《Linux那些事兒之我是USB》我是隨身碟（31）SCSI命令之我型我秀

usb_stor_control_thread()基本講完了，但是其中下面這幾行，正是高潮中的高潮。所謂的批量傳輸，所謂的Bulk-Only協議。正是在這裡體現出來的。

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           }

所謂的US_DEBUG，我們前面已經講過，無非就是列印條是資訊的。而眼下這句話就是執行usb_stor_show_command(us->srb)這個函數，鑒於這個函數是我們自己寫的，而且有意義，所以也就列出來。這個函數定義於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(" %02x", srb->cmnd[i]);

158      US_DEBUGPX("\n");

159 }

這個函數，很簡單，就是把要執行的SCSI命令列印出來。列出這個函數沒別的意思，讓不熟悉SCSI的讀者知道基本上會遇到一些什麼命令。顯然，剛才說的那個INQUIRY也包含在其中。

不過別看這個函數很簡單，你要是不熟悉SCSI協議的話，你還真的解釋不了這個函數。比如你說srb->cmnd[]這個數組到底是什麼內容?有什麼格式？為什麼函數一開始只判斷cmnd[0]？實不相瞞，這裡邊還真有學問。首先，在SCSI的規範裡邊定義了一些命令，每個命令都有一定的格式，命令的位元組數也有好幾種，有的命令是6個位元組的，有的命令是10個位元組的，有的命令是12個位元組的。4.33.1、圖4.33.2和圖4.33.3所示，SCSI命令就該是這個樣子。

人們把這樣幾個位元組的命令稱之為CDB（Command DescriptorBlock，命令描述符塊）。而我們為CDB準備了一個字元數組，結構體struct scsi_cmnd中的unsigned char cmnd[16]，最大就12個位元組，為什麼不申請一個12個位元組的數組？

既然這個CDB有16個位元組，那麼為什麼我們每次都判斷cmnd[0]就夠了？仔細看這三幅圖，注意到Operation code了嗎？沒錯，三幅圖中的第1個位元組都被稱為Operationcode，換而言之，不管是什麼樣子的命令，都必須在第1個位元組裡簽上自己的名字，向世人說明你是誰。於是在include/scsi/scsi.h中，定義了好多宏，比如#define INQUIRY 0x12，#define READ_6 0x08，#define FORMAT_UNIT 0x04，實際上作業碼就相當於SCSI命令的序號，SCSI命令總共也就那麼多，8位的作業碼已經足夠表示了，因此，我們只要用一個位元組就可以判斷出這是哪個命令了。

好了，命令說完了，開始進入真正處理命令的部分了。