一、代码模块位置
1、AudioFlinger
1. frameworks/base/services/audioflinger/
2. +-- Android.mk
3. +-- AudioBufferProvider.h
4. +-- AudioFlinger.cpp
5. +-- AudioFlinger.h
6. +-- AudioMixer.cpp
7. +-- AudioMixer.h
8. +-- AudioPolicyService.cpp
9. +-- AudioPolicyService.h
10. +-- AudioResampler.cpp
11. +-- AudioResamplerCubic.cpp
12. +-- AudioResamplerCubic.h
13. +-- AudioResampler.h
14. +-- AudioResamplerSinc.cpp
15. +-- AudioResamplerSinc.h
两种Audio Hardware HAL接口定义：
1/ legacy：hardware/libhardware_legacy/include/hardware_legacy/AudioHardwareInterface.h
2/ 非legacy：hardware/libhardware/include/hardware/audio.h
2、audio_hw
1. hardware/libhardware_legacy/audio/
2. +-- A2dpAudioInterface.cpp
3. +-- A2dpAudioInterface.h
4. +-- Android.mk
5. +-- AudioDumpInterface.cpp
6. +-- AudioDumpInterface.h
7. +-- AudioHardwareGeneric.cpp
8. +-- AudioHardwareGeneric.h
9. +-- AudioHardwareInterface.cpp
10. +-- AudioHardwareStub.cpp
11. +-- AudioHardwareStub.h
12. +-- audio_hw_hal.cpp
13. +-- AudioPolicyCompatClient.cpp
14. +-- AudioPolicyCompatClient.h
15. +-- audio_policy_hal.cpp
16. +-- AudioPolicyManagerBase.cpp
17. +-- AudioPolicyManagerDefault.cpp
18. +-- AudioPolicyManagerDefault.h
事实上legacy也要封装成非legacy中的audio.h，确切的说需要一个联系legacy interface和not legacy interface的中间层，这里的audio_hw_hal.cpp就充当这样的一个角色了。因此，我们其实也可以把2.3之前的alsa_sound这一套东西也搬过来。
1. hardware/libhardware/modules/audio/
2. +-- Android.mk
3. +-- audio_hw.c
4. +-- audio_policy.c
1. device/samsung/tuna/audio/
2. +-- Android.mk
3. +-- audio_hw.c
4. +-- ril_interface.c
5. +-- ril_interface.h
3、tinyalsa
1. external/tinyalsa/
2. +-- Android.mk
3. +-- include
4. | +-- tinyalsa
5. | +-- asoundlib.h
6. +-- mixer.c ##类alsa-lib的control，作用音频部件开关、音量调节等
7. +-- pcm.c ##类alsa-lib的pcm，作用音频pcm数据回放录制
8. +-- README
9. +-- tinycap.c ##类alsa_arecord
10. +-- tinymix.c ##类alsa_amixer
11. +-- tinyplay.c ##类alsa_aplay
这其实是历史的必然了，alsa-lib太过复杂繁琐了，我看得也很不爽；更重要的商业上面的考虑，必须移除被GNU GPL授权证所约束的部份，alsa-lib并不是个例。
注意：上面的hardware/libhardware_legacy/audio/、hardware/libhardware/modules/audio/、device/samsung/tuna/audio/是同层的。之一是legacy audio，用于兼容2.2时代的alsa_sound；之二是stub audio接口；之三是Samsung Tuna的音频抽象层实现。调用层次：AudioFlinger -> audio_hw -> tinyalsa。
二、Audio Hardware HAL加载
1、AudioFlinger
1. //加载audio hardware hal
2. static int load_audio_interface(const char *if_name, const
3. audio_hw_device_t **dev)
4. {
5. int
6.
7. //根据classid和if_name找到指定的动态库并加载，这里加载的是音频动态库，如libaudio.primary.tuna.so
8. rc = hw_get_module_by_class(AUDIO_HARDWARE_MODULE_ID, if_name, mod);
9. if
10. goto
11.
12. //加载好的动态库模块必有个open方法，调用open方法打开音频设备模块
13. rc = audio_hw_device_open(*mod, dev);
14. "couldn't open audio hw device in %s.%s (%s)",
15. AUDIO_HARDWARE_MODULE_ID, if_name, strerror(-rc));
16. if
17. goto
18.
19. return
20.
21. out:
22. *mod = NULL;
23. *dev = NULL;
24. return
25. }
26.
27. //音频设备接口，hw_get_module_by_class需要根据这些字符串找到相关的音频模块库
28. static const char
29. "primary", //主音频设备，一般为本机codec
30. "a2dp", //a2dp设备，蓝牙高保真音频
31. "usb", //usb-audio设备，这个东东我2.3就考虑要实现了，现在终于支持了
32. };
33. #define ARRAY_SIZE(x) (sizeof((x))/sizeof(((x)[0])))
34.
35. // ----------------------------------------------------------------------------
36.
37. AudioFlinger::AudioFlinger()
38. : BnAudioFlinger(),
39. false), mNextUniqueId(1),
40. false)
41. {
42. }
43.
44. void
45. {
46. int
47.
48. Mutex::Autolock _l(mLock);
49.
50. /* TODO: move all this work into an Init() function */
51. mHardwareStatus = AUDIO_HW_IDLE;
52.
53. //打开audio_interfaces数组定义的所有音频设备
54. for (size_t
55. const
56. audio_hw_device_t *dev;
57.
58. rc = load_audio_interface(audio_interfaces[i], &mod, &dev);
59. if
60. continue;
61.
62. "Loaded %s audio interface from %s (%s)", audio_interfaces[i],
63. mod->name, mod->id);
64. //mAudioHwDevs是一个Vector，存储已打开的audio hw devices
65.
66. if
67. mPrimaryHardwareDev = dev;
68. "Using '%s' (%s.%s) as the primary audio interface",
69. mod->name, mod->id, audio_interfaces[i]);
70. }
71. }
72.
73. mHardwareStatus = AUDIO_HW_INIT;
74.
75. if
76. "Primary audio interface not found");
77. return;
78. }
79.
80. //对audio hw devices进行一些初始化，如mode、master volume的设置
81. for (size_t
82. audio_hw_device_t *dev = mAudioHwDevs[i];
83.
84. mHardwareStatus = AUDIO_HW_INIT;
85. rc = dev->init_check(dev);
86. if
87. AutoMutex lock(mHardwareLock);
88.
89. mMode = AUDIO_MODE_NORMAL;
90. mHardwareStatus = AUDIO_HW_SET_MODE;
91. dev->set_mode(dev, mMode);
92. mHardwareStatus = AUDIO_HW_SET_MASTER_VOLUME;
93. dev->set_master_volume(dev, 1.0f);
94. mHardwareStatus = AUDIO_HW_IDLE;
95. }
96. }
97. }
以上对AudioFlinger进行的分析，主要是通过hw_get_module_by_class()找到模块接口名字if_name相匹配的模块库，加载，然后audio_hw_device_open()调用模块的open方法，完成音频设备模块的初始化。
留意AudioFlinger的构造函数只有简单的私有变量的初始化操作了，把音频设备初始化放到onFirstRef()，Android终于改进了这一点，好的设计根本不应该把可能会失败的操作放到构造函数中。onFirstRef是RefBase类的一个虚函数，在构造sp的时候就会被调用。因此，在构造sp<AudioFlinger>的时候就会触发onFirstRef方法，从而完成音频设备模块初始化。
2、hw_get_module_by_class
我们接下来看看hw_get_module_by_class，实现在hardware/libhardware/ hardware.c中，它作用加载指定名字的模块库（.so文件），这个应该是用于加载所有硬件设备相关的库文件，并不只是音频设备。
1. int hw_get_module_by_class(const char *class_id, const char
2. const struct
3. {
4. int
5. int
6. const struct
7. char
8. char
9. char
10.
11. if
12. "%s.%s", class_id, inst);
13. else
14. strlcpy(name, class_id, PATH_MAX);
15.
16. //这里我们以音频库为例，AudioFlinger调用到这个函数时，
17. //class_id=AUDIO_HARDWARE_MODULE_ID="audio"，inst="primary"(或"a2dp"或"usb")
18. //那么此时name="audio.primary"
19.
20. /*
21. * Here we rely on the fact that calling dlopen multiple times on
22. * the same .so will simply increment a refcount (and not load
23. * a new copy of the library).
24. * We also assume that dlopen() is thread-safe.
25. */
26.
27. /* Loop through the configuration variants looking for a module */
28. for
29. if
30. //通过property_get找到厂家标记如"ro.product.board=tuna"，这时prop="tuna"
31. if
32. continue;
33. }
34. sizeof(path), "%s/%s.%s.so",
35. //#define HAL_LIBRARY_PATH2 "/vendor/lib/hw"
36. if (access(path, R_OK) == 0) break;
37.
38. sizeof(path), "%s/%s.%s.so",
39. //#define HAL_LIBRARY_PATH1 "/system/lib/hw"
40. if (access(path, R_OK) == 0) break;
41. else
42. sizeof(path), "%s/%s.default.so", //如没有指定的库文件，则加载default.so，即stub-device
43. HAL_LIBRARY_PATH1, name);
44. if (access(path, R_OK) == 0) break;
45. }
46. }
47. //到这里，完成一个模块库的完整路径名称，如path="/system/lib/hw/audio.primary.tuna.so"
48. //如何生成audio.primary.tuna.so？请看相关的Android.mk文件，其中有定义LOCAL_MODULE := audio.primary.tuna
49.
50. status = -ENOENT;
51. if
52. /* load the module, if this fails, we're doomed, and we should not try
53. * to load a different variant. */
54. //加载模块库
55. }
56.
57. return
58. }
1. /**
2. * Every hardware module must have a data structure named HAL_MODULE_INFO_SYM
3. * and the fields of this data structure must begin with hw_module_t
4. * followed by module specific information.
5. */
6. typedef struct
7. /** tag must be initialized to HARDWARE_MODULE_TAG */
8. uint32_t tag;
9.
10. /** major version number for the module */
11. uint16_t version_major;
12.
13. /** minor version number of the module */
14. uint16_t version_minor;
15.
16. /** Identifier of module */
17. const char
18.
19. /** Name of this module */
20. const char
21.
22. /** Author/owner/implementor of the module */
23. const char
24.
25. /** Modules methods */
26. struct
27.
28. /** module's dso */
29. void* dso;
30.
31. /** padding to 128 bytes, reserved for future use */
32. uint32_t reserved[32-7];
33.
34. } hw_module_t;
35.
36. typedef struct
37. /** Open a specific device */
38. int (*open)(const struct hw_module_t* module, const char* id,
39. struct
40.
41. } hw_module_methods_t;
例如，在audio_hw.c中，它是这样定义的：
1. static struct
2. .open = adev_open,
3. };
4.
5. struct
6. .common = {
7. .tag = HARDWARE_MODULE_TAG,
8. .version_major = 1,
9. .version_minor = 0,
10. .id = AUDIO_HARDWARE_MODULE_ID,
11. "Tuna audio HW HAL",
12. "The Android Open Source Project",
13. .methods = &hal_module_methods,
14. },
15. };
3、audio_hw
好了，经过一番周折，又dlopen又dlsym的，终于进入我们的audio_hw。这部分没什么好说的，按照hardware/libhardware/include/hardware/audio.h定义的接口实现就行了。这些接口全扔到一个结构体里面的，这样做的好处是：不必用大量的dlsym来获取各个接口函数的地址，只需找到这个结构体即可，从易用性和可扩充性来说，都是首选方式。
接口定义如下：
1. struct
2. struct
3.
4. /**
5. * used by audio flinger to enumerate what devices are supported by
6. * each audio_hw_device implementation.
7. *
8. * Return value is a bitmask of 1 or more values of audio_devices_t
9. */
10. const struct
11.
12. /**
13. * check to see if the audio hardware interface has been initialized.
14. * returns 0 on success, -ENODEV on failure.
15. */
16. int (*init_check)(const struct
17.
18. /** set the audio volume of a voice call. Range is between 0.0 and 1.0 */
19. int (*set_voice_volume)(struct audio_hw_device *dev, float
20.
21. /**
22. * set the audio volume for all audio activities other than voice call.
23. * Range between 0.0 and 1.0. If any value other than 0 is returned,
24. * the software mixer will emulate this capability.
25. */
26. int (*set_master_volume)(struct audio_hw_device *dev, float
27.
28. /**
29. * setMode is called when the audio mode changes. AUDIO_MODE_NORMAL mode
30. * is for standard audio playback, AUDIO_MODE_RINGTONE when a ringtone is
31. * playing, and AUDIO_MODE_IN_CALL when a call is in progress.
32. */
33. int (*set_mode)(struct audio_hw_device *dev, int
34.
35. /* mic mute */
36. int (*set_mic_mute)(struct audio_hw_device *dev, bool
37. int (*get_mic_mute)(const struct audio_hw_device *dev, bool
38.
39. /* set/get global audio parameters */
40. int (*set_parameters)(struct audio_hw_device *dev, const char
41.
42. /*
43. * Returns a pointer to a heap allocated string. The caller is responsible
44. * for freeing the memory for it.
45. */
46. char * (*get_parameters)(const struct
47. const char
48.
49. /* Returns audio input buffer size according to parameters passed or
50. * 0 if one of the parameters is not supported
51. */
52. size_t (*get_input_buffer_size)(const struct
53. int
54. int
55.
56. /** This method creates and opens the audio hardware output stream */
57. int (*open_output_stream)(struct
58. int
59. uint32_t *sample_rate,
60. struct
61.
62. void (*close_output_stream)(struct
63. struct
64.
65. /** This method creates and opens the audio hardware input stream */
66. int (*open_input_stream)(struct
67. int
68. uint32_t *sample_rate,
69. audio_in_acoustics_t acoustics,
70. struct
71.
72. void (*close_input_stream)(struct
73. struct
74.
75. /** This method dumps the state of the audio hardware */
76. int (*dump)(const struct audio_hw_device *dev, int
77. };
78. typedef struct
注：这是比较标准的C接口设计方法了，但是个人感觉还是用C++比较好，直观易读。2.3之前都是用C++实现这些接口设计的，到了4.0，不知道为何采纳用C？不会理由是做底层的不懂C++吧？！
三、Audio Hardware HAL的legacy实现
之前提到两种Audio Hardware HAL接口定义：
1/ legacy：hardware/libhardware_legacy/include/hardware_legacy/AudioHardwareInterface.h
2/ 非legacy：hardware/libhardware/include/hardware/audio.h
前者是2.3及之前的音频设备接口定义，后者是4.0的接口定义。
为了兼容以前的设计，4.0实现一个中间层：hardware/libhardware_legacy/audio/audio_hw_hal.cpp，结构与其他的audio_hw.c大同小异，差别在于open方法：
1. static int legacy_adev_open(const hw_module_t* module, const char* name,
2. hw_device_t** device)
3. {
4. ......
5.
6. ladev->hwif = createAudioHardware();
7. if
8. ret = -EIO;
9. goto
10. }
11.
12. ......
13. }
因此老一套的alsa-lib、alsa-utils和alsa_sound也可以照搬过来，这里的文件被编译成静态库的，因此你需要修改alsa_sound里面的Android.mk文件，链接这个静态库。还有alsa_sound的命名空间原来是“android”，现在需要改成“android_audio_legacy”。
四、a2dp Audio HAL的实现
4.0的a2dp audio hal放到bluez里实现了，我找了好一会才找到：
external/Bluetooth/bluez/audio/android_audio_hw.c
大致与上面提到的audio_hw.c类似，因为都是基于audio.h定义的接口来实现的。
如果需要编译这个库，须在BoardConfig.mk里定义：
BOARD_HAVE_BLUETOOTH := true
开始还提到现在支持3种audio设备了，分别是primary、a2dp和usb。目前剩下usb audio hal我没有找到，不知是否需要自己去实现？其实alsa-driver都支持大部分的usb-audio设备了，因此上层也可调用tinyalsa的接口，就像samsung tuna的audio_hw.c那样。
五、音质改进？？？
可使用audio echo cancel和更好的resampler（SRC）？？？
--to be continued…
