audio_route.c 是 android 提供的一个 audio route的so 库，其位于 /system/media/audio_route 目录下。

libaudioroute.so 这个动态库的主要功能有一下几点：

1.解析 /system/etc/mixer_paths.xml 配置文件

2. 对audio 的ctl 访问方式进行封装，方便提供给hardware层的audio_hw 进行调用。

driver 层的codec 会通过 alsa-driver 接口提供多个ctl 节点，用于配置codec的ctl节点。

在android 系统中，可以将这些ctl 节点，设置在 mixer_paths.xml 。可以组合成一个path,

在调用对用 path 的name的时候，所有的ctl 都会进行调用。

audio_route.c 的源码：http://androidxref.com/5.1.1_r6/xref/system/media/audio_route/

mixer_paths.xml 距离：http://androidxref.com/5.1.1_r6/xref/device/htc/flounder/ 下 mixer_paths_0.xml

调用 libaudioroute.so的hardware层代码。位于 device/htc 目录下。

http://androidxref.com/5.1.1_r6/xref/device/htc/flounder/audio/hal/audio_hw.c

代码分析

//xml的回调函数static void start_tag(void *data, const XML_Char *tag_name,                      const XML_Char **attr){    const XML_Char *attr_name = NULL;    const XML_Char *attr_id = NULL;    const XML_Char *attr_value = NULL;    struct config_parse_state *state = data;    struct audio_route *ar = state->ar;    unsigned int i;    unsigned int ctl_index;    struct mixer_ctl *ctl;    int value;    unsigned int id;    struct mixer_value mixer_value;    enum mixer_ctl_type type;    /* Get name, id and value attributes (these may be empty) */    for (i = 0; attr[i]; i += 2) {        if (strcmp(attr[i], "name") == 0)            attr_name = attr[i + 1];        if (strcmp(attr[i], "id") == 0)            attr_id = attr[i + 1];        else if (strcmp(attr[i], "value") == 0)            attr_value = attr[i + 1];    } //这里查找的是名字为path的tag    /* Look at tags */    if (strcmp(tag_name, "path") == 0) {        if (attr_name == NULL) {            ALOGE("Unnamed path!");        } else {            if (state->level == 1) {                /* top level path: create and stash the path */                //创建一个path                state->path = path_create(ar, (char *)attr_name);            } else {                //如果是嵌套的path,则使用调用path_add_path                /* nested path */                struct mixer_path *sub_path = path_get_by_name(ar, attr_name);                path_add_path(ar, state->path, sub_path);            }        }    }//这里解析ctl tag ,ctl tag 是带有name  和value的    else if (strcmp(tag_name, "ctl") == 0) {        /* Obtain the mixer ctl and value */        ctl = mixer_get_ctl_by_name(ar->mixer, attr_name);        if (ctl == NULL) {            ALOGE("Control '%s' doesn't exist - skipping", attr_name);            goto done;        }        switch (mixer_ctl_get_type(ctl)) {        case MIXER_CTL_TYPE_BOOL:        case MIXER_CTL_TYPE_INT:            value = (int) strtol((char *)attr_value, NULL, 0);            break;        case MIXER_CTL_TYPE_ENUM:            value = mixer_enum_string_to_value(ctl, (char *)attr_value);            break;        default:            value = 0;            break;        }        /* locate the mixer ctl in the list */        for (ctl_index = 0; ctl_index < ar->num_mixer_ctls; ctl_index++) {            if (ar->mixer_state[ctl_index].ctl == ctl)                break;        }        if (state->level == 1) {            /* top level ctl (initial setting) */            type = mixer_ctl_get_type(ctl);            if (is_supported_ctl_type(type)) {                /* apply the new value */                if (attr_id) {                    /* set only one value */                    id = atoi((char *)attr_id);                    if (id < ar->mixer_state[ctl_index].num_values)                        ar->mixer_state[ctl_index].new_value[id] = value;                    else                        ALOGE("value id out of range for mixer ctl '%s'",                              mixer_ctl_get_name(ctl));                } else {                    /* set all values the same */                    for (i = 0; i < ar->mixer_state[ctl_index].num_values; i++)                        ar->mixer_state[ctl_index].new_value[i] = value;                }            }        } else {            /* nested ctl (within a path) */            mixer_value.ctl_index = ctl_index;            mixer_value.value = value;            if (attr_id)                mixer_value.index = atoi((char *)attr_id);            else                mixer_value.index = -1;            path_add_value(ar, state->path, &mixer_value);        }    }done:    state->level++;}//audio_route的初始化函数，传递参数为card 和 xml_paths  struct audio_route *audio_route_init(unsigned int card, const char *xml_path){    struct config_parse_state state;    XML_Parser parser;    FILE *file;    int bytes_read;    void *buf;    int i;  //解析的结果保存在ar中    struct audio_route *ar;    ar = calloc(1, sizeof(struct audio_route));    if (!ar)        goto err_calloc; // 调用tinyalsa 提供的mixer_open 函数    ar->mixer = mixer_open(card);    if (!ar->mixer) {        ALOGE("Unable to open the mixer, aborting.");        goto err_mixer_open;    }    ar->mixer_path = NULL;    ar->mixer_path_size = 0;    ar->num_mixer_paths = 0;    /* allocate space for and read current mixer settings */    if (alloc_mixer_state(ar) < 0)        goto err_mixer_state;//根据传递进来的参数 mixer_paths 决定是否使用默认的 /system/etc/mixer_paths.xml 文件    /* use the default XML path if none is provided */    if (xml_path == NULL)        xml_path = MIXER_XML_PATH;    file = fopen(xml_path, "r");    if (!file) {        ALOGE("Failed to open %s", xml_path);        goto err_fopen;    } // 创建一个 xml 的handler，    parser = XML_ParserCreate(NULL);    if (!parser) {        ALOGE("Failed to create XML parser");        goto err_parser_create;    }    memset(&state, 0, sizeof(state));    state.ar = ar;    XML_SetUserData(parser, &state);// 设置解析的callback 函数，分别是 start_tag and end_tag     XML_SetElementHandler(parser, start_tag, end_tag);    for (;;) {        buf = XML_GetBuffer(parser, BUF_SIZE);        if (buf == NULL)            goto err_parse;        bytes_read = fread(buf, 1, BUF_SIZE, file);        if (bytes_read < 0)            goto err_parse; // 进行数据的解析        if (XML_ParseBuffer(parser, bytes_read,                            bytes_read == 0) == XML_STATUS_ERROR) {            ALOGE("Error in mixer xml (%s)", MIXER_XML_PATH);            goto err_parse;        }        if (bytes_read == 0)            break;    }    /* apply the initial mixer values, and save them so we can reset the       mixer to the original values */    audio_route_update_mixer(ar);    save_mixer_state(ar);    XML_ParserFree(parser);    fclose(file);    return ar;err_parse:    XML_ParserFree(parser);err_parser_create:    fclose(file);err_fopen:    free_mixer_state(ar);err_mixer_state:    mixer_close(ar->mixer);err_mixer_open:    free(ar);    ar = NULL;err_calloc:    return NULL;}

下边分析一下 mixer_paths.xml

//初始化设置 ，ctl 包含了 name 和value , 这里的name ，就是对用codec 提供的控件的名字              //定义的path 为headphones

在hardware 层调用

audio_route_apply_and_update_path 或者 audio_route_reset_and_update_path 的时候，会根据提供的名字更新对应path下的ctl控件的值。

实例代码，可以查看http://androidxref.com/5.1.1_r6/xref/hardware/qcom/audio/hal/msm8974/platform.c 里边对audio_route的使用。

audio_route 分析

代码分析

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