Linux3.10.0块IO子系统流程（6）-- 派发SCSI命令到低层驱动

在SCSI策略例程中最后调用scsi_dispatch_cmd将SCSI命令描述符派发给低层驱动进行处理

 

  1 /**
  2 * scsi_dispatch_command - Dispatch a command to the low-level driver.
  3 * @cmd: command block we are dispatching.
  4 *
  5 * Return: nonzero return request was rejected and device's queue needs to be
  6 * plugged.
  7 */
  8 int scsi_dispatch_cmd(struct scsi_cmnd *cmd)
  9 {
 10     struct Scsi_Host *host = cmd->device->host;
 11     unsigned long timeout;
 12     int rtn = 0;
 13 
 14     // 递增SCSI设备的IO请求统计数 
 15     atomic_inc(&cmd->device->iorequest_cnt);
 16 
 17     /* 
 18      * check if the device is still usable 
 19      * 检测设备是否还可用。在此时，可能别的地方已经将设备状态置为SDEV_DEL，如果这样，我们将所有命令出错，将命令的结果标记为DID_NO_CONNECT，调用scsi_done结束
 20      */
 21     if (unlikely(cmd->device->sdev_state == SDEV_DEL)) {
 22         /* in SDEV_DEL we error all commands. DID_NO_CONNECT
 23          * returns an immediate error upwards, and signals
 24          * that the device is no longer present */
 25         cmd->result = DID_NO_CONNECT << 16;
 26         scsi_done(cmd);
 27         /* return 0 (because the command has been processed) */
 28         goto out;
 29     }
 30 
 31     /* Check to see if the scsi lld made this device blocked. 检查SCSI低层驱动是否已经阻塞了这个设备*/
 32     if (unlikely(scsi_device_blocked(cmd->device))) {
 33         /*
 34          * in blocked state, the command is just put back on
 35          * the device queue.  The suspend state has already
 36          * blocked the queue so future requests should not
 37          * occur until the device transitions out of the
 38          * suspend state.
 39          */
 40 
 41         scsi_queue_insert(cmd, SCSI_MLQUEUE_DEVICE_BUSY);
 42 
 43         SCSI_LOG_MLQUEUE(3, printk("queuecommand : device blocked \n"));
 44 
 45         /*
 46          * NOTE: rtn is still zero here because we don't need the
 47          * queue to be plugged on return (it's already stopped)
 48          */
 49         goto out;
 50     }
 51 
 52     /*
 53      * If SCSI-2 or lower, store the LUN value in cmnd.
 54      */
 55     if (cmd->device->scsi_level <= SCSI_2 &&
 56         cmd->device->scsi_level != SCSI_UNKNOWN) {
 57         cmd->cmnd[1] = (cmd->cmnd[1] & 0x1f) |
 58                    (cmd->device->lun << 5 & 0xe0);
 59     }
 60 
 61     /*
 62      * We will wait MIN_RESET_DELAY clock ticks after the last reset so we can avoid the drive not being ready.
 63      * 如果主机适配器的resetting被设置为1，表示其last_Reset域有效。也就是说，后者记录了主机适配器上次复位的时间。
 64      * 在复位后必须有2秒钟的延时，才能向这个主机适配器发送命令。
 65      */
 66     timeout = host->last_reset + MIN_RESET_DELAY;
 67 
 68     if (host->resetting && time_before(jiffies, timeout)) {
 69         int ticks_remaining = timeout - jiffies;
 70         /*
 71          * NOTE: This may be executed from within an interrupt
 72          * handler!  This is bad, but for now, it'll do.  The irq
 73          * level of the interrupt handler has been masked out by the
 74          * platform dependent interrupt handling code already, so the
 75          * sti() here will not cause another call to the SCSI host's
 76          * interrupt handler (assuming there is one irq-level per
 77          * host).
 78          */
 79         while (--ticks_remaining >= 0)
 80             mdelay(1 + 999 / HZ);
 81         host->resetting = 0;
 82     }
 83 
 84     // 在提交SCSI命令之前输出信息
 85     scsi_log_send(cmd);
 86 
 87     /*
 88      * Before we queue this command, check if the command length exceeds what the host adapter can handle.
 89      * 在将命令排入队列之前，检查命令长度是否超过了主机适配器可以处理的最大长度，如果这样，将命令结果标记为DID_ABORT并返回
 90      */
 91     if (cmd->cmd_len > cmd->device->host->max_cmd_len) {
 92         SCSI_LOG_MLQUEUE(3,
 93             printk("queuecommand : command too long. "
 94                    "cdb_size=%d host->max_cmd_len=%d\n",
 95                    cmd->cmd_len, cmd->device->host->max_cmd_len));
 96         cmd->result = (DID_ABORT << 16);
 97 
 98         scsi_done(cmd);
 99         goto out;
100     }
101 
102     /* 在获取锁的过程中，可能别的地方已经将主机适配器的状态设置为“删除”。如果这样，标记为DID_NO_CONNECT并返回*/
103     if (unlikely(host->shost_state == SHOST_DEL)) {
104         cmd->result = (DID_NO_CONNECT << 16);
105         scsi_done(cmd);
106     } else {
107         trace_scsi_dispatch_cmd_start(cmd);
108         cmd->scsi_done = scsi_done;
109         rtn = host->hostt->queuecommand(host, cmd);    // 调用主机适配器模板的queuecommand回调，将SCSI命令排入低层设备驱动的队列，调用时传入scsi_done，以便低层驱动完成后回调
110     }
111 
112     if (rtn) {
113         trace_scsi_dispatch_cmd_error(cmd, rtn);
114         if (rtn != SCSI_MLQUEUE_DEVICE_BUSY &&
115             rtn != SCSI_MLQUEUE_TARGET_BUSY)
116             rtn = SCSI_MLQUEUE_HOST_BUSY;
117 
118         scsi_queue_insert(cmd, rtn);
119 
120         SCSI_LOG_MLQUEUE(3,
121             printk("queuecommand : request rejected\n"));
122     }
123 
124 out:
125     SCSI_LOG_MLQUEUE(3, printk("leaving scsi_dispatch_cmnd()\n"));
126     return rtn;
127 }

 

posted @ 2018-07-05 06:51  陆小呆  阅读(1880)  评论(0)  编辑  收藏  举报