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Subject[PATCH] use mmiowb in qla1280.c
Date
There are a few spots in qla1280.c that don't need a full PCI write flush to 
the device, but rather a simple write ordering guarantee. This patch changes
some of the PIO reads that cause write flushes into mmiowb calls instead,
which is a lighter weight way of ensuring ordering.

Jes and James, can you ack this and/or push it in via the SCSI BK tree?

Thanks,
Jesse

Signed-off-by: Jeremy Higdon <jeremy@sgi.com>
Signed-off-by: Jesse Barnes <jbarnes@sgi.com>


===== drivers/scsi/qla1280.c 1.69 vs edited =====
--- 1.69/drivers/scsi/qla1280.c 2004-10-20 06:46:21 -07:00
+++ edited/drivers/scsi/qla1280.c 2004-10-21 16:06:11 -07:00
@@ -3400,7 +3400,8 @@
sp->flags |= SRB_SENT;
ha->actthreads++;
WRT_REG_WORD(&reg->mailbox4, ha->req_ring_index);
- (void) RD_REG_WORD(&reg->mailbox4); /* PCI posted write flush */
+ /* Enforce mmio write ordering; see comment in qla1280_isp_cmd(). */
+ mmiowb();

out:
if (status)
@@ -3668,7 +3669,8 @@
sp->flags |= SRB_SENT;
ha->actthreads++;
WRT_REG_WORD(&reg->mailbox4, ha->req_ring_index);
- (void) RD_REG_WORD(&reg->mailbox4); /* PCI posted write flush */
+ /* Enforce mmio write ordering; see comment in qla1280_isp_cmd(). */
+ mmiowb();

out:
if (status)
@@ -3778,9 +3780,21 @@
} else
ha->request_ring_ptr++;

- /* Set chip new ring index. */
+ /*
+ * Update request index to mailbox4 (Request Queue In).
+ * The mmiowb() ensures that this write is ordered with writes by other
+ * CPUs. Without the mmiowb(), it is possible for the following:
+ * CPUA posts write of index 5 to mailbox4
+ * CPUA releases host lock
+ * CPUB acquires host lock
+ * CPUB posts write of index 6 to mailbox4
+ * On PCI bus, order reverses and write of 6 posts, then index 5,
+ * causing chip to issue full queue of stale commands
+ * The mmiowb() prevents future writes from crossing the barrier.
+ * See Documentation/DocBook/deviceiobook.tmpl for more information.
+ */
WRT_REG_WORD(&reg->mailbox4, ha->req_ring_index);
- (void) RD_REG_WORD(&reg->mailbox4); /* PCI posted write flush */
+ mmiowb();

LEAVE("qla1280_isp_cmd");
}
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