分类： LINUX
2012-03-30 14:28:25
usb_stor_control_thread()基本讲完了，但是其中下面这几行，正是高潮中的高潮。所谓的批量传输，所谓的Bulk-Only协议。正是在这里体现出来的。
371 /* we've got a command, let's do it! */
372 else {
373 US_DEBUG(usb_stor_show_command(us->srb));
374 us->proto_handler(us->srb,us);
375 }
所谓的US_DEBUG，我们前面已经讲过，无非就是打印条是信息的。而眼下这句话就是执行 usb_stor_show_command(us->srb)这个函数，鉴于这个函数是我们自己写的，而且有意义，所以也就列出来。这个函数定义 于drivers/usb/storage/debug.c中：
56 voidusb_stor_show_command(struct scsi_cmnd *srb)
57 {
58 char*what = NULL;
59 inti;
60
61 switch(srb->cmnd[0]) {
62 caseTEST_UNIT_READY: what = "TEST_UNIT_READY"; break;
63 caseREZERO_UNIT: what = "REZERO_UNIT"; break;
64 caseREQUEST_SENSE: what = "REQUEST_SENSE"; break;
65 caseFORMAT_UNIT: what = "FORMAT_UNIT"; break;
66 caseREAD_BLOCK_LIMITS: what = "READ_BLOCK_LIMITS"; break;
67 caseREASSIGN_BLOCKS: what = "REASSIGN_BLOCKS"; break;
68 caseREAD_6: what = "READ_6"; break;
69 caseWRITE_6: what = "WRITE_6"; break;
70 caseSEEK_6: what = "SEEK_6"; break;
71 caseREAD_REVERSE: what = "READ_REVERSE"; break;
72 caseWRITE_FILEMARKS: what = "WRITE_FILEMARKS"; break;
73 caseSPACE: what = "SPACE"; break;
74 caseINQUIRY: what = "INQUIRY"; break;
75 caseRECOVER_BUFFERED_DATA: what ="RECOVER_BUFFERED_DATA"; break;
76 caseMODE_SELECT: what = "MODE_SELECT"; break;
77 caseRESERVE: what = "RESERVE"; break;
78 caseRELEASE: what = "RELEASE"; break;
79 caseCOPY: what = "COPY"; break;
80 caseERASE: what = "ERASE"; break;
81 caseMODE_SENSE: what = "MODE_SENSE"; break;
82 caseSTART_STOP: what = "START_STOP"; break;
83 caseRECEIVE_DIAGNOSTIC: what = "RECEIVE_DIAGNOSTIC"; break;
84 case SEND_DIAGNOSTIC: what ="SEND_DIAGNOSTIC"; break;
85 caseALLOW_MEDIUM_REMOVAL: what = "ALLOW_MEDIUM_REMOVAL"; break;
86 caseSET_WINDOW: what = "SET_WINDOW"; break;
87 caseREAD_CAPACITY: what = "READ_CAPACITY"; break;
88 caseREAD_10: what = "READ_10"; break;
89 caseWRITE_10: what = "WRITE_10"; break;
90 caseSEEK_10: what = "SEEK_10"; break;
91 caseWRITE_VERIFY: what = "WRITE_VERIFY"; break;
92 caseVERIFY: what = "VERIFY"; break;
93 caseSEARCH_HIGH: what = "SEARCH_HIGH"; break;
94 caseSEARCH_EQUAL: what = "SEARCH_EQUAL"; break;
95 caseSEARCH_LOW: what = "SEARCH_LOW"; break;
96 caseSET_LIMITS: what = "SET_LIMITS"; break;
97 caseREAD_POSITION: what = "READ_POSITION"; break;
98 case SYNCHRONIZE_CACHE: what ="SYNCHRONIZE_CACHE"; break;
99 case LOCK_UNLOCK_CACHE: what ="LOCK_UNLOCK_CACHE"; break;
100 case READ_DEFECT_DATA: what ="READ_DEFECT_DATA"; break;
101 case MEDIUM_SCAN: what ="MEDIUM_SCAN"; break;
102 case COMPARE: what = "COMPARE";break;
103 case COPY_VERIFY: what ="COPY_VERIFY"; break;
104 case WRITE_BUFFER: what ="WRITE_BUFFER"; break;
105 case READ_BUFFER: what ="READ_BUFFER"; break;
106 case UPDATE_BLOCK: what ="UPDATE_BLOCK"; break;
107 case READ_LONG: what = "READ_LONG";break;
108 case WRITE_LONG: what ="WRITE_LONG"; break;
109 case CHANGE_DEFINITION: what ="CHANGE_DEFINITION"; break;
110 case WRITE_SAME: what ="WRITE_SAME"; break;
111 case GPCMD_READ_SUBCHANNEL: what = "READSUBCHANNEL"; break;
112 case READ_TOC: what = "READ_TOC";break;
113 case GPCMD_READ_HEADER: what = "READHEADER"; break;
114 case GPCMD_PLAY_AUDIO_10: what = "PLAYAUDIO (10)"; break;
115 case GPCMD_PLAY_AUDIO_MSF: what = "PLAYAUDIO MSF"; break;
116 case GPCMD_GET_EVENT_STATUS_NOTIFICATION:
117 what = "GET EVENT/STATUSNOTIFICATION"; break;
118 case GPCMD_PAUSE_RESUME: what ="PAUSE/RESUME"; break;
119 case LOG_SELECT: what ="LOG_SELECT"; break;
120 case LOG_SENSE: what = "LOG_SENSE";break;
121 case GPCMD_STOP_PLAY_SCAN: what = "STOPPLAY/SCAN"; break;
122 case GPCMD_READ_DISC_INFO: what = "READDISC INFORMATION"; break;
123 case GPCMD_READ_TRACK_RZONE_INFO:
124 what = "READ TRACK INFORMATION";break;
125 case GPCMD_RESERVE_RZONE_TRACK: what ="RESERVE TRACK"; break;
126 case GPCMD_SEND_OPC: what = "SENDOPC"; break;
127 case MODE_SELECT_10: what ="MODE_SELECT_10"; break;
128 case GPCMD_REPAIR_RZONE_TRACK: what ="REPAIR TRACK"; break;
129 case 0x59: what = "READ MASTER CUE";break;
130 case MODE_SENSE_10: what ="MODE_SENSE_10"; break;
131 case GPCMD_CLOSE_TRACK: what = "CLOSETRACK/SESSION"; break;
132 case 0x5C: what = "READ BUFFER CAPACITY";break;
133 case 0x5D: what = "SEND CUESHEET"; break;
134 case GPCMD_BLANK: what = "BLANK";break;
135 case REPORT_LUNS: what = "REPORTLUNS"; break;
136 case MOVE_MEDIUM: what = "MOVE_MEDIUM orPLAY AUDIO (12)"; break;
137 case READ_12: what = "READ_12";break;
138 case WRITE_12: what = "WRITE_12";break;
139 case WRITE_VERIFY_12: what ="WRITE_VERIFY_12"; break;
140 case SEARCH_HIGH_12: what ="SEARCH_HIGH_12"; break;
141 case SEARCH_EQUAL_12: what ="SEARCH_EQUAL_12"; break;
142 case SEARCH_LOW_12: what ="SEARCH_LOW_12"; break;
143 case SEND_VOLUME_TAG: what ="SEND_VOLUME_TAG"; break;
144 case READ_ELEMENT_STATUS: what ="READ_ELEMENT_STATUS"; break;
145 case GPCMD_READ_CD_MSF: what = "READ CDMSF"; break;
146 case GPCMD_SCAN: what = "SCAN";break;
147 case GPCMD_SET_SPEED: what = "SET CDSPEED"; break;
148 case GPCMD_MECHANISM_STATUS: what ="MECHANISM STATUS"; break;
149 case GPCMD_READ_CD: what = "READCD"; break;
150 case 0xE1: what = "WRITE CONTINUE";break;
151 case WRITE_LONG_2: what ="WRITE_LONG_2"; break;
152 default: what = "(unknown command)";break;
153 }
154 US_DEBUGP("Command %s (%d Bytes)\n",what, srb->cmd_len);
155 US_DEBUGP("");
156 for (i = 0; i < srb->cmd_len &&i < 16; i )
157 US_DEBUGPX(" x", srb->cmnd[i]);
158 US_DEBUGPX("\n");
159 }
这个函数，很简单，就是把要执行的SCSI命令打印出来。列出这个函数没别的意思，让不熟悉SCSI的读者知道基本上会遇到一些什么命令。显然，刚才说的那个INQUIRY也包含在其中。
不过别看这个函数很简单，你要是不熟悉SCSI协议的话，你还真的解释不了这个函数。比如你说srb->cmnd[]这个数组到底是什么内 容?有什么格式？为什么函数一开始只判断cmnd[0]？实不相瞒，这里边还真有学问。首先，在SCSI的规范里边定义了一些命令，每个命令都有一定的格 式，命令的字节数也有好几种，有的命令是6个字节的，有的命令是10个字节的，有的命令是12个字节的。如图4.33.1、图4.33.2和图 4.33.3所示，SCSI命令就该是这个样子。
人们把这样几个字节的命令称之为CDB（Command DescriptorBlock，命令描述符块）。而我们为CDB准备了一个字符数组，结构体struct scsi_cmnd中的unsigned char cmnd[16]，最大就12个字节，为什么不申请一个12个字节的数组？
既然这个CDB有16个字节，那么为什么我们每次都判断cmnd[0]就够了？仔细看这三幅图，注意到Operation code了吗？没错，三幅图中的第1个字节都被称为Operationcode，换而言之，不管是什么样子的命令，都必须在第1个字节里签上自己的名字， 向世人说明你是谁。于是在include/scsi/scsi.h中，定义了好多宏，比如#define INQUIRY 0x12，#define READ_6 0x08，#define FORMAT_UNIT 0x04，实际上操作码就相当于SCSI命令的序列号，SCSI命令总共也就那么多，8位的操作码已经足够表示了，因此，我们只要用一个字节就可以判断出 这是哪个命令了。
好了，命令说完了，开始进入真正处理命令的部分了。