Android中HAL如何向上層提供接口總結

參考文獻:

http://www.linuxidc.com/Linux/2011-07/38980.htm

http://www.linuxidc.com/Linux/2012-01/50741.htm

建議閱讀本文時先浏覽以上兩篇文章，本文是對上兩篇文章在HAL對上層接口話題的一個總結.

1 什麼是HAL

HAL的全稱是Hardware Abstraction Layer,即硬件抽象層.其架構圖如下:

Android的HAL是為了保護一些硬件提供商的知識產權而提出的，是為了避開linux的GPL束縛。思路是把控制硬件的動作都放到了Android HAL中，而linux driver僅僅完成一些簡單的數據交互作用，甚至把硬件寄存器空間直接映射到user space。而Android是基於Aparch的license，因此硬件廠商可以只提供二進制代碼，所以說Android只是一個開放的平台，並不是一個開源的平台。也許也正是因為Android不遵從GPL，所以Greg Kroah-Hartman才在2.6.33內核將Andorid驅動從linux中刪除。GPL和硬件廠商目前還是有著無法彌合的裂痕。Android想要把這個問題處理好也是不容易的。

總結下來，Android HAL存在的原因主要有：

1. 並不是所有的硬件設備都有標准的linux kernel的接口

2. KERNEL DRIVER涉及到GPL的版權。某些設備制造商並不原因公開硬件驅動，所以才去用HAL方式繞過GPL。

3. 針對某些硬件，Android有一些特殊的需求.

2 與接口相關的幾個結構體

首先來看三個與HAL對上層接口有關的幾個結構體：

1. struct hw_module_t; //模塊類型
2. struct hw_module_methods_t; //模塊方法
3. struct hw_device_t; //設備類型

這幾個數據結構是在Android工作目錄/hardware/libhardware/include/hardware/hardware.h文件中定義.

3 解釋

一般來說，在寫HAL相關代碼時都得包含這個hardware.h頭文件，所以有必要先了解一下這個頭文件中的內容.

1. /*
2. * Copyright (C) 2008 The Android Open Source Project
3. *
4. * Licensed under the Apache License, Version 2.0 (the "License");
5. * you may not use this file except in compliance with the License.
6. * You may obtain a copy of the License at
7. *
8. * http://www.apache.org/licenses/LICENSE-2.0
9. *
10. * Unless required by applicable law or agreed to in writing, software
11. * distributed under the License is distributed on an "AS IS" BASIS,
12. * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13. * See the License for the specific language governing permissions and
14. * limitations under the License.
15. */
17. #ifndef ANDROID_INCLUDE_HARDWARE_HARDWARE_H
18. #define ANDROID_INCLUDE_HARDWARE_HARDWARE_H
20. #include <stdint.h>
21. #include <sys/cdefs.h>
23. #include <cutils/native_handle.h>
24. #include <system/graphics.h>
26. __BEGIN_DECLS
28. /*
29. * Value for the hw_module_t.tag field
30. */
32. #define MAKE_TAG_CONSTANT(A,B,C,D) (((A) << 24) | ((B) << 16) | ((C) << 8) | (D))
34. #define HARDWARE_MODULE_TAG MAKE_TAG_CONSTANT('H', 'W', 'M', 'T')
35. #define HARDWARE_DEVICE_TAG MAKE_TAG_CONSTANT('H', 'W', 'D', 'T')
37. struct hw_module_t;
38. struct hw_module_methods_t;
39. struct hw_device_t;
41. /**
42. * Every hardware module must have a data structure named HAL_MODULE_INFO_SYM
43. * and the fields of this data structure must begin with hw_module_t
44. * followed by module specific information.
45. */
46. //每一個硬件模塊都每必須有一個名為HAL_MODULE_INFO_SYM的數據結構變量，它的第一個成員的類型必須為hw_module_t
47. typedef struct hw_module_t {
48. /** tag must be initialized to HARDWARE_MODULE_TAG */
49. uint32_t tag;
51. /** major version number for the module */
52. uint16_t version_major;
54. /** minor version number of the module */
55. uint16_t version_minor;
57. /** Identifier of module */
58. const char *id;
60. /** Name of this module */
61. const char *name;
63. /** Author/owner/implementor of the module */
64. const char *author;
66. /** Modules methods */
67. //模塊方法列表,指向hw_module_methods_t*
68. struct hw_module_methods_t* methods;
70. /** module's dso */
71. void* dso;
73. /** padding to 128 bytes, reserved for future use */
74. uint32_t reserved[32-7];
76. } hw_module_t;
78. typedef struct hw_module_methods_t { //硬件模塊方法列表的定義，這裡只定義了一個open函數
79. /** Open a specific device */
80. int (*open)(const struct hw_module_t* module, const char* id, //注意這個open函數明確指出第三個參數的類型為struct hw_device_t**
81. struct hw_device_t** device);
82. } hw_module_methods_t;
84. /**
85. * Every device data structure must begin with hw_device_t
86. * followed by module specific public methods and attributes.
87. */
88. //每一個設備數據結構的第一個成員函數必須是hw_device_t類型，其次才是各個公共方法和屬性
89. typedef struct hw_device_t {
90. /** tag must be initialized to HARDWARE_DEVICE_TAG */
91. uint32_t tag;
93. /** version number for hw_device_t */
94. uint32_t version;
96. /** reference to the module this device belongs to */
97. struct hw_module_t* module;
99. /** padding reserved for future use */
100. uint32_t reserved[12];
102. /** Close this device */
103. int (*close)(struct hw_device_t* device);
105. } hw_device_t;
107. /**
108. * Name of the hal_module_info
109. */
110. #define HAL_MODULE_INFO_SYM HMI
112. /**
113. * Name of the hal_module_info as a string
114. */
115. #define HAL_MODULE_INFO_SYM_AS_STR "HMI"
117. /**
118. * Get the module info associated with a module by id.
119. *
120. * @return: 0 == success, <0 == error and *module == NULL
121. */
122. int hw_get_module(const char *id, const struct hw_module_t **module);
124. /**
125. * Get the module info associated with a module instance by class 'class_id'
126. * and instance 'inst'.
127. *
128. * Some modules types necessitate multiple instances. For example audio supports
129. * multiple concurrent interfaces and thus 'audio' is the module class
130. * and 'primary' or 'a2dp' are module interfaces. This implies that the files
131. * providing these modules would be named audio.primary.<variant>.so and
132. * audio.a2dp.<variant>.so
133. *
134. * @return: 0 == success, <0 == error and *module == NULL
135. */
136. int hw_get_module_by_class(const char *class_id, const char *inst,
137. const struct hw_module_t **module);
139. __END_DECLS
141. #endif /* ANDROID_INCLUDE_HARDWARE_HARDWARE_H */

由以上內容可以看出(typedef struct hw_module_t ,typedef struct hw_device_t)，如果我們要寫一個自定義設備的驅動的HAL層時，我們得首先自定義兩個數據結構: