Linux那些事儿之我是U盘(42)我型我show--scsi命令集体show

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Bulk传输,所谓的Bulk-Only协议.正是在这里体现出来的.
372 /* we've got a command, let's do it! */
373 else {
374 US_DEBUG(usb_stor_show_command(us->srb));
375 us->proto_handler(us->srb, us);
376 }
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,而且有点意义,所以也就列出来.这个函数定义于drivers/usb/storage/debug.c中,
55 void usb_stor_show_command(struct scsi_cmnd *srb)
56 {
57 char *what = NULL;
58 int i;
59
60 switch (srb->cmnd[0]) {
61 case TEST_UNIT_READY: what = "TEST_UNIT_READY"; break;
62 case REZERO_UNIT: what = "REZERO_UNIT"; break;
63 case REQUEST_SENSE: what = "REQUEST_SENSE"; break;
64 case FORMAT_UNIT: what = "FORMAT_UNIT"; break;
65 case READ_BLOCK_LIMITS: what = "READ_BLOCK_LIMITS"; break;
66 case REASSIGN_BLOCKS: what = "REASSIGN_BLOCKS"; break;
67 case READ_6: what = "READ_6"; break;
68 case WRITE_6: what = "WRITE_6"; break;
69 case SEEK_6: what = "SEEK_6"; break;
70 case READ_REVERSE: what = "READ_REVERSE"; break;
71 case WRITE_FILEMARKS: what = "WRITE_FILEMARKS"; break;
72 case SPACE: what = "SPACE"; break;
73 case INQUIRY: what = "INQUIRY"; break;
74 case RECOVER_BUFFERED_DATA: what = "RECOVER_BUFFERED_DATA"; break;
75 case MODE_SELECT: what = "MODE_SELECT"; break;
76 case RESERVE: what = "RESERVE"; break;
77 case RELEASE: what = "RELEASE"; break;
78 case COPY: what = "COPY"; break;
79 case ERASE: what = "ERASE"; break;
80 case MODE_SENSE: what = "MODE_SENSE"; break;
81 case START_STOP: what = "START_STOP"; break;
82 case RECEIVE_DIAGNOSTIC: what = "RECEIVE_DIAGNOSTIC"; break;
83 case SEND_DIAGNOSTIC: what = "SEND_DIAGNOSTIC"; break;
84 case ALLOW_MEDIUM_REMOVAL: what = "ALLOW_MEDIUM_REMOVAL"; break;
85 case SET_WINDOW: what = "SET_WINDOW"; break;
86 case READ_CAPACITY: what = "READ_CAPACITY"; break;
87 case READ_10: what = "READ_10"; break;
88 case WRITE_10: what = "WRITE_10"; break;
89 case SEEK_10: what = "SEEK_10"; break;
90 case WRITE_VERIFY: what = "WRITE_VERIFY"; break;
91 case VERIFY: what = "VERIFY"; break;
92 case SEARCH_HIGH: what = "SEARCH_HIGH"; break;
93 case SEARCH_EQUAL: what = "SEARCH_EQUAL"; break;
94 case SEARCH_LOW: what = "SEARCH_LOW"; break;
95 case SET_LIMITS: what = "SET_LIMITS"; break;
96 case READ_POSITION: what = "READ_POSITION"; break;
97 case SYNCHRONIZE_CACHE: what = "SYNCHRONIZE_CACHE"; break;
98 case LOCK_UNLOCK_CACHE: what = "LOCK_UNLOCK_CACHE"; break;
99 case READ_DEFECT_DATA: what = "READ_DEFECT_DATA"; break;
100 case MEDIUM_SCAN: what = "MEDIUM_SCAN"; break;
101 case COMPARE: what = "COMPARE"; break;
102 case COPY_VERIFY: what = "COPY_VERIFY"; break;
103 case WRITE_BUFFER: what = "WRITE_BUFFER"; break;
104 case READ_BUFFER: what = "READ_BUFFER"; break;
105 case UPDATE_BLOCK: what = "UPDATE_BLOCK"; break;
106 case READ_LONG: what = "READ_LONG"; break;
107 case WRITE_LONG: what = "WRITE_LONG"; break;
108 case CHANGE_DEFINITION: what = "CHANGE_DEFINITION"; break;
109 case WRITE_SAME: what = "WRITE_SAME"; break;
110 case GPCMD_READ_SUBCHANNEL: what = "READ SUBCHANNEL"; break;
111 case READ_TOC: what = "READ_TOC"; break;
112 case GPCMD_READ_HEADER: what = "READ HEADER"; break;
113 case GPCMD_PLAY_AUDIO_10: what = "PLAY AUDIO (10)"; break;
114 case GPCMD_PLAY_AUDIO_MSF: what = "PLAY AUDIO MSF"; break;
115 case GPCMD_GET_EVENT_STATUS_NOTIFICATION:
116 what = "GET EVENT/STATUS NOTIFICATION"; break;
117 case GPCMD_PAUSE_RESUME: what = "PAUSE/RESUME"; break;
118 case LOG_SELECT: what = "LOG_SELECT"; break;
119 case LOG_SENSE: what = "LOG_SENSE"; break;
120 case GPCMD_STOP_PLAY_SCAN: what = "STOP PLAY/SCAN"; break;
121 case GPCMD_READ_DISC_INFO: what = "READ DISC INFORMATION"; break;
122 case GPCMD_READ_TRACK_RZONE_INFO:
123 what = "READ TRACK INFORMATION"; break;
124 case GPCMD_RESERVE_RZONE_TRACK: what = "RESERVE TRACK"; break;
125 case GPCMD_SEND_OPC: what = "SEND OPC"; break;
126 case MODE_SELECT_10: what = "MODE_SELECT_10"; break;
127 case GPCMD_REPAIR_RZONE_TRACK: what = "REPAIR TRACK"; break;
128 case 0x59: what = "READ MASTER CUE"; break;
129 case MODE_SENSE_10: what = "MODE_SENSE_10"; break;
130 case GPCMD_CLOSE_TRACK: what = "CLOSE TRACK/SESSION"; break;
131 case 0x5C: what = "READ BUFFER CAPACITY"; break;
132 case 0x5D: what = "SEND CUE SHEET"; break;
133 case GPCMD_BLANK: what = "BLANK"; break;
134 case MOVE_MEDIUM: what = "MOVE_MEDIUM or PLAY AUDIO (12)"; break;
135 case READ_12: what = "READ_12"; break;
136 case WRITE_12: what = "WRITE_12"; break;
137 case WRITE_VERIFY_12: what = "WRITE_VERIFY_12"; break;
138 case SEARCH_HIGH_12: what = "SEARCH_HIGH_12"; break;
139 case SEARCH_EQUAL_12: what = "SEARCH_EQUAL_12"; break;
140 case SEARCH_LOW_12: what = "SEARCH_LOW_12"; break;
141 case SEND_VOLUME_TAG: what = "SEND_VOLUME_TAG"; break;
142 case READ_ELEMENT_STATUS: what = "READ_ELEMENT_STATUS"; break;
143 case GPCMD_READ_CD_MSF: what = "READ CD MSF"; break;
144 case GPCMD_SCAN: what = "SCAN"; break;
145 case GPCMD_SET_SPEED: what = "SET CD SPEED"; break;
146 case GPCMD_MECHANISM_STATUS: what = "MECHANISM STATUS"; break;
147 case GPCMD_READ_CD: what = "READ CD"; break;
148 case 0xE1: what = "WRITE CONTINUE"; break;
149 case WRITE_LONG_2: what = "WRITE_LONG_2"; break;
150 default: what = "(unknown command)"; break;
151 }
152 US_DEBUGP("Command %s (%d bytes)/n", what, srb->cmd_len);
153 US_DEBUGP("");
154 for (i = 0; i < srb->cmd_len && i < 16; i++)
155 US_DEBUGPX(" %02x", srb->cmnd[i]);
156 US_DEBUGPX("/n");
157 }
相信即使是天下无贼里边的傻根也能看懂这个函数,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的同志们知道基本上会遇到些啥命令.显然,刚才说的那个INQUIRY也包含在其中的.
不过别看这个函数很easy,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?为啥函数一开始只判断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,有的命令是六个字节的,有的命令是10个字节的,有的命令是12个字节的.你看,你看,下面就是摘自scsi规范里边的几张图,scsi命令就该是这个样子.

这是6字节的,

这是10个字节的,

这是12个字节的.
之所以有这好几种命令描述块,没什么特别的,也许只是想体现生物的多样性而已,又或许,印证了张爱玲的那句话,也许每一个男子全都有过这样的两个女人,至少两个.娶了红玫瑰,久而久之,红的变成了墙上的一抹蚊子血,白的还是”窗前明月光”;娶了白玫瑰,白的便是衣服上的一粒饭粘子,红的却是心口上的一颗朱砂痣.
江湖中人们把这样几个字节的命令称之为CDB, command descriptor block, 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],你说最大就12个字节,干嘛不申请一个12个字节的数组?给你一个建议:我记得复旦并没有一门课程叫做可持续性发展,但是我记得我上海交大的同学有修过这样一门课程,有机会的话,去交大听一下吧.
Ok,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,注意到那个Operation code了吗?没错,三幅图中的第一个字节都被称为Operation code,换言之,不管你是什么样子的命令, 你都必须在第一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,定义了好多好多宏,比如#define INQUIRY 0x12,又比如#define READ_6 0x08,再比如#define FORMAT_UNIT 0x04,够了,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. 因为你的第一个字节就相当于你的眼睛,不管你埋藏的多深,你会发现最终总是你的眼睛背叛了你的心,这一点郑中基大概感受颇深吧.
好了,命令说完了,开始进入真正处理命令的部分了.