RDMA/srpt: Report the SCSI residual to the initiator

The code added by this patch is similar to the code that already exists in
ibmvscsis_determine_resid(). This patch has been tested by running the
following command:

strace sg_raw -r 1k /dev/sdb 12 00 00 00 60 00 -o inquiry.bin |&
    grep resid=

Link: https://lore.kernel.org/r/20191105214632.183302-1-bvanassche@acm.org
Fixes: a42d985b ("ib_srpt: Initial SRP Target merge for v3.3-rc1")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Honggang Li <honli@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/ulp/srpt/ib_srpt.c

@@ -1362,9 +1362,11 @@ static int srpt_build_cmd_rsp(struct srpt_rdma_ch *ch,
 			      struct srpt_send_ioctx *ioctx, u64 tag,
 			      int status)
 {
+	struct se_cmd *cmd = &ioctx->cmd;
 	struct srp_rsp *srp_rsp;
 	const u8 *sense_data;
 	int sense_data_len, max_sense_len;
+	u32 resid = cmd->residual_count;
 
 	/*
 	 * The lowest bit of all SAM-3 status codes is zero (see also
@@ -1386,6 +1388,28 @@ static int srpt_build_cmd_rsp(struct srpt_rdma_ch *ch,
 	srp_rsp->tag = tag;
 	srp_rsp->status = status;
 
+	if (cmd->se_cmd_flags & SCF_UNDERFLOW_BIT) {
+		if (cmd->data_direction == DMA_TO_DEVICE) {
+			/* residual data from an underflow write */
+			srp_rsp->flags = SRP_RSP_FLAG_DOUNDER;
+			srp_rsp->data_out_res_cnt = cpu_to_be32(resid);
+		} else if (cmd->data_direction == DMA_FROM_DEVICE) {
+			/* residual data from an underflow read */
+			srp_rsp->flags = SRP_RSP_FLAG_DIUNDER;
+			srp_rsp->data_in_res_cnt = cpu_to_be32(resid);
+		}
+	} else if (cmd->se_cmd_flags & SCF_OVERFLOW_BIT) {
+		if (cmd->data_direction == DMA_TO_DEVICE) {
+			/* residual data from an overflow write */
+			srp_rsp->flags = SRP_RSP_FLAG_DOOVER;
+			srp_rsp->data_out_res_cnt = cpu_to_be32(resid);
+		} else if (cmd->data_direction == DMA_FROM_DEVICE) {
+			/* residual data from an overflow read */
+			srp_rsp->flags = SRP_RSP_FLAG_DIOVER;
+			srp_rsp->data_in_res_cnt = cpu_to_be32(resid);
+		}
+	}
+
 	if (sense_data_len) {
 		BUILD_BUG_ON(MIN_MAX_RSP_SIZE <= sizeof(*srp_rsp));
 		max_sense_len = ch->max_ti_iu_len - sizeof(*srp_rsp);