access(params);runtime->format = params_format(params);runtime->subformat = params_subformat(params);runtime->channels = params_channels(params);runtime->rate = params_rate(params);runtime->period_size = params_period_size(params);runtime->periods = params_periods(params);runtime->buffer_size = params_buffer_size(params);runtime->info = params->info;runtime->rate_num = params->rate_num;runtime->rate_den = params->rate_den;...return 0;_error:...
}int snd_pcm_hw_refine(struct snd_pcm_substream *substream,struct snd_pcm_hw_params *params)
{.../* 对所有 掩码类型参数 做可能的调整 */err = constrain_mask_params(substream, params);.../* 对所有 区间类型参数 做可能的调整 */err = constrain_interval_params(substream, params);.../* 对 所有类型(掩码类 + 区间类) 的所有参数 按 约束规则 做可能的调整 */err = constrain_params_by_rules(substream, params);...
}

这列只看 constrain_interval_params() 的调整参数的过程，其它的类似：

static int constrain_interval_params(struct snd_pcm_substream *substream,struct snd_pcm_hw_params *params)
{// 这里的 ，正是前面 2.2 分析 PCM 打开过程中， snd_pcm_hw_constraint_minmax() 设置的struct snd_pcm_hw_constraints *constrs =&substream->runtime->hw_constraints;struct snd_interval *i;unsigned int k;...for (k = SNDRV_PCM_HW_PARAM_FIRST_INTERVAL; k <= SNDRV_PCM_HW_PARAM_LAST_INTERVAL; k++) {i = hw_param_interval(params, k);.../* 对区间类型参数做可能的调整 */changed = snd_interval_refine(i, constrs_interval(constrs, k));...}return 0;
}

到此，对声卡硬件参数管理设置过程分析已毕。对应硬件参数，声卡还有软件参数，对这部分感兴趣的读者，可自行阅读相关源码。