Date: Wed, 7 Jul 1999 10:40:45 -0600 (MDT) From: "Justin T. Gibbs" <gibbs@narnia.plutotech.com> To: Karl-Heinz Herrmann <k.-h.herrmann@fz-juelich.de> Cc: aic7xxx@FreeBSD.org Subject: Re: multi scsi questions Message-ID: <199907071640.KAA00893@narnia.plutotech.com> In-Reply-To: <XFMail.990707182935.k.-h.herrmann@fz-juelich.de>
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Confussion abounds... > Sorry, I don't get the complete picture. > > 1) Do you want to put the two controllers on *one* SCSI-Bus? > 2) Or do you wan't to use both cards, having two separate SCSI-Buses with > their respective own devices? > > In Case 1) I don't think this is working just like that yet (And why do you > want it anyway?) Multi-initiator environments are becoming more and more common these days. This has been supported in FreeBSD (and just about any comercial UNIX) for some time now. I would bet that Linux already supports it or could be easily modified to do so. > Only Widecontroller have ID's above 7, and only wide devices can use ID's > above 7. You can't use a non-wide device on ID's like 11. > > And it seems[1], that non-wide devices "see" a call to a wide device with > ID 9 on the same SCSI-Bus as a call to ID 2 -- If there is a non-wide > device with ID 2 it will/can get confused. This is not the case. SCSI uses the data bus lines as a bitmask of device ids wishing to access the bus. The narrow devices cannot see the high byte, so cannot see that the devices with IDs above 7 are interrested in the bus. What the narrow devices can see is that the bus is 'busy' and that 'someone' has won arbitration. To get around this problem, parallel SCSI (without the fair arbitration extension of SPI-3) uses a prioritized arbitration scheme based on the ID of the requester. The priority, from highest to lowest, is as follows: 7->0, 15->8. When a device with an ID lower than 8 arbitrates along with a device with an ID greater than 7, the device with the high ID knows that it has lost the arbitration and will get off the bus. The device with the low device ID doesn't need to see the high byte for this to work. -- Justin To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe aic7xxx" in the body of the message
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