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    SubjectRe: [PATCH] queue barrier support
    On Wed, Feb 13 2002, James Bottomley wrote:
    > said:
    > > ChangeSet@1.297, 2002-02-13 13:42:39+01:00,
    > > Add support for SCSI drivers to indicate support for ordered tags
    > >
    > > torvalds-2002020517305 \ 6-16047-c1d11a41ed024864/cset@
    > > index.html|ChangeSet@-1h
    > > ChangeSet@1.298, 2002-02-13 13:43:04+01:00,
    > > Add ordered tag support to the aic7xxx scsi driver
    > The rest of the aic7xxx code uses MSG_ORDERED_TASK rather than
    > MSG_ORDERED_Q_TAG. You have to scan through the headers to see that
    > these are #defined the same.

    Ah, the MSG_ORDERED_TASK did probably not exist when I originally did
    the patch (a year or so ago, iirc). I'll change the define to follow the
    rest of the code.

    > A problem (that is probably only an issue for older drives) is that
    > while technically the standard requires all 3 types of TAG to be
    > supported if tag queueing is, some drives really only have simple tag
    > support in their firmware, so you may need to add a blacklist for
    > ordered tags on certain drives.

    Yes you are right.

    > A further issue is that you haven't added anything to the error
    > recovery code for this. If error recovery is activated for the device
    > at the reset level, all tags will be discarded by the device. The eh
    > will retry the failing command and then the other tagged commands will
    > be re-issued from the scsi_bottom_half_handler (assuming the low level
    > device driver immediately fails them with DID_RESET) in the order in
    > which the low level driver failed them. Thus you have potentially
    > completely messed up the ordering when the commands all get retried.

    I already have this fixed locally by just maintaining a fifo list of
    queued commands so we can retry them in the correct order. For 2.5
    there's a busy and free list (so no more scanning for a free command
    either), I would imagine that the easiest for 2.4 is to just maintain a
    busy list in addition to the current array of pending/free commands.

    Jens Axboe

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