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alsa-lib如何解析asound.conf_asound.conf怎么被解析?

作者：花生_TL007 | 2024-03-27 18:05:45

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asound.conf怎么被解析?

1. 简介

无论是在录音还是在放音，都要打开一个PCM流，具体对应的函数原型为：

  1. int snd_pcm_open(snd_pcm_t **pcmp, const char *name,  snd_pcm_stream_t stream, int mode)；

 本文以录音为例介绍一下它的流程。录音时传入name==AndroidCapture
 先解一下各个参数的函义：

  • pcmp: 是带回的pcm流
  • name是要找开的流即放音还是录音，放音对应AndroidPlayback_Speaker_normal，录音对应AndroidCapture。放音的字符串会因为当前使用设备和设备状况不一样而有所不同，不同之处在下划线后面那部分。录音则因为大多数板子成型后，都只有一个录音设备或者接口，因此录音串对应的就是AndroidCapture
  • stream对应流的类型，是放音(0)还是录音(1).
  • mode是指设备打开模式，阻塞或非阻塞。

2. 打开流程(snd_pcm_open_conf)

       首先会根据传入的name,到配置树中查找对应的结点。 snd_config_search_definition(root, "pcm", name, &pcm_conf); 此函数将在配置树中查找对应的结点，然后将查找到的结点复制一份到
pcm_conf中。 之后，系统会调用snd_pcm_open_conf(pcmp, name, root, pcm_conf, stream, mode);进一步分析，pcm_conf即为刚才根据传入name到配置树查找到的结点。

 

  1. static int snd_pcm_open_conf(snd_pcm_t **pcmp, const char *name,
  2. 			     snd_config_t *pcm_root, snd_config_t *pcm_conf,
  3. 			     snd_pcm_stream_t stream, int mode)
  4. {
  5. 	const char *str;
  6. 	char *buf = NULL, *buf1 = NULL;
  7. 	int err;
  8. 	snd_config_t *conf, *type_conf = NULL;
  9. 	snd_config_iterator_t i, next;
  10. 	const char *id;
  11. 	const char *lib = NULL, *open_name = NULL;
  12. 	int (*open_func)(snd_pcm_t **, const char *, 
  13. 			 snd_config_t *, snd_config_t *, 
  14. 			 snd_pcm_stream_t, int) = NULL;
  15. #ifndef PIC
  16. 	extern void *snd_pcm_open_symbols(void);
  17. #endif
  18. 	void *h = NULL;
  19. 	if (snd_config_get_type(pcm_conf) != SND_CONFIG_TYPE_COMPOUND) {
  20. 		char *val;
  21. 		id = NULL;
  22. 		snd_config_get_id(pcm_conf, &id);
  23. 		val = NULL;
  24. 		snd_config_get_ascii(pcm_conf, &val);
  25. 		SNDERR("Invalid type for PCM %s%sdefinition (id: %s, value: %s)", name ? name : "", name ? " " : "", id, val);
  26. 		free(val);
  27. 		return -EINVAL;
  28. 	}
  29. 	err = snd_config_search(pcm_conf, "type", &conf);
  30. 	if (err < 0) {
  31. 		SNDERR("type is not defined");
  32. 		return err;
  33. 	}
  34. 	err = snd_config_get_id(conf, &id);
  35. 	if (err < 0) {
  36. 		SNDERR("unable to get id");
  37. 		return err;
  38. 	}
  39. 	err = snd_config_get_string(conf, &str);
  40. 	if (err < 0) {
  41. 		SNDERR("Invalid type for %s", id);
  42. 		return err;
  43. 	}
  44. 	err = snd_config_search_definition(pcm_root, "pcm_type", str, &type_conf);
  45. 	if (err >= 0) {
  46. 		if (snd_config_get_type(type_conf) != SND_CONFIG_TYPE_COMPOUND) {
  47. 			SNDERR("Invalid type for PCM type %s definition", str);
  48. 			goto _err;
  49. 		}
  50. 		snd_config_for_each(i, next, type_conf) {
  51. 			snd_config_t *n = snd_config_iterator_entry(i);
  52. 			const char *id;
  53. 			if (snd_config_get_id(n, &id) < 0)
  54. 				continue;
  55. 			if (strcmp(id, "comment") == 0)
  56. 				continue;
  57. 			if (strcmp(id, "lib") == 0) {
  58. 				err = snd_config_get_string(n, &lib);
  59. 				if (err < 0) {
  60. 					SNDERR("Invalid type for %s", id);
  61. 					goto _err;
  62. 				}
  63. 				continue;
  64. 			}
  65. 			if (strcmp(id, "open") == 0) {
  66. 				err = snd_config_get_string(n, &open_name);
  67. 				if (err < 0) {
  68. 					SNDERR("Invalid type for %s", id);
  69. 					goto _err;
  70. 				}
  71. 				continue;
  72. 			}
  73. 			SNDERR("Unknown field %s", id);
  74. 			err = -EINVAL;
  75. 			goto _err;
  76. 		}
  77. 	}
  78. 	if (!open_name) {
  79. 		buf = malloc(strlen(str) + 32);
  80. 		if (buf == NULL) {
  81. 			err = -ENOMEM;
  82. 			goto _err;
  83. 		}
  84. 		open_name = buf;
  85. 		sprintf(buf, "_snd_pcm_%s_open", str); //生成真正的open函数
  86. 	}
  87. 	if (!lib) {
  88. 		const char *const *build_in = build_in_pcms;
  89. 		while (*build_in) {
  90. 			if (!strcmp(*build_in, str))
  91. 				break;
  92. 			build_in++;
  93. 		}
  94. 		if (*build_in == NULL) {
  95. 			buf1 = malloc(strlen(str) + sizeof(ALSA_PLUGIN_DIR) + 32);
  96. 			if (buf1 == NULL) {
  97. 				err = -ENOMEM;
  98. 				goto _err;
  99. 			}
  100. 			lib = buf1;
  101. 			sprintf(buf1, "%s/libasound_module_pcm_%s.so", ALSA_PLUGIN_DIR, str);
  102. 		}
  103. 	}
  104. #ifndef PIC
  105. 	snd_pcm_open_symbols();	/* this call is for static linking only */
  106. #endif
  107. 	open_func = snd_dlobj_cache_lookup(open_name);
  108. 	if (open_func) {
  109. 		err = 0;
  110. 		goto _err;
  111. 	}
  112. 	h = snd_dlopen(lib, RTLD_NOW);
  113. 	if (h)
  114. 		open_func = snd_dlsym(h, open_name, SND_DLSYM_VERSION(SND_PCM_DLSYM_VERSION));
  115. 	err = 0;
  116. 	if (!h) {
  117. 		SNDERR("Cannot open shared library %s",
  118. 		       lib ? lib : "[builtin]");
  119. 		err = -ENOENT;
  120. 	} else if (!open_func) {
  121. 		SNDERR("symbol %s is not defined inside %s", open_name,
  122. 		       lib ? lib : "[builtin]");
  123. 		snd_dlclose(h);
  124. 		err = -ENXIO;
  125. 	}
  126.        _err:
  127. 	if (err >= 0) {
  128. 		err = open_func(pcmp, name, pcm_root, pcm_conf, stream, mode);
  129. 		if (err >= 0) {
  130. 			if (h /*&& (mode & SND_PCM_KEEP_ALIVE)*/) {
  131. 				snd_dlobj_cache_add(open_name, h, open_func);
  132. 				h = NULL;
  133. 			}
  134. 			(*pcmp)->dl_handle = h;
  135. 			err = 0;
  136. 		} else {
  137. 			if (h)
  138. 				snd_dlclose(h);
  139. 		}
  140. 	}
  141. 	if (type_conf)
  142. 		snd_config_delete(type_conf);
  143. 	free(buf);
  144. 	free(buf1);
  145. 	return err;
  146. }
  • snd_pcm_open_conf函数的主要功能：

        分析传递过来的配置树pcm_conf的type子结点对应的字符串：hooks，然后调用由type值(hooks)构成的字符串函数（_snd_pcm_hooks_open）的过程。在我们系统中后面还要调用此函数，而且在查找lib名时，都能在build_in_pcms中找到，所以对我们来说这个函数的主要功能就是查找pcm_conf子结点type对应的字符串<如plug>构成的函数。


       snd_pcm_open_conf会根据配置树的情况，获取子节点中名为type的结点（它是一个叶子结点），并获取它的值hooks，使用str变量存储。然后会在root对应配置树中查找名为pcm_type hooks的结点。查找成功后，会在查找到的结点中搜索所索要的lib,open_name字符串。（后面要打开）。 在本系统中没有找到pcm_type hooks的结点。那么lib和open_name均为空，下面就要对lib/open_name初始化，（如果找到pcm_type empyt结点则会用该结点的孩子结点初始化）。首先对lib初始化，首先查找str是否包含在：

  1. static const char *const build_in_pcms[] = {
  2. 	"adpcm", "alaw", "copy", "dmix", "file", "hooks", "hw", "ladspa", "lfloat",
  3. 	"linear", "meter", "mulaw", "multi", "null", "empty", "plug", "rate", "route", "share",
  4. 	"shm", "dsnoop", "dshare", "asym", "iec958", "softvol", "mmap_emul",
  5. 	NULL
  6. };

     这个数组中，如果在，就将lib设为null.如果不在，设置lib为默认库
 sprintf(lib, "%s/libasound_module_pcm_%s.so", ALSA_PLUGIN_DIR, str); 即/usr/lib/alsa-lib/libasound_module_pcm_hooks.so；如果查找发现hooks在数组中，所以lib为null。下面对open_name初始化sprintf(open_name, "_snd_pcm_%s_open", open_name);即_snd_pcm_hooks_open函数。


为了搞清楚配置选项在代码中的表现形式，可用如下码查看，此函数在conf.c中定义：

  1. int  show_snd_config(snd_config_t *config)
  2. { 
  3.     assert(config);
  4.     switch (config->type) {
  5.     case SND_CONFIG_TYPE_COMPOUND:
  6.         {
  7.             int err;
  8.             struct list_head *i;
  9.             i = config->u.compound.fields.next;
  10.             SNDERR("Start Compound---> node id =%s\n",config->id?config->id:"null");
  11.             
  12.             while (i != &config->u.compound.fields) {
  13.                 struct list_head *nexti = i->next;
  14.                 snd_config_t *leaf = snd_config_iterator_entry(i);
  15.                 
  16.                 err = show_snd_config(leaf);
  17.                 if (err < 0)
  18.                     return err;
  19.                 
  20.                 i = nexti;
  21.             }
  22.             SNDERR("End Compound---> ---> node id =%s\n",config->id?config->id:"null");
  23.             break;
  24.         }
  25.     case SND_CONFIG_TYPE_STRING:
  26.         SNDERR("SND_CONFIG_TYPE_STRING---> leaf id=%s,str=%s\n",config->id?config->id:"null",config->u.string?config->u.string:"null");
  27.         break;
  28.     
  29.     case SND_CONFIG_TYPE_INTEGER:
  30.         SNDERR("SND_CONFIG_TYPE_ INTGER --->leaf id=%s,num=%ld\n",config->id?config->id:"null",config->u.integer); 
  31.         break;
  32.     
  33.     case SND_CONFIG_TYPE_REAL:
  34.         SNDERR("SND_CONFIG_TYPE_ REAL--->leaf id=%s,num=%f\n",config->id?config->id:"null",config->u.real);  
  35.         break;
  36.     
  37.     case   SND_CONFIG_TYPE_INTEGER64:
  38.         SNDERR("SND_CONFIG_TYPE_INTEGER64--->leaf id=%s,num=%ld\n",config->id?config->id:"null",config->u.integer64); 
  39.         break;
  40.     
  41.     case SND_CONFIG_TYPE_POINTER:
  42.         SNDERR("SND_CONFIG_TYPE_POINTER--->leaf id=%s,num=%p\n",config->id?config->id:"null",config->u.ptr); 
  43.         break;
  44.     
  45.     default:
  46.         break;
  47.     }
  48.     return 0;
  49. }

    对于音频播放，其在asound.conf的配置项如下：

  1. pcm.AndroidPlayback_Speaker_normal {
  2. 	type plug
  3. 	slave.pcm {
  4. 		type hw
  5. 		card 0
  6. 		device 0
  7. 	}
  8. }

对于此节点，使用show_snd_config结果如下：

  1. pcm/pcm.c:2280:(snd_pcm_open) ***2280:start:name:AndroidPlayback_Speaker_normal
  2. pcm/pcm.c:2235:(snd_pcm_open_noupdate) ***Begin:2235:name=AndroidPlayback_Speaker_normal, stream type:0
  3. pcm/pcm.c:2249:(snd_pcm_open_noupdate) 2222222 pcm_conf
  4. conf.c:2504:(show_snd_config) Start Compound---> node id =AndroidPlayback_Speaker_normal
  5. conf.c:2520:(show_snd_config) SND_CONFIG_TYPE_STRING---> leaf id=type,str=plug
  6. conf.c:2504:(show_snd_config) Start Compound---> node id =slave
  7. conf.c:2504:(show_snd_config) Start Compound---> node id =pcm          
  8. conf.c:2520:(show_snd_config) SND_CONFIG_TYPE_STRING---> leaf id=type,str=hw
  9. conf.c:2524:(show_snd_config) SND_CONFIG_TYPE_ INTGER --->leaf id=card,num=0
  10. conf.c:2524:(show_snd_config) SND_CONFIG_TYPE_ INTGER --->leaf id=device,num=0
  11. conf.c:2516:(show_snd_config) End Compound---> ---> node id =pcm      
  12. conf.c:2516:(show_snd_config) End Compound---> ---> node id =slave     
  13. conf.c:2516:(show_snd_config) End Compound---> ---> node id =AndroidPlayback_Speaker_normal
  14. pcm/pcm.c:2260:(snd_pcm_open_noupdate) ***2260:name=AndroidPlayback_Speaker_normal,hop=0
  15. pcm/pcm.c:2079:(snd_pcm_open_conf) ***Begin: 2079                
  16. pcm/pcm.c:2100:(snd_pcm_open_conf) ***2100:id=type
  17. pcm/pcm.c:2107:(snd_pcm_open_conf) ***2107:str=plug
  18. pcm/pcm.c:2152:(snd_pcm_open_conf) ***2152:buf=_snd_pcm_plug_open
  19. pcm/pcm.c:2156:(snd_pcm_open_conf) ***2156:str=plug                   
  20. pcm/pcm.c:2177:(snd_pcm_open_conf) ***2177:open_name=_snd_pcm_plug_open
  21. pcm/pcm.c:2199:(snd_pcm_open_conf) ***2199:cal open_func<_snd_pcm_plug_open>
  22. pcm/pcm_plug.c:1227:(_snd_pcm_plug_open) ****Begin:1227
  23. pcm/pcm.c:2079:(snd_pcm_open_conf) ***Begin: 2079
  24. pcm/pcm.c:2100:(snd_pcm_open_conf) ***2100:id=type
  25. pcm/pcm.c:2107:(snd_pcm_open_conf) ***2107:str=hw
  26. pcm/pcm.c:2152:(snd_pcm_open_conf) ***2152:buf=_snd_pcm_hw_open
  27. pcm/pcm.c:2156:(snd_pcm_open_conf) ***2156:str=hw                   
  28. pcm/pcm.c:2177:(snd_pcm_open_conf) ***2177:open_name=_snd_pcm_hw_open
  29. pcm/pcm.c:2199:(snd_pcm_open_conf) ***2199:call open_func<_snd_pcm_hw_open>
  30. pcm/pcm_hw.c:1397:(_snd_pcm_hw_open) ***Begin:1397                   
  31. pcm/pcm_hw.c:1272:(snd_pcm_hw_open) ***Begin:1272:err=0                
  32. pcm/pcm_hw.c:1273:(snd_pcm_hw_open) ***1273:name=(null),card=0,device=0,subdevice=-1
  33. control/control_hw.c:375:(snd_ctl_hw_open) ***375:card:0
  34. pcm/pcm_hw.c:1129:(snd1_pcm_hw_open_fd) ***Begin:1129:ret=0            
  35. pcm/pcm_hw.c:1237:(snd1_pcm_hw_open_fd) ***End:1237:ret=0              
  36. pcm/pcm_hw.c:1327:(snd_pcm_hw_open) ***End:1327:ret=0
  37. pcm/pcm_hw.c:1519:(_snd_pcm_hw_open) ***Begin:1519:err=0
  38. pcm/pcm.c:2218:(snd_pcm_open_conf) ***END: 2218:err=0
  39. pcm/pcm_plug.c:1328:(_snd_pcm_plug_open) ****End: 1328: err=0
  40. pcm/pcm.c:2218:(snd_pcm_open_conf) ***END: 2218:err=0
  41. pcm/pcm.c:2264:(snd_pcm_open_noupdate) ***End:2264:err=0
  42.  
  43.

     所以与此节点对应的open函数为_snd_pcm_plug_open，通过函数sprintf(buf, "_snd_pcm_%s_open", str);生成。

 3. _snd_pcm_hooks_open <如_snd_pcm_plug_open>

 

  1. /**
  2.  * \brief Creates a new Plug PCM
  3.  * \param pcmp Returns created PCM handle
  4.  * \param name Name of PCM
  5.  * \param root Root configuration node
  6.  * \param conf Configuration node with Plug PCM description
  7.  * \param stream Stream type
  8.  * \param mode Stream mode
  9.  * \retval zero on success otherwise a negative error code
  10.  * \warning Using of this function might be dangerous in the sense
  11.  *          of compatibility reasons. The prototype might be freely
  12.  *          changed in future.
  13.  */
  14. int _snd_pcm_plug_open(snd_pcm_t **pcmp, const char *name,
  15. 		       snd_config_t *root, snd_config_t *conf, 
  16. 		       snd_pcm_stream_t stream, int mode)
  17. {
  18. 	snd_config_iterator_t i, next;
  19. 	int err;
  20. 	snd_pcm_t *spcm;
  21. 	snd_config_t *slave = NULL, *sconf;
  22. 	snd_config_t *tt = NULL;
  23. 	enum snd_pcm_plug_route_policy route_policy = PLUG_ROUTE_POLICY_DEFAULT;
  24. 	snd_pcm_route_ttable_entry_t *ttable = NULL;
  25. 	unsigned int csize, ssize;
  26. 	unsigned int cused, sused;
  27. 	snd_pcm_format_t sformat = SND_PCM_FORMAT_UNKNOWN;
  28. 	int schannels = -1, srate = -1;
  29. 	const snd_config_t *rate_converter = NULL;
  30.  
  31.     SNDERR("****Begin:%d\n",__LINE__);
  32. 	snd_config_for_each(i, next, conf) {
  33. 		snd_config_t *n = snd_config_iterator_entry(i);
  34. 		const char *id;
  35. 		if (snd_config_get_id(n, &id) < 0)
  36. 			continue;
  37. 		if (snd_pcm_conf_generic_id(id))
  38. 			continue;
  39. 		if (strcmp(id, "slave") == 0) {
  40. 			slave = n;
  41. 			continue;
  42. 		}
  43. #ifdef BUILD_PCM_PLUGIN_ROUTE
  44. 		if (strcmp(id, "ttable") == 0) {
  45. 			route_policy = PLUG_ROUTE_POLICY_NONE;
  46. 			if (snd_config_get_type(n) != SND_CONFIG_TYPE_COMPOUND) {
  47. 				SNDERR("Invalid type for %s", id);
  48. 				return -EINVAL;
  49. 			}
  50. 			tt = n;
  51. 			continue;
  52. 		}
  53. 		if (strcmp(id, "route_policy") == 0) {
  54. 			const char *str;
  55. 			if ((err = snd_config_get_string(n, &str)) < 0) {
  56. 				SNDERR("Invalid type for %s", id);
  57. 				return -EINVAL;
  58. 			}
  59. 			if (tt != NULL)
  60. 				SNDERR("Table is defined, route policy is ignored");
  61. 			if (!strcmp(str, "default"))
  62. 				route_policy = PLUG_ROUTE_POLICY_DEFAULT;
  63. 			else if (!strcmp(str, "average"))
  64. 				route_policy = PLUG_ROUTE_POLICY_AVERAGE;
  65. 			else if (!strcmp(str, "copy"))
  66. 				route_policy = PLUG_ROUTE_POLICY_COPY;
  67. 			else if (!strcmp(str, "duplicate"))
  68. 				route_policy = PLUG_ROUTE_POLICY_DUP;
  69. 			continue;
  70. 		}
  71. #endif
  72. #ifdef BUILD_PCM_PLUGIN_RATE
  73. 		if (strcmp(id, "rate_converter") == 0) {
  74. 			rate_converter = n;
  75. 			continue;
  76. 		}
  77. #endif
  78. 		SNDERR("Unknown field %s", id);
  79. 		return -EINVAL;
  80. 	}
  81. 	if (!slave) {
  82. 		SNDERR("slave is not defined");
  83. 		return -EINVAL;
  84. 	}
  85. 	err = snd_pcm_slave_conf(root, slave, &sconf, 3,
  86. 				 SND_PCM_HW_PARAM_FORMAT, SCONF_UNCHANGED, &sformat,
  87. 				 SND_PCM_HW_PARAM_CHANNELS, SCONF_UNCHANGED, &schannels,
  88. 				 SND_PCM_HW_PARAM_RATE, SCONF_UNCHANGED, &srate);
  89. 	if (err < 0){
  90.         SNDERR("****End: %d: err=%d\n",__LINE__,err);
  91. 		return err;
  92.     }
  93. #ifdef BUILD_PCM_PLUGIN_ROUTE
  94. 	if (tt) {
  95. 		err = snd_pcm_route_determine_ttable(tt, &csize, &ssize);
  96. 		if (err < 0) {
  97. 			snd_config_delete(sconf);
  98.             SNDERR("****End: %d: err=%d\n",__LINE__,err);
  99. 			return err;
  100. 		}
  101. 		ttable = malloc(csize * ssize * sizeof(*ttable));
  102. 		if (ttable == NULL) {
  103. 			snd_config_delete(sconf);
  104.             SNDERR("****End: %d: err=%d\n",__LINE__,err);
  105. 			return err;
  106. 		}
  107. 		err = snd_pcm_route_load_ttable(tt, ttable, csize, ssize, &cused, &sused, -1);
  108. 		if (err < 0) {
  109. 			snd_config_delete(sconf);
  110.             SNDERR("****End: %d: err=%d\n",__LINE__,err);
  111. 			return err;
  112. 		}
  113. 	}
  114. #endif
  115. 	
  116. #ifdef BUILD_PCM_PLUGIN_RATE
  117. 	if (! rate_converter)
  118. 		rate_converter = snd_pcm_rate_get_default_converter(root);
  119. #endif
  120.  
  121. 	err = snd_pcm_open_slave(&spcm, root, sconf, stream, mode, conf);
  122. 	snd_config_delete(sconf);
  123. 	if (err < 0){
  124.         SNDERR("****End: %d: err=%d\n",__LINE__,err);
  125. 		return err;
  126.     }
  127. 	err = snd_pcm_plug_open(pcmp, name, sformat, schannels, srate, rate_converter,
  128. 				route_policy, ttable, ssize, cused, sused, spcm, 1);
  129. 	if (err < 0)
  130. 		snd_pcm_close(spcm);
  131.     
  132.     SNDERR("****End: %d: err=%d\n",__LINE__,err);
  133. 	return err;
  134. }

这个函数主要完成三件事：

    1）在传入的配置树中查找子孩子ID为slave的结点

    2）然后调用snd_pcm_open_slave和snd_pcm_hooks_open

    3）最后再分析配置树是否有hooks结点，有的话调用snd_pcm_hook_add_conf。

    为了方便大家理解，还是先看一下传入_snd_pcm_hooks_open配置树的情况， 先来看一下snd_pcm_open_slave(&spcm, root, sconf, stream, mode, conf);函数，在这里sconf即为slave结点中ID为pcm对应的配置树。root对应的是总的配置树（系统启动时会根据配置文件asound.conf/alsa-lib.conf加载构建）。

      snd_pcm_open_slave－－>

         snd_pcm_open_named_slave-->

                 snd_pcm_open_noupdate

      由此可见snd_pcm_open_slave仍然会走到snd_pcm_open_noupdate这个函数处。然后会根据传入的参数name=default,在总的配置树中查找pcm default结点。snd_config_search_definition最终根据pcm default串查找对应的树。之后会调用 snd_pcm_open_conf(pcmp, name, root, pcm_conf, stream, mode)前面已经详细介过本函数了，本次调用只是传递过来的配置树不一样。

       snd_pcm_open_conf(pcmp, name, root, pcm_conf, stream, mode)根据当前传入树pcm_conf的情况会调用如下函数_snd_pcm_empty_open。这个函数会从当前传入树取下slave孩子结点的pcm结点对应的配置树做为根结点，并调用 snd_pcm_open_named_slave.
      snd_pcm_open_named_slave-->

          snd_pcm_open_conf

      可以看出又回到了snd_pcm_open_conf函数，只是传入的树不一样。snd_pcm_open_conf会调用_snd_pcm_plug_open.
     函数_snd_pcm_plug_open会调 用snd_pcm_open_slave及snd_pcm_plug_open。
     在传入树中有这样的子树，调用

      snd_pcm_open_slave-->

          nd_pcm_open_named_slave-->

               snd_pcm_open_conf-->

                  _snd_pcm_hw_open.

4. _snd_pcm_hw_open

     _snd_pcm_hw_open-->snd_pcm_hw_open，此函数中会根据树中的card号码，调用snd_ctl_hw_open打开/dev/snd/ControlC0,然后再打开/dev/snd/pcmC0D0c,完成后调用snd_ctl_close关闭/dev/snd/ControlC0.并调用snd_pcm_hw_open_fd，传入fd为打开的/dev/snd/pcmC0D0c.该函数完成PCM流的创建，首先创建pcm的私有数据hw (snd_pcm_hw_t结构)，然后用参数（参数获取是通过ioctl命令发送到fd,然后内核将请求信息带回最后赋值给hw的成员中）初始化它，接着创建pcm(_snd_pcm结构)，并将hw赋值给pcm->private_data成员。最后再调用snd_pcm_hw_mmap_status/snd_pcm_hw_mmap_control两个函数完成映射。需要注意在这里pcm有一个类型，本处创建的pcm类型为SND_PCM_TYPE_HW.
 对pcm的初始化代码如下：

  1. //创建类型为SND_PCM_TYPE_HW的pcm流，并完成一些基本初始化。
  2.  ret = snd_pcm_new(&pcm, SND_PCM_TYPE_HW, name, info.stream, mode);
  3.  if (ret < 0) {
  4.   free(hw);
  5.   close(fd);
  6.   return ret;
  7.  }
  8.  //初始化ops函数集
  9.  pcm->ops = &snd_pcm_hw_ops;
  10.  //初始化fast_ops函数集
  11.  pcm->fast_ops = &snd_pcm_hw_fast_ops;
  12.  pcm->private_data = hw;//如上所述私有数据赋值
  13.  pcm->poll_fd = fd;/dev/snd/pcmC0D0c
  14.  pcm->poll_events = info.stream == SND_PCM_STREAM_PLAYBACK ? POLLOUT : POLLIN;
  15.  pcm->monotonic = monotonic;
  16.


5. 配置树

配置树如下图所示：


转自：http://blog.chinaunix.net/uid-27134408-id-3272774.html

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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