《Linux那些事儿之我是USB》我是U盘（33）迷雾重重的批量传输（二）

其实故事已经讲了很久，但如果你觉得到这里你已经把故事都看明白了，那么你错了。不仅仅是错了。不信，我们就继续看，先看512行，us->transport()，这个函数指针同样是在storage_probe时被赋值，对于U盘，它遵守的是Bulk-Only协议，因此us->transport()被赋值为usb_stor_Bulk_transport()。来看usb_stor_Bulk_transport()，它同样来自drivers/usb/storage/transport.c：

941 int usb_stor_Bulk_transport(struct scsi_cmnd*srb, struct us_data *us)

942 {

943 struct bulk_cb_wrap *bcb = (structbulk_cb_wrap *) us->iobuf;

944 struct bulk_cs_wrap *bcs = (structbulk_cs_wrap *) us->iobuf;

945 unsigned int transfer_length =srb->request_bufflen;

946 unsigned int residue;

947 int result;

948 int fake_sense = 0;

949 unsigned int cswlen;

950 unsigned int cbwlen = US_BULK_CB_WRAP_LEN;

951

952 /* Take care of BULK32 devices; set extra Byteto 0 */

953 if ( unlikely(us->flags &US_FL_BULK32)) {

954 cbwlen = 32;

955 us->iobuf[31] = 0;

956 }

957

958 /* set up the command wrapper */

959 bcb->Signature =cpu_to_le32(US_BULK_CB_SIGN);

960 bcb->DataTransferLength =cpu_to_le32(transfer_length);

961 bcb->Flags = srb->sc_data_direction ==DMA_FROM_DEVICE ? 1 <<7 : 0;

962 bcb->Tag= ++us->tag;

963 bcb->Lun= srb->device->lun;

964 if (us->flags & US_FL_SCM_MULT_TARG)

965 bcb->Lun |= srb->device->id <<4;

966 bcb->Length = srb->cmd_len;

967

968 /* copy the command payload */

969 memset(bcb->CDB, 0, sizeof(bcb->CDB));

970 memcpy(bcb->CDB, srb->cmnd,bcb->Length);

971

972 /* send it to out endpoint */

973 US_DEBUGP("Bulk Command S 0x%x T 0x%x L%d F %d Trg %d LUN %d CL %d\n",

974 le32_to_cpu(bcb->Signature), bcb->Tag,

975 le32_to_cpu(bcb->DataTransferLength),bcb->Flags,

976 (bcb->Lun >>4), (bcb->Lun & 0x0F),

977 bcb->Length);

978 result = usb_stor_bulk_transfer_buf(us,us->send_bulk_pipe,

979 bcb, cbwlen,NULL);

980 US_DEBUGP("Bulk command transferresult=%d\n", result);

981 if (result != USB_STOR_XFER_GOOD)

982 return USB_STOR_TRANSPORT_ERROR;

983

984 /* DATA STAGE */

985 /* send/receive data payload, if there is any*/

986

987 /* Some USB-IDE converter chips need a 100usdelay between the

988 *command phase and the data phase. Somedevices need a little

989 * morethan that, probably because of clock rate inaccuracies. */

990 if (unlikely(us->flags &US_FL_GO_SLOW))

991 udelay(125);

992

993 if (transfer_length) {

994 unsigned int pipe = srb->sc_data_direction== DMA_FROM_DEVICE ?

995 us->recv_bulk_pipe : us->send_bulk_pipe;

996 result = usb_stor_bulk_transfer_sg(us, pipe,

997 srb->request_buffer, transfer_length,

998 srb->use_sg, &srb->resid);

999 US_DEBUGP("Bulk data transfer result0x%x\n", result);

1000 if (result == USB_STOR_XFER_ERROR)

1001 return USB_STOR_TRANSPORT_ERROR;

1002

1003 /* If the device tried to send back more datathan the

1004 *amount requested, the spec requires us to transfer

1005 * theCSW anyway. Since there's no pointretrying the

1006 * thecommand, we'll return fake sense data indicating

1007 * IllegalRequest, Invalid Field in CDB.

1008 */

1009 if (result == USB_STOR_XFER_LONG)

1010 fake_sense = 1;

1011 }

1012

1013 /* See flow chart on pg 15 of the Bulk OnlyTransport spec for

1014 * anexplanation of how this code works.

1015 */

1016

1017 /* get CSW for device status */

1018 US_DEBUGP("Attempting to getCSW...\n");

1019 result = usb_stor_bulk_transfer_buf(us,us->recv_bulk_pipe,

1020 bcs,US_BULK_CS_WRAP_LEN, &cswlen);

1021

1022 /* Some broken devices add unnecessaryzero-length packets to the

1023 * endof their data transfers. Such packetsshow up as 0-length

1024 *CSWs. If we encounter such a thing, tryto read the CSW again.

1025 */

1026 if (result == USB_STOR_XFER_SHORT &&cswlen == 0) {

1027 US_DEBUGP("Received 0-length CSW;retrying...\n");

1028 result = usb_stor_bulk_transfer_buf(us,us->recv_bulk_pipe,

1029 bcs,US_BULK_CS_WRAP_LEN, &cswlen);

1030 }

1031

1032 /* did the attempt to read the CSW fail? */

1033 if (result == USB_STOR_XFER_STALLED) {

1034

1035 /* get the status again */

1036 US_DEBUGP("Attempting to get CSW (2ndtry)...\n");

1037 result = usb_stor_bulk_transfer_buf(us,us->recv_bulk_pipe,

1038 bcs,US_BULK_CS_WRAP_LEN, NULL);

1039 }

1040

1041 /* if we still have a failure at this point,we're in trouble */

1042 US_DEBUGP("Bulk status result =%d\n", result);

1043 if(result != USB_STOR_XFER_GOOD)

1044 return USB_STOR_TRANSPORT_ERROR;

1045

1046 /* check bulk status */

1047 residue = le32_to_cpu(bcs->Residue);

1048 US_DEBUGP("Bulk Status S 0x%x T 0x%x R%u Stat 0x%x\n",

1049 le32_to_cpu(bcs->Signature), bcs->Tag,

1050 residue,bcs->Status);

1051 if (bcs->Tag != us->tag ||bcs->Status > US_BULK_STAT_PHASE) {

1052 US_DEBUGP("Bulk logical error\n");

1053 return USB_STOR_TRANSPORT_ERROR;

1054 }

1055

1056 /* Some broken devices report odd signatures,so we do not check them

1057 * forvalidity against the spec. We store the first one we see,

1058 * andcheck subsequent transfers for validity against this signature.

1059 */

1060 if (!us->bcs_signature) {

1061 us->bcs_signature = bcs->Signature;

1062 if (us->bcs_signature !=cpu_to_le32(US_BULK_CS_SIGN))

1063 US_DEBUGP("Learnt BCSsignature 0x%08X\n",

1064 le32_to_cpu(us->bcs_signature));

1065 } else if (bcs->Signature !=us->bcs_signature) {

1066 US_DEBUGP("Signature mismatch: got %08X,expecting %08X\n",

1067 le32_to_cpu(bcs->Signature),

1068 le32_to_cpu(us->bcs_signature));

1069 return USB_STOR_TRANSPORT_ERROR;

1070 }

1071

1072 /* try to compute the actual residue, basedon how much data

1073 * wasreally transferred and what the device tells us */

1074 if (residue) {

1075 if(!(us->flags & US_FL_IGNORE_RESIDUE)) {

1076 residue = min(residue,transfer_length);

1077 srb->resid =max(srb->resid, (int) residue);

1078 }

1079 }

1080

1081 /* based on the status code, we report goodor bad */

1082 switch (bcs->Status) {

1083 case US_BULK_STAT_OK:

1084 /* device babbled -- return fake sense data */

1085 if (fake_sense) {

1086 memcpy(srb->sense_buffer,

1087 usb_stor_sense_invalidCDB,

1088 sizeof(usb_stor_sense_invalidCDB));

1089 returnUSB_STOR_TRANSPORT_NO_SENSE;

1090 }

1091

1092 /* command good -- note that data could beshort */

1093 return USB_STOR_TRANSPORT_GOOD;

1094

1095 case US_BULK_STAT_FAIL:

1096 /* command failed */

1097 return USB_STOR_TRANSPORT_FAILED;

1098

1099 case US_BULK_STAT_PHASE:

1100 /* phase error -- note that a transport resetwill be

1101 *invoked by the invoke_transport() function

1102 */

1103 return USB_STOR_TRANSPORT_ERROR;

1104 }

1105

1106 /* we should never get here, but if we do,we're in trouble */

1107 return USB_STOR_TRANSPORT_ERROR;

1108 }

这个函数也不是好“惹”的。但正是这个函数掀开了我们批量传输的新篇章。

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