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使用虚拟 codec 节点将SAI或I2S控制器注册为ASLA声卡
打开 dep
打开 DepAlsaBridge

注册虚拟声卡

设备树

参考文档：

dts

/{
    //虚拟codec节点
    dummy_codec: dummy-codec {
        status = "okay";
        compatible = "linux,dummy-codec";		/*虽然是用来占位的，但驱动中也需要对音频格式做支持*/
        #sound-dai-cells = <0>;
    };
    
    sound:sound{
        compatible = "simple-audio-card";
        simple-audio-card,name = "dummy-audio-card";

        simple-audio-card,dai-link@0 {
		    format = "i2s";
            bitclock-master = <&cpu0_audio>;
            frame-master = <&cpu0_audio>;
            convert-channels = <16>;
            mclk-fs = <256>;

		    cpu0_audio:cpu {
			    sound-dai = <&sai2a>;
                dai-tdm-slot-num = <16>;
                dai-tdm-slot-width = <32>;
		    };

		    codec {
			    sound-dai = <&dummy_codec>;
		    };
	    };

        simple-audio-card,dai-link@1 {
            format = "i2s";
            bitclock-master = <&cpu1_audio>;
            frame-master = <&cpu1_audio>;
            convert-channels = <16>;
            mclk-fs = <256>;

		    cpu1_audio:cpu {
			    sound-dai = <&sai2b>;
                dai-tdm-slot-num = <16>;
                dai-tdm-slot-width = <32>;
		    };

		    codec {
			    sound-dai = <&dummy_codec>;
		    };
	    };

        simple-audio-card,dai-link@2 {
            format = "i2s";
            bitclock-master = <&cpu2_audio>;
            frame-master = <&cpu2_audio>;
            convert-channels = <2>;
            mclk-fs = <256>;

		    cpu2_audio:cpu {
			    sound-dai = <&i2s3>;
		    };
            
		    codec {
			    sound-dai = <&dummy_codec>;
		    };
	    };
    };
}

&sai2 {
    pinctrl-names = "default", "sleep";
    pinctrl-0 = <&sai2a_pins_mx>, <&sai2b_pins_mx>;
    pinctrl-1 = <&sai2a_sleep_pins_mx>, <&sai2b_sleep_pins_mx>;
    status = "okay";

	sai2a:audio-controller@402a0004{
        #clock-cells = <0>;		/* Set SAI2A as master clock provider */
        dma-names = "tx"; 		/*此配置应在芯片配置中，但st在6.1内核源码中未添加*/
		status = "okay";
	};

	sai2b:audio-controller@402a0024{
        #clock-cells = <0>;
        dma-names = "rx";
		status = "okay";
	};
};

&i2s3{
	pinctrl-names = "default", "sleep";
	pinctrl-0 = <&i2s3_pins_mx>;
	pinctrl-1 = <&i2s3_sleep_pins_mx>;
    #clock-cells = <0>;
	status = "okay";
};

内核配置

参考

AUD-MAN-DEP_Programmers-Guide-v2.24 中 Appendix1:KernelandPlatformConfiguration 章节

关于cgroup

OpenSTLinux 使用 systemd 引导，默认挂载 cgroups v2 可在uboot的配置文件中添加内核引导选项来使用 cgroups-v1
```
systemd.unified_cgroup_hierarchy=no
systemd.legacy_systemd_cgroup_controller=yes
```
使用buildroot构建系统可添加 cgroupfs-mount 包来挂载cgroups

asound.config

用sai注册声卡，由于sai有两个子模块，且传输方向分别为 tx 和 rx，所以要满足 DepAlsaBridge 就需要把两个子模块都用上

bash

root@stm32mp2:~# ls /proc/asound/card0/
id  pcm0p  pcm1c  pcm2c  pcm2p
root@stm32mp2:~# aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: dummyaudiocard [dummy-audio-card], device 0: 402a0004.audio-controller-dummy-codec-pcm dummy-codec-pcm-0 [402a0004.audio-controller-dummy-codec-pcm dummy-codec-pcm-0]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: dummyaudiocard [dummy-audio-card], device 2: 400c0000.audio-controller-dummy-codec-pcm dummy-codec-pcm-2 [400c0000.audio-controller-dummy-codec-pcm dummy-codec-pcm-2]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
root@stm32mp2:~# arecord -l
**** List of CAPTURE Hardware Devices ****
card 0: dummyaudiocard [dummy-audio-card], device 1: 402a0024.audio-controller-dummy-codec-pcm dummy-codec-pcm-1 [402a0024.audio-controller-dummy-codec-pcm dummy-codec-pcm-1]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: dummyaudiocard [dummy-audio-card], device 2: 400c0000.audio-controller-dummy-codec-pcm dummy-codec-pcm-2 [400c0000.audio-controller-dummy-codec-pcm dummy-codec-pcm-2]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
root@stm32mp2:~#

通过 ls /proc/asound/card0/ 可以看到 pcm0 只有 Playback 而 pcm1只有 Capture 。可以通过修改 /etc/asound.config

pcm.sai2 {
    type asym
    playback.pcm {
        type hw
        card 0
        device 0
    }
    capture.pcm {
        type hw
        card 0
        device 1
    }
}

ctl.sai2 {
    type hw
    card 0
}

pcm.!default pcm.sai2
ctl.!default ctl.sai2

在使用 DepAlsaBridge 应用程序时通过 -d sai2 指定声卡即可

dummy_codec 模块编译

单独编译

makefile

# 定义模块目标
obj-m := dummy-codec.o

# 内核构建目录路径
KDIR := /opt/1/MP257/Developer-Package/stm32mp2-openstlinux-6.6-yocto-scarthgap-mpu-v24.11.06/sources/aarch64-ostl-linux/linux-stm32mp-6.6.48-stm32mp-r1-r0/build/

CROSS_COMPILE := aarch64-ostl-linux-

all:
	$(MAKE) ARCH=arm64 CROSS_COMPILE=$(CROSS_COMPILE) -C $(KDIR) M=$(PWD) modules

clean:
	$(MAKE) ARCH=arm64 CROSS_COMPILE=$(CROSS_COMPILE) -C $(KDIR) M=$(PWD) clean

在内核源码中编译
1. 复制模块源码
  将模块源码复制到以下路径：
  <KERNEL_SOURCE_PATH>/sound/soc/codecs
2. 修改配置文件
  - 编辑 Makefile，添加模块的编译规则。
  - 修改 Kconfig，确保模块可以被配置。
3. 启用模块
  使用以下命令打开内核配置菜单并选择模块
  bash
```
make menuconfig
```

Debug

参考

Linux声卡状态检查与音频通道管理-CSDN博客

Procfs filesystem

列出PCM音频设备：
bash
```
cat /proc/asound/pcm
```
获取PCM音频设备的硬件参数（此处为“0”卡的“0”设备）：
bash
```
cat /proc/asound/card0/pcm0p/sub0/hw_params
```

Debugfs filesystem

列出DAIs
bash
```
cat /sys/kernel/debug/asoc/dais
```

注册虚拟声卡 ​

设备树 ​

内核配置 ​

asound.config ​

dummy_codec 模块编译 ​

Debug ​

Procfs filesystem ​

Debugfs filesystem ​