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    SubjectRe: PATCH: Different SCSI ordering for WIDE SCSI chains.
    Rolf Fokkens wrote:
    >... For backward compatibility you can connect non-wide
    > devices on a wide-chain too, but their ID's range from 0 to 7.
    > Furhermore I heard that non-wide take actually two ID's when connected
    > on a WIDE chain: the configured ID and that ID+8. They can be addressed
    > by their own ID, but the other ID is lost.
    >
    >... I changed the scanning order for wide scsi
    > devices in the kernel. I changed it to:
    > 0,8,1,9,2,10,3,11,4,12,5,13,6,14,7,15. This means that wide devices are
    > always checked in pairs. Now I can use both ID 1 and 9 for my external
    > wide devices and still connect an external non-wide devices.
    >

    When mixing narrow and wide devices you must only use the first eight
    scsi ids. If you don't, scsi arbitration will not work correctly. The
    scsi bus uses the data bits to arbitrate for the bus. A narrow device
    does not see the upper eight arbitration bits and can fowl things up.
    Your wide devices can still work in wide mode, but their addresses
    should be set between 0 and 7 where the narrow devices can see them for
    arbitration.

    Other thoughts:
    The Scsi arbitration priority is ordered as follows:
    7,6,5,4,3,2,1,0,15,14,13,12,11,10,9,8, where 7 is granted the bus first.
    This is one reason why many host adapter's id are set to 7. The other
    reason is for a typical 0,1,... scan order. It might make sense to scan
    from 7,6..0,15,14,..8 to make the more important devices first in the
    device list.

    <>< Lance.

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