《Linux那些事儿之我是USB》我是U盘（32）迷雾重重的批量传输（一）

374行，us->proto_handler()其实是一个函数指针，知道它指向什么吗?我们早在storage_probe()中，确切地说，在get_protocol()就赋了值，当时只知道是get protocol，却不知道究竟干什么用，现在该用上了，一个指针要是没什么用人家才不会为它赋值呢。当初我们就讲了，对于U盘，proto_handler被赋值为usb_stor_transparent_scsi_command，所以我们来看后者吧。后者定义于drivers/usb/storage/protocol.c：

140 void usb_stor_transparent_scsi_command(structscsi_cmnd *srb,

141                                        structus_data *us)

142 {

143    /* send the command to the transport layer */

144     usb_stor_invoke_transport(srb, us);

145 }

usb_stor_invoke_transport()这个函数可不简单。咱们先做好思想准备，接下来就去见识一下它的庐山真面目。它来自drivers/usb/storage/transport.c：

505 void usb_stor_invoke_transport(struct scsi_cmnd*srb, struct us_data *us)

506 {

507    int need_auto_sense;

508      intresult;

509

510      /* send the command to the transport layer */

511    srb->resid = 0;

512    result = us->transport(srb, us);

513

514      /* if the command gets aborted by the higherlayers, we need to

515    *short-circuit all other processing

516    */

517      if(test_bit(US_FLIDX_TIMED_OUT, &us->flags)) {

518      US_DEBUGP("-- command wasaborted\n");

519           srb->result = DID_ABORT << 16;

520         goto Handle_Errors;

521     }

522

523      /* if there is a transport error, reset anddon't auto-sense */

524      if (result == USB_STOR_TRANSPORT_ERROR) {

525         US_DEBUGP("-- transport indicates error,resetting\n");

526         srb->result = DID_ERROR << 16;

527           gotoHandle_Errors;

528      }

529

530      /* if the transport provided its own sense data,don't auto-sense */

531     if (result == USB_STOR_TRANSPORT_NO_SENSE) {

532           srb->result = SAM_STAT_CHECK_CONDITION;

533         return;

534    }

535

536      srb->result = SAM_STAT_GOOD;

537

538    /* Determine if we need to auto-sense

539      *

540     * Inormally don't use a flag like this, but it's almost impossible

541     * tounderstand what's going on here if I don't.

542    */

543     need_auto_sense = 0;

544

545    /*

546      * Ifwe're running the CB transport, which is incapable

547      * ofdetermining status on its own, we will auto-sense

548    *unless the operation involved a data-in transfer. Devices

549      * cansignal most data-in errors by stalling the bulk-in pipe.

550     */

551    if ((us->protocol == US_PR_CB ||us->protocol == US_PR_DPCM_USB) &&

552                        srb->sc_data_direction != DMA_FROM_DEVICE) {

553      US_DEBUGP("-- CB transport devicerequiring auto-sense\n");

554           need_auto_sense = 1;

555      }

556

557      /*

558    * If wehave a failure, we're going to do a REQUEST_SENSE

559    *automatically. Note that wedifferentiate between a command

560      *"failure" and an "error" in the transport mechanism.

561      */

562     if (result == USB_STOR_TRANSPORT_FAILED) {

563         US_DEBUGP("-- transport indicates commandfailure\n");

564           need_auto_sense = 1;

565      }

566

567    /*

568      * Ashort transfer on a command where we don't expect it

569      * isunusual, but it doesn't mean we need to auto-sense.

570    */

571    if ((srb->resid > 0) &&

572            !((srb->cmnd[0] == REQUEST_SENSE) ||

573              (srb->cmnd[0] == INQUIRY) ||

574              (srb->cmnd[0] == MODE_SENSE) ||

575              (srb->cmnd[0] == LOG_SENSE) ||

576              (srb->cmnd[0] == MODE_SENSE_10))) {

577         US_DEBUGP("-- unexpectedly shorttransfer\n");

578     }

579

580     /* Now, if we need to do the auto-sense, let'sdo it */

581    if (need_auto_sense) {

582         int temp_result;

583      void* old_request_buffer;

584         unsigned short old_sg;

585         unsigned old_request_bufflen;

586           unsigned char old_sc_data_direction;

587         unsigned char old_cmd_len;

588          unsigned char old_cmnd[MAX_COMMAND_SIZE];

589           int old_resid;

590

591         US_DEBUGP("Issuingauto-REQUEST_SENSE\n");

592

593         /* save the old command */

594           memcpy(old_cmnd, srb->cmnd,MAX_COMMAND_SIZE);

595         old_cmd_len = srb->cmd_len;

596

597         /* set the command and the LUN */

598         memset(srb->cmnd, 0, MAX_COMMAND_SIZE);

599         srb->cmnd[0] = REQUEST_SENSE;

600           srb->cmnd[1] = old_cmnd[1] & 0xE0;

601          srb->cmnd[4] = 18;

602

603           /* FIXME: we must do the protocol translationhere */

604      if (us->subclass == US_SC_RBC ||us->subclass == US_SC_SCSI)

605                 srb->cmd_len= 6;

606           else

607                srb->cmd_len= 12;

608

609         /* set the transfer direction */

610           old_sc_data_direction =srb->sc_data_direction;

611         srb->sc_data_direction = DMA_FROM_DEVICE;

612

613          /* use the new buffer we have */

614         old_request_buffer = srb->request_buffer;

615           srb->request_buffer = us->sensebuf;

616

617           /* set the buffer length for transfer */

618         old_request_bufflen = srb->request_bufflen;

619           srb->request_bufflen = US_SENSE_SIZE;

620

621           /* set up for no scatter-gather use */

622         old_sg = srb->use_sg;

623           srb->use_sg = 0;

624

625         /* issue the auto-sense command */

626         old_resid = srb->resid;

627         srb->resid = 0;

628         temp_result = us->transport(us->srb,us);

629

630           /*let's clean up right away */

631         memcpy(srb->sense_buffer, us->sensebuf,US_SENSE_SIZE);

632           srb->resid = old_resid;

633           srb->request_buffer = old_request_buffer;

634         srb->request_bufflen = old_request_bufflen;

635          srb->use_sg= old_sg;

636          srb->sc_data_direction =old_sc_data_direction;

637         srb->cmd_len = old_cmd_len;

638           memcpy(srb->cmnd, old_cmnd,MAX_COMMAND_SIZE);

639

640           if (test_bit(US_FLIDX_TIMED_OUT,&us->flags)) {

641               US_DEBUGP("-- auto-senseaborted\n");

642                srb->result = DID_ABORT<< 16;

643               goto Handle_Errors;

644         }

645           if (temp_result != USB_STOR_TRANSPORT_GOOD) {

646               US_DEBUGP("-- auto-sense failure\n");

647

648                /* we skip the reset if thishappens to be a

649               * multi-target device, since failure of an

650                * auto-sense is perfectly valid

651               */

652                srb->result = DID_ERROR <<16;

653               if (!(us->flags &US_FL_SCM_MULT_TARG))

654                     goto Handle_Errors;

655                return;

656           }

657

658           US_DEBUGP("-- Result from auto-senseis %d\n", temp_result);

659           US_DEBUGP("-- code: 0x%x, key: 0x%x,ASC: 0x%x, ASCQ: 0x%x\n",

660                          srb->sense_buffer[0],

661                          srb->sense_buffer[2] & 0xf,

662                          srb->sense_buffer[12],

663                          srb->sense_buffer[13]);

664 #ifdef CONFIG_USB_STORAGE_DEBUG

665         usb_stor_show_sense(

666                          srb->sense_buffer[2] & 0xf,

667                          srb->sense_buffer[12],

668                          srb->sense_buffer[13]);

669 #endif

670

671            /*set the result so the higher layers expect this data */

672           srb->result = SAM_STAT_CHECK_CONDITION;

673

674           /* If things are really okay, then let's showthat. Zero

675           * outthe sense buffer so the higher layers won't realize

676      * we didan unsolicited auto-sense. */

677         if (result == USB_STOR_TRANSPORT_GOOD&&

678                      /* Filemark 0, ignore EOM, ILI 0, no sense */

679                           (srb->sense_buffer[2] & 0xaf) == 0 &&

680                         /* No ASC or ASCQ */

681                           srb->sense_buffer[12] == 0 &&

682                           srb->sense_buffer[13] == 0) {

683               srb->result = SAM_STAT_GOOD;

684               srb->sense_buffer[0] = 0x0;

685           }

686      }

687

688      /* Did we transfer less than the minimumamount required? */

689      if (srb->result == SAM_STAT_GOOD&&

690                         srb->request_bufflen- srb->resid < srb->underflow)

691           srb->result = (DID_ERROR << 16) | (SUGGEST_RETRY<< 24);

692

693    return;

694

695    /* Error and abort processing: try toresynchronize with the device

696      * byissuing a port reset. If that fails, trya class-specific

697    *device reset. */

698  Handle_Errors:

699

700      /* Set the RESETTING bit, and clear theABORTING bit so that

701      * thereset may proceed. */

702    scsi_lock(us_to_host(us));

703    set_bit(US_FLIDX_RESETTING,&us->flags);

704    clear_bit(US_FLIDX_ABORTING,&us->flags);

705     scsi_unlock(us_to_host(us));

706

707     /* We must release the device lock becausethe pre_reset routine

708    * willwant to acquire it. */

709     mutex_unlock(&us->dev_mutex);

710      result = usb_stor_port_reset(us);

711    mutex_lock(&us->dev_mutex);

712

713      if (result < 0) {

714           scsi_lock(us_to_host(us));

715         usb_stor_report_device_reset(us);

716           scsi_unlock(us_to_host(us));

717         us->transport_reset(us);

718      }

719      clear_bit(US_FLIDX_RESETTING,&us->flags);

720 }

这段代码的复杂，调用关系一层又一层，让很多新手看了感觉无可奈何。

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