Writing Device Drivers

Command Completion

When the host bus adapter driver is through with the command, the driver invokes the packet's completion callback routine. The driver then passes a pointer to the scsi_pkt(9S) structure as a parameter. After decoding the packet, the completion routine takes the appropriate action.

Example 17–5 presents a simple completion callback routine. This code checks for transport failures. In case of failure, the routine gives up rather than retrying the command. If the target is busy, extra code is required to resubmit the command at a later time.

If the command results in a check condition, the target driver needs to send a request sense command unless auto request sense has been enabled.

Otherwise, the command succeeded. At the end of processing for the command, the command destroys the packet and calls biodone(9F).

In the event of a transport error, such as a bus reset or parity problem, the target driver can resubmit the packet by using scsi_transport(9F). No values in the packet need to be changed prior to resubmitting.

The following example does not attempt to retry incomplete commands.

Note –

Normally, the target driver's callback function is called in interrupt context. Consequently, the callback function should never sleep.

Example 17–5 Completion Routine for a SCSI Driver

static void
xxcallback(struct scsi_pkt *pkt)
    struct buf        *bp;
    struct xxstate    *xsp;
    minor_t           instance;
    struct scsi_status *ssp;
     * Get a pointer to the buf(9S) structure for the command
     * and to the per-instance data structure.
    bp = (struct buf *)pkt->pkt_private;
    instance = getminor(bp->b_edev);
    xsp = ddi_get_soft_state(statep, instance);
     * Figure out why this callback routine was called
    if (pkt->pkt_reason != CMP_CMPLT) {
        bp->b_resid = bp->b_bcount;
        bioerror(bp, EIO);
        scsi_destroy_pkt(pkt);          /* Release resources */
        biodone(bp);                    /* Notify waiting threads */ ;
    } else {
         * Command completed, check status.
         * See scsi_status(9S)
        ssp = (struct scsi_status *)pkt->pkt_scbp;
        if (ssp->sts_busy) {
            /* error, target busy or reserved */
        } else if (ssp->sts_chk) {
            /* Send a request sense command. */
        } else {
            bp->b_resid = pkt->pkt_resid;  /* Packet completed OK */