Writing Device Drivers

tran_start() Entry Point

The tran_start(9E) entry point for a SCSI HBA driver is called to transport a SCSI command to the addressed target. The SCSI command is described entirely within the scsi_pkt(9S) structure, which the target driver allocated through the HBA driver's tran_init_pkt(9E) entry point. If the command involves a data transfer, DMA resources must also have been allocated for the scsi_pkt(9S) structure.

The tran_start() entry point is called when a target driver calls scsi_transport(9F).

tran_start() should perform basic error checking along with any initialization that is required by the command. The FLAG_NOINTR flag in the pkt_flags field of the scsi_pkt(9S) structure can affect the behavior of tran_start(). If FLAG_NOINTR is not set, tran_start() must queue the command for execution on the hardware and return immediately. Upon completion of the command, the HBA driver should call the pkt completion routine.

If the FLAG_NOINTR is set, then the HBA driver should not call the pkt completion routine.

The following example demonstrates how to handle the tran_start(9E) entry point. The ISP hardware provides a queue per-target device. For devices that can manage only one active outstanding command, the driver is typically required to manage a per-target queue. The driver then starts up a new command upon completion of the current command in a round-robin fashion.

Example 18–8 HBA Driver tran_start(9E) Entry Point

static int
    struct scsi_address    *ap,
    struct scsi_pkt        *pkt)
    struct isp_cmd         *sp;
    struct isp             *isp;
    struct isp_request     *req;
    u_long                 cur_lbolt;
    int                    xfercount;
    int                    rval = TRAN_ACCEPT;
    int                    i;

    sp = (struct isp_cmd *)pkt->pkt_ha_private;
    isp = (struct isp *)ap->a_hba_tran->tran_hba_private;

    sp->cmd_flags = (sp->cmd_flags & ~CFLAG_TRANFLAG) |
    pkt->pkt_reason = CMD_CMPLT;
    * set up request in cmd_isp_request area so it is ready to
    * go once we have the request mutex
    req = &sp->cmd_isp_request;

    req->req_header.cq_entry_type = CQ_TYPE_REQUEST;
    req->req_header.cq_entry_count = 1;
    req->req_header.cq_flags        = 0;
    req->req_header.cq_seqno = 0;
    req->req_reserved = 0;
    req->req_token = (opaque_t)sp;
    req->req_target = TGT(sp);
    req->req_lun_trn = LUN(sp);
    req->req_time = pkt->pkt_time;
    ISP_SET_PKT_FLAGS(pkt->pkt_flags, req->req_flags);
    * Set up data segments for dma transfers.
    if (sp->cmd_flags & CFLAG_DMAVALID) {

        if (sp->cmd_flags & CFLAG_CMDIOPB) {
            (void) ddi_dma_sync(sp->cmd_dmahandle,
            sp->cmd_dma_offset, sp->cmd_dma_len,

        ASSERT(sp->cmd_cookiecnt > 0 &&
            sp->cmd_cookiecnt <= ISP_NDATASEGS);

        xfercount = 0;
        req->req_seg_count = sp->cmd_cookiecnt;
        for (i = 0; i < sp->cmd_cookiecnt; i++) {
            req->req_dataseg[i].d_count =
            req->req_dataseg[i].d_base =
            xfercount +=

        for (; i < ISP_NDATASEGS; i++) {
            req->req_dataseg[i].d_count = 0;
            req->req_dataseg[i].d_base = 0;

        pkt->pkt_resid = xfercount;

        if (sp->cmd_flags & CFLAG_DMASEND) {
            req->req_flags |= ISP_REQ_FLAG_DATA_WRITE;
        } else {
            req->req_flags |= ISP_REQ_FLAG_DATA_READ;
    } else {
        req->req_seg_count = 0;
        req->req_dataseg[0].d_count = 0;
    * Set up cdb in the request
    req->req_cdblen = sp->cmd_cdblen;
    bcopy((caddr_t)pkt->pkt_cdbp, (caddr_t)req->req_cdb,
    * Start the cmd.  If NO_INTR, must poll for cmd completion.
    if ((pkt->pkt_flags & FLAG_NOINTR) == 0) {
        rval = isp_i_start_cmd(isp, sp);
    } else {
        rval = isp_i_polled_cmd_start(isp, sp);
    return (rval);