DAPM之六：dapm机制深入分析（下）
dapm触发时的入口函数是dapm_power_widgets，稍后详细分析这个函数，这里仅说其作用：检查每个dapm widget，如果该widget处在一条complete paths中，则power up这个widget，否则power down。
dapm触发依据
1、dapm widgets建立时，详见snd_soc_dapm_new_widgets；
2、上层通过alsa_amixer等工具改变codec音频路径时，此时与此相关的widgets状态要重置，详见dapm_mixer_update_power和dapm_mux_update_power；
amixer-应用层[alsa_amixer cset name='Left Output Mixer Left Input Mixer Switch' 1]
|->snd_ctl_ioctl-系统调用
|->snd_ctl_elem_write_user-内核钩子函数
|->snd_ctl_elem_wirte-
|->snd_ctl_find_id-遍历kcontrol链表找到name字段匹配的kctl
|->kctl->put()-调用kctl的成员函数put()
|->snd_soc_dapm_put_volsw
|->dapm_mixer_update_power
|->更新path->connect状态
|->dapm_power_widgets 触发dapm，重置相关的widgets
3、发生stream事件时，会触发snd_soc_dapm_stream_even。什么叫stream事件？准备或关闭一个pcm stream通道（snd_pcm_prepare/snd_pcm_close）这些都属于stream事件。另外suspend或resume时，也会触发snd_soc_dapm_stream_event处理。
snd_pcm_prepare
|->soc_pcm_prepare
|->处理platform、codec-dai、cpu-dai的prepare回调函数
|->snd_soc_dapm_stream_event
|->遍历codec每个dapm widget，如果该widget的stream name与传递进来的stream参数相匹配，如果匹配则置widget->active为真
|->dapm_power_widgets 触发dapm，重置相关的widgets
dapm_power_widgets
1、初始化两个链表up_list和down_list，如字面意思，up_list指向要power up的widgets，down_list指向要power down的widgets；
2、遍历所有widgets，检查是否需要对其进行power操作；要power up的则插入到up_list，要power down的则插入到down_list；
3、先power down down_list上widgets，再power up up_list上的widgets；
4、设置codec的偏置（bias）电压。
/*
* Scan each dapm widget for complete audio path.
* A complete path is a route that has valid endpoints i.e.:-
*
* o DAC to output pin.
* o Input Pin to ADC.
* o Input pin to Output pin (bypass, sidetone)
* o DAC to ADC (loopback).
*/
static int dapm_power_widgets(struct snd_soc_codec *codec, int event)
{
struct snd_soc_device *socdev = codec->socdev;
struct snd_soc_dapm_widget *w;
//初始化两个链表up_list和down_list，up_list指向要power up的widgets，down_list指向要power down的widgets
LIST_HEAD(up_list);
LIST_HEAD(down_list);
int ret = 0;
int power;
int sys_power = 0;
/* Check which widgets we need to power and store them in
* lists indicating if they should be powered up or down.
*/
//遍历所有的dapm widgets，检查是否需要对widget开关；'开'则把该widget插入到up_list，'关'则插入到down_list
list_for_each_entry(w, &codec->dapm_widgets, list) {
switch (w->id) {
case snd_soc_dapm_pre:
//属machine specific pre widget，插入到down_list最前方
dapm_seq_insert(w, &down_list, dapm_down_seq);
break;
case snd_soc_dapm_post:
//属machine specific post widget，插入到up_list最后方
dapm_seq_insert(w, &up_list, dapm_up_seq);
break;
default:
//其他类型的widgets，则调用自身的power_check函数进行检查需要开关。
//关于power_check，具体见《DAPM之五：dapm机制深入分析（上）》第四、第五小节。非常重要的一个函数。
if (!w->power_check)
continue;
/* If we're suspending then pull down all the
* power. */
switch (event) {
case SND_SOC_DAPM_STREAM_SUSPEND:
//上面注释很清楚了，如果是suspend事件，则pull down所有widgets。
power = 0;
break;
default:
power = w->power_check(w);
if (power)
sys_power = 1;
break;
}
//w->power保存widget当前的power状态，如果当前状态和设置状态一致，那么显然不用重复设置widget
if (w->power == power)
continue;
//将widget插入到up_list或down_list中
if (power)
dapm_seq_insert(w, &up_list, dapm_up_seq);
else
dapm_seq_insert(w, &down_list, dapm_down_seq);
//更新w->power的状态
w->power = power;
break;
}
}
/* If there are no DAPM widgets then try to figure out power from the
* event type.
*/
if (list_empty(&codec->dapm_widgets)) {
switch (event) {
case SND_SOC_DAPM_STREAM_START:
case SND_SOC_DAPM_STREAM_RESUME:
sys_power = 1;
break;
case SND_SOC_DAPM_STREAM_SUSPEND:
sys_power = 0;
break;
case SND_SOC_DAPM_STREAM_NOP:
sys_power = codec->bias_level != SND_SOC_BIAS_STANDBY;
break;
default:
break;
}
}
/* If we're changing to all on or all off then prepare */
if ((sys_power && codec->bias_level == SND_SOC_BIAS_STANDBY) ||
(!sys_power && codec->bias_level == SND_SOC_BIAS_ON)) {
ret = snd_soc_dapm_set_bias_level(socdev,
SND_SOC_BIAS_PREPARE);
if (ret != 0)
pr_err("Failed to prepare bias: %d\n", ret);
}
//先power down链表down_list上的widgets，接着power up链表up_list上的widgets
//按照这样的次序，目的是避免产生pop音
//dapm_seq_run核心函数，见其详细分析
/* Power down widgets first; try to avoid amplifying pops. */
dapm_seq_run(codec, &down_list, event, dapm_down_seq);
/* Now power up. */
dapm_seq_run(codec, &up_list, event, dapm_up_seq);
/* If we just powered the last thing off drop to standby bias */
if (codec->bias_level == SND_SOC_BIAS_PREPARE && !sys_power) {
ret = snd_soc_dapm_set_bias_level(socdev,
SND_SOC_BIAS_STANDBY);
if (ret != 0)
pr_err("Failed to apply standby bias: %d\n", ret);
}
/* If we just powered up then move to active bias */
if (codec->bias_level == SND_SOC_BIAS_PREPARE && sys_power) {
ret = snd_soc_dapm_set_bias_level(socdev,
SND_SOC_BIAS_ON);
if (ret != 0)
pr_err("Failed to apply active bias: %d\n", ret);
}
pop_dbg(codec->pop_time, "DAPM sequencing finished, waiting %dms\n",
codec->pop_time);
return 0;
}
dapm_seq_run
/* Apply a DAPM power sequence.
*
* We walk over a pre-sorted list of widgets to apply power to. In
* order to minimise the number of writes to the device required
* multiple widgets will be updated in a single write where possible.
* Currently anything that requires more than a single write is not
* handled.
*/
static void dapm_seq_run(struct snd_soc_codec *codec, struct list_head *list,
int event, int sort[])
{
struct snd_soc_dapm_widget *w, *n;
//创建pending链表
LIST_HEAD(pending);
int cur_sort = -1;
int cur_reg = SND_SOC_NOPM;
int ret;
//遍历list（即up_list或down_list），根据成员power_list找到挂到list上的每一个widget
list_for_each_entry_safe(w, n, list, power_list) {
ret = 0;
/* Do we need to apply any queued changes? */
if (sort[w->id] != cur_sort || w->reg != cur_reg) {
if (!list_empty(&pending))
dapm_seq_run_coalesced(codec, &pending);
INIT_LIST_HEAD(&pending);
cur_sort = -1;
cur_reg = SND_SOC_NOPM;
}
switch (w->id) {
//为什么类型为pre/post的widget只执行event回调函数？看看它们的原型就明白了。
//#define SND_SOC_DAPM_PRE(wname, wevent)，显然这些widget只含有stream name和event回调函数。
case snd_soc_dapm_pre:
if (!w->event)
list_for_each_entry_safe_continue(w, n, list,
power_list);
if (event == SND_SOC_DAPM_STREAM_START)
ret = w->event(w,
NULL, SND_SOC_DAPM_PRE_PMU);
else if (event == SND_SOC_DAPM_STREAM_STOP)
ret = w->event(w,
NULL, SND_SOC_DAPM_PRE_PMD);
break;
case snd_soc_dapm_post:
if (!w->event)
list_for_each_entry_safe_continue(w, n, list,
power_list);
if (event == SND_SOC_DAPM_STREAM_START)
ret = w->event(w,
NULL, SND_SOC_DAPM_POST_PMU);
else if (event == SND_SOC_DAPM_STREAM_STOP)
ret = w->event(w,
NULL, SND_SOC_DAPM_POST_PMD);
break;
case snd_soc_dapm_input:
case snd_soc_dapm_output:
case snd_soc_dapm_hp:
case snd_soc_dapm_mic:
case snd_soc_dapm_line:
case snd_soc_dapm_spk:
/* No register support currently */
//这里就比较奇怪了，input/output/hp/mic/line/spk这些widgets也只有stream name和event，
//但是仍然调用dapm_generic_apply_power试图控制widget的开关？根据这里的注释，应该是预留的。
ret = dapm_generic_apply_power(w);
break;
default:
/* Queue it up for application */
//遇到非以上类型的widget，则插入到pending链表，进一步调用dapm_seq_run_coalesced处理。
//这里设计很巧妙！下面详细解析这点。
cur_sort = sort[w->id];
cur_reg = w->reg;
list_move(&w->power_list, &pending);
break;
}
if (ret < 0)
pr_err("Failed to apply widget power: %d\n",
ret);
}
if (!list_empty(&pending))
dapm_seq_run_coalesced(codec, &pending);
}从dapm_seq_run的分析，我们可以看出，mixer/mux类型的widgets处理是不同的。
/* Do we need to apply any queued changes? */
if (sort[w->id] != cur_sort || w->reg != cur_reg) {
if (!list_empty(&pending))
dapm_seq_run_coalesced(codec, &pending);
INIT_LIST_HEAD(&pending);
cur_sort = -1;
cur_reg = SND_SOC_NOPM;
}
/* Queue it up for application */ cur_sort = sort[w->id]; cur_reg = w->reg; list_move(&w->power_list, &pending);
结合这两段代码理解：遇到操作对象是同一个reg的widgets，则把他们放入pending链表中，随后调用dapm_seq_run_coalesced进行处理。这样做的意义何在？见dapm_seq_run注释：In order to minimise the number of writes to the device required multiple widgets will be updated in a single write where possible.保证了同reg但不同widgets的一次性读写。这设计是相当巧妙高效的。
dapm_seq_run_coalesced就不累述了，比较简单，对同reg但不同widgets进行一次读写。