hw mixer还是sw mixer实现setMasterVolume？

昨天分析了一把snd_kcontrol，可以认为上层应用的确是通过名称标识name来遍历底层的snd_kcontrol链表，从而找到相匹配的kcontrol。见snd_ctl_find_id函数

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/** 
 * snd_ctl_find_id - find the control instance with the given id 
 * @card: the card instance 
 * @id: the id to search 
 * 
 * Finds the control instance with the given id from the card. 
 * 
 * Returns the pointer of the instance if found, or NULL if not. 
 * 
 * The caller must down card->controls_rwsem before calling this function 
 * (if the race condition can happen). 
 */ 
struct snd_kcontrol *snd_ctl_find_id(struct snd_card *card,  
                     struct snd_ctl_elem_id *id)  
{  
    struct snd_kcontrol *kctl;  
 
    if (snd_BUG_ON(!card || !id))  
        return NULL;  
    if (id->numid != 0)  
        return snd_ctl_find_numid(card, id->numid);  
    list_for_each_entry(kctl, &card->controls, list) {  
        if (kctl->id.iface != id->iface)  
            continue;  
        if (kctl->id.device != id->device)  
            continue;  
        if (kctl->id.subdevice != id->subdevice)  
            continue;  
        if (strncmp(kctl->id.name, id->name, sizeof(kctl->id.name)))  
            continue;  
        if (kctl->id.index > id->index)  
            continue;  
        if (kctl->id.index + kctl->count <= id->index)  
            continue;  
        return kctl;  
    }  
    return NULL;  
} 
/**
 * snd_ctl_find_id - find the control instance with the given id
 * @card: the card instance
 * @id: the id to search
 *
 * Finds the control instance with the given id from the card.
 *
 * Returns the pointer of the instance if found, or NULL if not.
 *
 * The caller must down card->controls_rwsem before calling this function
 * (if the race condition can happen).
 */
struct snd_kcontrol *snd_ctl_find_id(struct snd_card *card,
         struct snd_ctl_elem_id *id)
{
 struct snd_kcontrol *kctl;

 if (snd_BUG_ON(!card || !id))
  return NULL;
 if (id->numid != 0)
  return snd_ctl_find_numid(card, id->numid);
 list_for_each_entry(kctl, &card->controls, list) {
  if (kctl->id.iface != id->iface)
   continue;
  if (kctl->id.device != id->device)
   continue;
  if (kctl->id.subdevice != id->subdevice)
   continue;
  if (strncmp(kctl->id.name, id->name, sizeof(kctl->id.name)))
   continue;
  if (kctl->id.index > id->index)
   continue;
  if (kctl->id.index + kctl->count <= id->index)
   continue;
  return kctl;
 }
 return NULL;
}

今天继续跟踪Android音频系统时，发现无论如何都找不到Android与snd_kcontrol的联结点，无论是name还是numid（alsa_amixer controls打印出来的那个numid，即是内核层snd_kcontrol链表元素的numid）都找不到相关关键字。但是它确实可以进行调节音量等控制。后来我修改内核的Codec驱动，将音量控制的kcontrol.name换成其他字符串，重新编译，Android还是可以进行音量控制。

看起来有点像是通过numid来控制的，《Android音频HAL移植》一文有提到：“设备的切换就需要和驱动联调。例如，目前earpiece的id为10，那么就可以通过ALSA的amixer设置earpiece的开和关，以及大小。而id的值就需要做驱动的同事提供。”但是还不能就此肯定。目前也没有找到保存这些值的脚本文件。

 

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继续以上问题，我在调节音量时，打印Codec寄存器的值，发现volume寄存器的值根本不会发生变化，但是音量确确实实是变化的。那时就在怀疑我们Android的音量调节不是通过硬件来实现的，而是有自身的sw mixer机制。晚上和Vic一起吃饭时，聊起这个，肯定了我的猜测。

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status_t AudioFlinger::setMasterVolume(float value)  
{  
    // check calling permissions  
    if (!settingsAllowed()) {  
        return PERMISSION_DENIED;  
    }  
 
    // when hw supports master volume, don't scale in sw mixer  
    AutoMutex lock(mHardwareLock);  
    mHardwareStatus = AUDIO_HW_SET_MASTER_VOLUME;  
    if (mAudioHardware->setMasterVolume(value) == NO_ERROR) {  
        value = 1.0f;  
    }  
    mHardwareStatus = AUDIO_HW_IDLE;  
 
    mMasterVolume = value;  
    for (uint32_t i = 0; i < mPlaybackThreads.size(); i++)  
       mPlaybackThreads.valueAt(i)->setMasterVolume(value);  
 
    return NO_ERROR;  
} 
status_t AudioFlinger::setMasterVolume(float value)
{
    // check calling permissions
    if (!settingsAllowed()) {
        return PERMISSION_DENIED;
    }

    // when hw supports master volume, don't scale in sw mixer
    AutoMutex lock(mHardwareLock);
    mHardwareStatus = AUDIO_HW_SET_MASTER_VOLUME;
    if (mAudioHardware->setMasterVolume(value) == NO_ERROR) {
        value = 1.0f;
    }
    mHardwareStatus = AUDIO_HW_IDLE;

    mMasterVolume = value;
    for (uint32_t i = 0; i < mPlaybackThreads.size(); i++)
       mPlaybackThreads.valueAt(i)->setMasterVolume(value);

    return NO_ERROR;
}

应该是Android允许开发者在HAL层实现hw mixer，当AudioFlinger探测到存在hw mixer时，则调用mAudioHardware->setMasterVolume()对volume寄存器进行设置，也不会对volume值进行scale。至于sw mixer如何使用scale值的，我没有深入探究。

以后实现hw mixer看看（反正到时也要实现EQ）效果，现在这个音量设置不是线性的，用硬件控制应该更好一点。

PS：标题就不用管它了，默认下Android根本不去找底层的kcontrol接口，而是使用自身的sw mixer。

本文来自CSDN博客，转载请标明出处：http://blog.csdn.net/sepnic/archive/2011/01/19/6152792.aspx