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    SubjectRe: [PATCH] I/O space write barrier
    On Thu, 30 Sep 2004, Jeremy Higdon wrote:

    > Here's the qla1280 patch to go with Jesse's mmiowb patch.
    > Comments are verbose to satisfy a request from Mr. Bottomley.
    >
    > signed-off-by: Jeremy Higdon <jeremy@sgi.com>
    >
    > ===== drivers/scsi/qla1280.c 1.65 vs edited =====
    > --- 1.65/drivers/scsi/qla1280.c 2004-07-28 20:59:10 -07:00
    > +++ edited/drivers/scsi/qla1280.c 2004-09-29 23:43:30 -07:00
    > @@ -3397,8 +3397,22 @@
    > "qla1280_64bit_start_scsi: Wakeup RISC for pending command\n");
    > sp->flags |= SRB_SENT;
    > ha->actthreads++;
    > +
    > + /*
    > + * 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.
    > + */

    A pretty obvious note: instead of repeating this nice but pretty lengthy
    comment 3 times in the same file, wouldn't it be better to write at
    further locations something like

    /* Enforce IO-ordering. See comment in <function> for details. */

    Also helps if you later have to modify the comment, or move it, or add
    more mmiowb()s, or do some other modifications.

    Thanks
    Guennadi
    ---
    Guennadi Liakhovetski

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