Date: 12 Oct 1999 14:23:43 +0000 From: Randell Jesup <rjesup@wgate.com> To: "Justin T. Gibbs" <gibbs@narnia.plutotech.com> Cc: Gerard Roudier <groudier@club-internet.fr>, scsi@FreeBSD.ORG Subject: Re: Driver for GDT6517RD RAID controller Message-ID: <ybubta4zmhc.fsf@jesup.eng.tvol.net.jesup.eng.tvol.net> In-Reply-To: "Justin T. Gibbs"'s message of "Tue, 12 Oct 1999 10:42:52 -0600 (MDT)" References: <199910121642.KAA34505@narnia.plutotech.com>
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"Justin T. Gibbs" <gibbs@narnia.plutotech.com> writes:
>> You must distinguish between phase cognizant target mode and host target
>> mode. What I intend to implement in the future is something that avoids
>> stalling the phase engine as possible, thus host target mode. Note that if
>I agree phase cognizant isn't that interesting. The aic7xxx driver works
>in the fashion you describe.
Phase cognizant is mostly useful for making
<random-device>-workalikes, and (primarily) for testing the initiator's
SCSI implementation (by feeding it weird shit). I have no real interest in
either at this point.
>> As seen from SIM the handling of the target mode is in fact much simpler
>> tham initiator mode, since the part that acts as a target has the baton in
>> SCSI. So, a SIM that will only implement target mode should be
>> significantly simpler than common ones that only implement initiator mode
>> (given appropriate hardware).
>Target mode isn't difficult, but I would refrain from calling it simpler
>than the initiator role. There are several little gotchas mostly having
>to do with resource deadlocks when running in both roles at the same time
>and the initiator tells you that you cannot disconnect.
Well, you could always punt and return an error in that case
(though it's very not-nice to do so). I assume the problem becomes
something like: command comes in with disconnect-disabled. You need
to page (or otherwise access a disk) to fufill the request. The disk
you need to access is on the same bus. Ooops. I forget if this was
explicitly considered in the CAM subcommittee discussions, or if it was
just felt that this was something that could not be dealt with. I wouldn't
be surprised if we thought about it and agreed that it was not solvable,
and that an error return was perfectly reasonable, and that in this
sort of situation reselect-enabled was VERY strongly advised.
I'm going to dig out my old mail archives from the Section 10
(now renumbered) subcommittee. (A lot happened over conference calls too;
we never met in person.)
Thanks for the info.
--
Randell Jesup, Worldgate Communications, ex-Scala, ex-Amiga OS team ('88-94)
rjesup@wgate.com
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