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
/**
* scsi_dispatch_command - Dispatch a command to the low-level driver.
* @cmd: command block we are dispatching.
*
* Return: nonzero return request was rejected and device's queue needs to be
* plugged.
*/
int scsi_dispatch_cmd(struct scsi_cmnd *cmd)
{
struct Scsi_Host *host = cmd->device->host;
unsigned long timeout;
int rtn = ; // 递增SCSI设备的IO请求统计数
atomic_inc(&cmd->device->iorequest_cnt); /*
* check if the device is still usable
* 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
*/
if (unlikely(cmd->device->sdev_state == SDEV_DEL)) {
/* in SDEV_DEL we error all commands. DID_NO_CONNECT
* returns an immediate error upwards, and signals
* that the device is no longer present */
cmd->result = DID_NO_CONNECT << ;
scsi_done(cmd);
/* return 0 (because the command has been processed) */
goto out;
} /* 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*/
if (unlikely(scsi_device_blocked(cmd->device))) {
/*
* in blocked state, the command is just put back on
* the device queue. The suspend state has already
* blocked the queue so future requests should not
* occur until the device transitions out of the
* suspend state.
*/ scsi_queue_insert(cmd, SCSI_MLQUEUE_DEVICE_BUSY); SCSI_LOG_MLQUEUE(, printk("queuecommand : device blocked \n")); /*
* NOTE: rtn is still zero here because we don't need the
* queue to be plugged on return (it's already stopped)
*/
goto out;
} /*
* If SCSI-2 or lower, store the LUN value in cmnd.
*/
if (cmd->device->scsi_level <= SCSI_2 &&
cmd->device->scsi_level != SCSI_UNKNOWN) {
cmd->cmnd[] = (cmd->cmnd[] & 0x1f) |
(cmd->device->lun << & 0xe0);
} /*
* We will wait MIN_RESET_DELAY clock ticks after the last reset so we can avoid the drive not being ready.
* 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。
* 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。
*/
timeout = host->last_reset + MIN_RESET_DELAY; if (host->resetting && time_before(jiffies, timeout)) {
int ticks_remaining = timeout - jiffies;
/*
* NOTE: This may be executed from within an interrupt
* handler! This is bad, but for now, it'll do. The irq
* level of the interrupt handler has been masked out by the
* platform dependent interrupt handling code already, so the
* sti() here will not cause another call to the SCSI host's
* interrupt handler (assuming there is one irq-level per
* host).
*/
while (--ticks_remaining >= )
mdelay( + / HZ);
host->resetting = ;
} // 在提交SCSI命令之前输出信息
scsi_log_send(cmd); /*
* Before we queue this command, check if the command length exceeds what the host adapter can handle.
* 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
*/
if (cmd->cmd_len > cmd->device->host->max_cmd_len) {
SCSI_LOG_MLQUEUE(,
printk("queuecommand : command too long. "
"cdb_size=%d host->max_cmd_len=%d\n",
cmd->cmd_len, cmd->device->host->max_cmd_len));
cmd->result = (DID_ABORT << ); scsi_done(cmd);
goto out;
} /* 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*/
if (unlikely(host->shost_state == SHOST_DEL)) {
cmd->result = (DID_NO_CONNECT << );
scsi_done(cmd);
} else {
trace_scsi_dispatch_cmd_start(cmd);
cmd->scsi_done = scsi_done;
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
} if (rtn) {
trace_scsi_dispatch_cmd_error(cmd, rtn);
if (rtn != SCSI_MLQUEUE_DEVICE_BUSY &&
rtn != SCSI_MLQUEUE_TARGET_BUSY)
rtn = SCSI_MLQUEUE_HOST_BUSY; scsi_queue_insert(cmd, rtn); SCSI_LOG_MLQUEUE(,
printk("queuecommand : request rejected\n"));
} out:
SCSI_LOG_MLQUEUE(, printk("leaving scsi_dispatch_cmnd()\n"));
return rtn;
}