Change maxio to reflect variable hardware configurations.

If max_sg_length is 0, then we default to 16

If max_sg_length is less than CISS_MAX_SG_ELEMENTS, then
we will set round the value of max_sg_length to the nearest
power of 2 and use it to align maxio.

Else, we will use CISS_MAX_SG_ELEMENTS for our calculations.

Thanks to scottl for working me through the history and providing
the basis for this patch.

Submitted by:	scott
Obtained from:	Yahoo! Inc.
MFC after:	2 weeks

1 files changed, 17 insertions, 1 deletions

diff --git a/sys/dev/ciss/ciss.c b/sys/dev/ciss/ciss.c
index d4c3583..1dcf88b 100644
--- a/sys/dev/ciss/ciss.c
+++ b/sys/dev/ciss/ciss.c
@@ -2985,6 +2985,7 @@ ciss_cam_action(struct cam_sim *sim, union ccb *ccb)
     case XPT_PATH_INQ:
     {
 	struct ccb_pathinq	*cpi = &ccb->cpi;
+	int			sg_length;
 
 	debug(1, "XPT_PATH_INQ %d:%d:%d", cam_sim_bus(sim), ccb->ccb_h.target_id, ccb->ccb_h.target_lun);
 
@@ -3005,7 +3006,22 @@ ciss_cam_action(struct cam_sim *sim, union ccb *ccb)
 	cpi->transport_version = 2;
 	cpi->protocol = PROTO_SCSI;
 	cpi->protocol_version = SCSI_REV_2;
-	cpi->maxio = (min(CISS_MAX_SG_ELEMENTS - 1, sc->ciss_cfg->max_sg_length)) * PAGE_SIZE;
+	if (sc->ciss_cfg->max_sg_length == 0) {
+		sg_length = 16;
+	} else {
+	/* XXX Fix for ZMR cards that advertise max_sg_length == 32
+	 * Confusing bit here. max_sg_length is usually a power of 2. We always
+	 * need to subtract 1 to account for partial pages. Then we need to 
+	 * align on a valid PAGE_SIZE so we round down to the nearest power of 2. 
+	 * Add 1 so we can then subtract it out in the assignment to maxio.
+	 * The reason for all these shenanigans is to create a maxio value that
+	 * creates IO operations to volumes that yield consistent operations
+	 * with good performance.
+	 */
+		sg_length = sc->ciss_cfg->max_sg_length - 1;
+		sg_length = (1 << (fls(sg_length) - 1)) + 1;
+	}
+	cpi->maxio = (min(CISS_MAX_SG_ELEMENTS, sg_length) - 1) * PAGE_SIZE;
 	ccb->ccb_h.status = CAM_REQ_CMP;
 	break;
     }