/* * A word about residuals. * The scb is presented to the sequencer with the dataptr and datacnt * fields initialized to the contents of the first S/G element to * transfer. The sgptr field is initialized to the bus address for * the S/G element that follows the first in the in core S/G array * or'ed with the SG_FULL_RESID flag. Sgptr may point to an invalid * S/G entry for this transfer (single S/G element transfer with the * first elements address and length preloaded in the dataptr/datacnt * fields). If no transfer is to occur, sgptr is set to SG_LIST_NULL. * The SG_FULL_RESID flag ensures that the residual will be correctly * noted even if no data transfers occur. Once the data phase is entered, * the residual sgptr and datacnt are loaded from the sgptr and the * datacnt fields. After each S/G element's dataptr and length are * loaded into the hardware, the residual sgptr is advanced. After * each S/G element is expired, its datacnt field is checked to see * if the LAST_SEG flag is set. If so, SG_LIST_NULL is set in the * residual sg ptr and the transfer is considered complete. If the * sequencer determines that there is a residual in the transfer, or * there is non-zero status, it will set the SG_STATUS_VALID flag in * sgptr and DMA the scb back into host memory. To summarize: * * Sequencer: * o A residual has occurred if SG_FULL_RESID is set in sgptr, * or residual_sgptr does not have SG_LIST_NULL set. * * o We are transfering the last segment if residual_datacnt has * the SG_LAST_SEG flag set. * * Host: * o A residual can only have occurred if a completed scb has the * SG_STATUS_VALID flag set. Inspection of the SCSI status field, * the residual_datacnt, and the residual_sgptr field will tell * for sure. * * o residual_sgptr and sgptr refer to the "next" sg entry * and so may point beyond the last valid sg entry for the * transfer. */