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🤣本文内容🤣：🍭SS528开发板编译Linux内核ALSA驱动、移植alsa-lib、采集与播放usb耳机声音🍭
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一、概述

上篇文章【音视频|ALSA】ALSA是什么？ALSA框架详细介绍 简单地介绍了ALSA驱动；这篇文章主要是一篇实战文章，通过ALSA架构，在SS528开发板实现播放usb耳机声音。整个操作大概分为三步：

• 编译ALSA驱动
• 交叉编译alsa-lib库
• 基于alsa-lib库开发采集、播放音频程序

本文使用开发环境：

• 开发主机：Ubuntu18.04LTS
• 交叉编译工具：aarch64-mix210-linux-gcc
• 使用的Linux内核源码：ss528sdk自带的，SS528V100_SDK_V2.0.0.3/open_source/linux/linux-4.19.y
• 使用的alsa-lib库：ALSA官网下载的 alsa-lib-1.2.10

二、编译ALSA驱动

在Linux系统中，要播放usb接口的音频，首先需要先有一个可以识别到这个usb耳机的驱动，然后再针对这个驱动编写应用层代码。自己写的话，耗时耗力不讨好，而ALSA架构就提供了这样的驱动和应用层的库，下面介绍怎样在Linux源码编译ALSA驱动，并将其编译成模块，插入到板子的Linux系统。

2.1 配置和编译ALSA驱动模块

首先进入Linux源码目录，配置启用ALSA驱动：

cd linux-4.19.y
make ARCH=arm64 CROSS_COMPILE=aarch64-mix210-linux- menuconfig

参考下面语句配置：

	Device Drivers  ---><M> Sound card support  ---><M>   Advanced Linux Sound Architecture  --->[*]   PCM timer interface (NEW)[*]   Support old ALSA API (NEW)[*]   Sound Proc FS Support (NEW)[*]     Verbose procfs contents (NEW)[*]   Generic sound devices (NEW)  ---> [*]   PCI sound devices (NEW)  ---> (2048) Pre-allocated buffer size for HD-audio driver[*]   SPI sound devices (NEW)  ----[*]   USB sound devices (NEW)  ---><M>   USB Audio/MIDI driver

配置完成后，保存退出，执行下面语句编译模块：

make ARCH=arm64 CROSS_COMPILE=aarch64-mix210-linux- modules

编译完成后，在sound目录下，有8个ko生成，将它们复制到开发板的文件系统即可：

$ find ./sound/ -name "*.ko"
./sound/core/snd.ko
./sound/core/snd-hwdep.ko
./sound/core/snd-rawmidi.ko
./sound/core/snd-timer.ko
./sound/core/snd-pcm.ko
./sound/usb/snd-usbmidi-lib.ko
./sound/usb/snd-usb-audio.ko
./sound/soundcore.ko
$ mkdir /nfsroot/sound
$ cd sound/
$ cp soundcore.ko core/*.ko usb/*.ko /nfsroot/sound -far

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2.2 在ss528开发板插入ALSA驱动

说明：下面的打印都是在开发板文件系统的操作打印。

开发板在没用插入ALSA驱动之前，是没有/dev/snd目录、/prc/asound目录的：

/nfsroot/sound # ls /dev/snd
ls: /dev/snd: No such file or directory
/nfsroot/sound # ls /proc/asound
ls: /prc/asound: No such file or directory
/nfsroot/sound # 

插入ALSA驱动，参考下面命令：

insmod soundcore.ko
insmod snd.ko         
insmod snd-hwdep.ko   
insmod snd-timer.ko     
insmod snd-rawmidi.ko   
insmod snd-pcm.ko       
insmod snd-usbmidi-lib.ko
insmod snd-usb-audio.ko 

注意insmod的顺序，避免报错：insmod: can’t insert ‘snd.ko’: unknown symbol in module, or unknown parameter

插入成功后，可以看到/dev/snd目录、/prc/asound目录：

卸载ALSA驱动，参考下面语句：

rmmod snd-usb-audio
rmmod snd-usbmidi-lib
rmmod snd-pcm
rmmod snd-rawmidi
rmmod snd-timer
rmmod snd-hwdep
rmmod snd
rmmod soundcore

注意rmmod的顺序，避免报错：rmmod: can’t unload module ‘soundcore’: Resource temporarily unavailable

三、交叉编译 alsa-lib

3.1 下载 alsa-lib

在官网 https://www.alsa-project.org/files/pub/lib/ 可以下载到历史版本；
在Github https://github.com/alsa-project/alsa-lib/tags 可以下载到最新发布版本
本文下载的是 alsa-lib-1.2.10.tar.gz

3.2 交叉编译 alsa-lib

解压源码，如果里面没有configure文件，需要执行下面命令生成：

libtoolize --force --copy --automake
aclocal
autoheader
automake --foreign --copy --add-missing
autoconf

编译过程参考下面命令：

sudo mkdir /usr/lib/alsa-lib-1.2.10
sudo chown wkd:wkd /usr/lib/alsa-lib-1.2.10/ -R
tar zxf alsa-lib-1.2.10.tar.gz
cd alsa-lib-1.2.10/
./configure --prefix=/usr/lib/alsa-lib-1.2.10/ CC=aarch64-mix210-linux-gcc --host=aarch64-mix210-linux --enable-static=yes --enable-shared=no
make && make install

这里首先创建了/usr/lib/alsa-lib-1.2.10目录，因为链接了libasound.a后，程序需要到安装目录寻找配置文件share/alsa/alsa.conf，所以这里安装的目录需要和开发板存放的目录一致。

如果指定了--prefix为其他目录，就需要将share/alsa/alsa.conf复制到开发板同样的目录，否则运行程序会报错：Cannot access file /usr/lib/alsa-lib-1.2.10/share/alsa/alsa.conf，也可以通过选项--with-configdir来指定配置文件目录，它将影响 include/config.h中的 ALSA_CONFIG_DIR 目录。

四、开发ALSA应用层程序

4.1 编写应用层代码 alsa-capture-playback.c

代码参考：https://blog.csdn.net/u014056414/article/details/120989131

// alsa-capture-playback.c
// aarch64-mix210-linux-gcc alsa-capture-playback.c -I /usr/lib/alsa-lib-1.2.10/include/ -L /usr/lib/alsa-lib-1.2.10/lib/ -l asound -lpthread -ldl -lm -o alsa-capture-playback#include <stdio.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <alsa/asoundlib.h>#define PLAYBACK_FILE "48000Hz-16bit-2ch-ChengDu.pcm"  // 采样率:48000 位深度:16bit 双通道
#define CAPTURE_FINE "capture.pcm"//#define PCM_NAME	"hw:CARD=mycodec,DEV=0"
#define PCM_NAME	"hw:0,0"
#define RATE 		48000
#define FORMAT		SND_PCM_FORMAT_S16_LE
#define CHANNELS	1snd_pcm_hw_params_t *hw_params;int print_all_pcm_name(void) {char **hints;/* Enumerate sound devices */int err = snd_device_name_hint(-1, "pcm", (void***)&hints);if(err != 0)return -1;char** n = hints;while(*n != NULL) {char *name = snd_device_name_get_hint(*n, "NAME");if(name != NULL && 0 != strcmp("null", name)) {printf("pcm name : %s\n",name);free(name);}n++;}snd_device_name_free_hint((void**)hints);return 0;
}snd_pcm_t *open_sound_dev(snd_pcm_stream_t type,const char *name, unsigned int rate, int format,int channels,snd_pcm_uframes_t *period_frames) {int err;snd_pcm_t *handle;int dir = 0;printf("rate=%d, format=%d, channels=%d\n",rate,format,channels);if((err = snd_pcm_open(&handle, name, type, 0)) < 0) {printf("cannot snd_pcm_open (%s)\n",snd_strerror(err));return NULL;}if((err = snd_pcm_hw_params_malloc(&hw_params)) < 0) {printf("cannot allocate hardware parameter structure (%s)\n",snd_strerror(err));return NULL;}if((err = snd_pcm_hw_params_any(handle, hw_params)) < 0) {printf("cannot initialize hardware parameter structure (%s)\n",snd_strerror(err));return NULL;}if((err = snd_pcm_hw_params_set_access(handle, hw_params, SND_PCM_ACCESS_RW_INTERLEAVED)) < 0) {printf("cannot set access type (%s)\n",snd_strerror(err));return NULL;}if((err = snd_pcm_hw_params_set_format(handle, hw_params, format)) < 0) {printf("cannot set sample format (%s)\n",snd_strerror(err));return NULL;}if((err = snd_pcm_hw_params_set_rate_near(handle, hw_params, &rate, 0)) < 0) {printf("cannot set sample rate (%s)\n",snd_strerror(err));return NULL;}if((err = snd_pcm_hw_params_set_channels(handle, hw_params, channels)) < 0) {printf("cannot set channel count (%s) channels=%d\n",snd_strerror(err),channels);if(channels==2){channels = 1;}if(channels==1){channels = 2;}if((err = snd_pcm_hw_params_set_channels(handle, hw_params, channels)) < 0) {printf("cannot set channel count (%s)\n",snd_strerror(err));return NULL;}}#if 1unsigned int buffer_time, period_time;if (snd_pcm_hw_params_get_buffer_time_max(hw_params, &buffer_time, 0) < 0) {fprintf(stderr, "Error snd_pcm_hw_params_get_buffer_time_max\n");return NULL;}if(snd_pcm_stream(handle) == SND_PCM_STREAM_PLAYBACK){if (buffer_time > 50000) buffer_time = 50000;	//50ms 这个数越小数据取的越快}period_time = buffer_time / 4;if (snd_pcm_hw_params_set_buffer_time_near(handle, hw_params, &buffer_time, 0) < 0) {fprintf(stderr, "Error snd_pcm_hw_params_set_buffer_time_near\n");return NULL;}if (snd_pcm_hw_params_set_period_time_near(handle, hw_params, &period_time, 0) < 0) {fprintf(stderr, "Error snd_pcm_hw_params_set_period_time_near\n");return NULL;}#endifif((err = snd_pcm_hw_params(handle, hw_params)) < 0) {printf("cannot set parameters (%s)\n",snd_strerror(err));return NULL;}if(period_frames != NULL) {//获取一个周期有多少帧数据if((err =snd_pcm_hw_params_get_period_size(hw_params, period_frames, &dir)) < 0){printf("cannot get period size (%s)\n",snd_strerror(err));return NULL;}}snd_pcm_hw_params_free(hw_params);return handle;
}int main(void) {int p,err;snd_pcm_t *playback_handle;snd_pcm_t *capture_handle;int pfd,cfd;snd_pcm_uframes_t period_frames;int size2frames;printf("program running ...\n");//查看所有pcm的nameprint_all_pcm_name();playback_handle = open_sound_dev(SND_PCM_STREAM_PLAYBACK,PCM_NAME,RATE,FORMAT,CHANNELS,&period_frames);if(!playback_handle) {printf("cannot open for playback\n");return -1;}usleep(5);capture_handle = open_sound_dev(SND_PCM_STREAM_CAPTURE,PCM_NAME,RATE,FORMAT,CHANNELS,NULL);if(!capture_handle) {printf("cannot open for capuure\n");snd_pcm_close(playback_handle);return -1;}if((err = snd_pcm_prepare(playback_handle)) < 0) {printf("cannot prepare audio interface for use (%s)\n",snd_strerror(err));goto out;}if((err = snd_pcm_prepare(capture_handle)) < 0) {printf("cannot prepare audio interface for use (%s)\n",snd_strerror(err));goto out;}//打开要播放的PCM文件pfd = open(PLAYBACK_FILE,O_RDONLY,0644);if(pfd < 0){printf("open %s error!!!\n",PLAYBACK_FILE);goto out;}//新建一个进程, 子进程播放, 父进程录音p = fork();if(p < 0) {printf("fork error!!!\n");goto out;}if(p==0) {char *pbuf;int size,period_bytes;period_bytes = snd_pcm_frames_to_bytes(playback_handle,period_frames);pbuf = malloc(period_bytes);int i = 0;for(; i<100; i++){printf("[%s %d]times = %d\n",__FILE__,__LINE__,i);lseek(pfd, 0, SEEK_SET);while( size = read(pfd, pbuf, period_bytes)) {//解决最后一个周期数据问题if(size < period_bytes) {memset(pbuf+size, 0, period_bytes-size);}//size2frames = snd_pcm_bytes_to_frames(playback_handle,size);size2frames = period_frames;//向PCM写入数据，播放err = snd_pcm_writei(playback_handle, pbuf, size2frames);if(err == -EPIPE) {snd_pcm_prepare(playback_handle);fprintf(stderr, "<<< snd_pcm_writei --> Buffer Underrun >>> \n");err = snd_pcm_writei(playback_handle, pbuf, size2frames);if(err != size2frames) {printf("write to audio interface failede err:%d (size2frames:%d)\n",err,size2frames);free(pbuf);close(pfd);exit(-1);}}else if(err != size2frames) {printf("write to audio interface failede err:%d (size2frames:%d)\n",err,size2frames);free(pbuf);close(pfd);exit(-1);}//printf("process:playback wrote %d frames\n",size2frames);usleep(100);}}free(pbuf);close(pfd);sleep(1);	//等待一下，给点时间父进程录音exit(0);}char *cbuf;const int frames_size = snd_pcm_frames_to_bytes(capture_handle,period_frames);cbuf = malloc(frames_size);memset(cbuf, 0, frames_size);//打开录音的保存文件cfd = open(CAPTURE_FINE,O_RDWR | O_TRUNC | O_CREAT,0644);if(cfd < 0){printf("open %s error!!!\n",CAPTURE_FINE);goto out;}while(waitpid(p, NULL, WNOHANG) == 0) {	//查看一下子进程是否已经退出//向PCM读一周期数据if((size2frames = snd_pcm_readi(capture_handle, cbuf, period_frames)) < 0) {printf("read from audio interface failed (%d)\n",size2frames);free(cbuf);close(cfd);goto out;}//printf("--process:capture read %d frames\n",size2frames);write(cfd,cbuf,snd_pcm_frames_to_bytes(capture_handle,size2frames));memset(cbuf,0,frames_size);usleep(100);}free(cbuf);close(cfd);out:snd_pcm_close(playback_handle);snd_pcm_close(capture_handle);printf("program finish ...\n");return 0;
}

复制上面代码保存为alsa-capture-playback.c，然后参考下面命令编译：

aarch64-mix210-linux-gcc alsa-capture-playback.c -I /usr/lib/alsa-lib-1.2.10/include/ -L /usr/lib/alsa-lib-1.2.10/lib/ -l asound -lpthread -ldl -lm -o alsa-capture-playback

4.2 运行程序 alsa-capture-playback

首先，在开发板插入驱动，参考上面 2.2 节；

其次，复制alsa-lib交叉编译生成的 /usr/lib/alsa-lib-1.2.3.2/share/alsa/alsa.conf 文件到开发板同样的路径；

最后，复制应用程序 alsa-capture-playback 和任意 48KHZ的双通道16bit的pcm文件 48000Hz-16bit-2ch-ChengDu.pcm到开发板任一目录，执行alsa-capture-playback；

顺利执行的话，可以在耳机听到播放的pcm文件声音，并且执行的目录下会生成record.pcm。我使用的pcm文件是48KHZ的双通道16bit的，点击 下载链接 可以下载，音频打开如下图：

五、总结

本篇文章介绍了在Linux开发板使用ALSA架构播放usb耳机声音的实例，包括了：编译安装ALSA驱动、交叉编译alsa-lib、实现ALSA应用层程序。

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