Re: why Target cannot change its mind

[Neal Palmer <neal@dinigroup.com>]

Dude, just follow the spec.  There are already enough broken devices on
the planet.  Don't make another one because you are too lazy to think
through a small state machine.

On Fri, 13 Apr 2001, Weng wrote:

> Hi Brian,
> I disagree with your claim:
> " I think the harm is in the complexity of logic needed in the master to
> support TRDY changing state when the master is inserting a wait state."
>
> First of all, I emphasize the following assertion:
> "In other words, when Either Master or Target is inserting wait cycles, NOT
> FOR LAST DATA (FRAME# = '0'), the opposite partner can change its state
> (IRDY# or TRDY#) without any harm if no accompanying STOP# change !!! That's
> all."
>
> My above assertion is more relax than PCI specs at least in paper. In designing implementation, it would be much easier to design.
>
> I don't want to challenge the specs, nor to violate it in my design. I am just curious of trying to know why PCI specify a tougher limit on
> both Master and Target on data transfer.
>
> I cannot imagine what damage will happen when one side says wait a minute, another side says, hay, if you want a wait, I want it too, let make a deal 1 clock or 2 later.
>
> Weng Tianxiang
>
> wtx@umem.com
> wengtianxiang@yahoo.com
>
> Micro Memory Inc.
> 9540 Vassar Avenue
> Chartsworth, CA 91311
> Tel: 818-998-0070
> Fax: 818-998-4459
>
>
>
>
> ----- Original Message -----
> From: Brian Small <small@quicklogic.com>
> To: <wtx@umem.com>; <pci-sig@znyx.com>
> Sent: Friday, April 13, 2001 2:55 PM
> Subject: RE: Why cannot Target change its mind
>
>
> > Weng,
> >
> > I think the harm is in the complexity of logic needed in the master to
> > support TRDY changing state when the master is inserting a wait state.
> >
> > In any case, it certainly would break a great deal of PCI masters if
> targets
> > began to behave in this way.  This is because all previous masters were
> > designed to the current spec, which assumes that once TRDY is low, TRDY
> will
> > stay low until the master completes the data cycle by making IRDY low.
> >
> > - Brian Small
> >
> > -----Original Message-----
> > From: Weng [mailto:wtx@umem.com]
> > Sent: Friday, April 13, 2001 2:34 PM
> > To: pci-sig@znyx.com
> > Subject: Re: Why cannot Target change its mind
> >
> >
> >
> > Hi,
> > I disagree with your following opinions:
> > 1. "Anyway, such behaviour by the target might lead to bus cycles wastage
> > or,
> > for example, let the master (some kind of bridge for example) prefetch
> > data and throw them away..."
> > 2. "In my opinion you misread the specs here. The assertion of IRDY# must
> > also be considered as a commitment for a data phase by the master as seen
> by
> > the target. In this situation, catastroph may happen if the master wants
> > to read to non-prefetchable and then changes its mind, since the target
> > may have read the data (with side effect) and then will not be able to
> > deliver it to the master."
> >
> > I don't see any harm will happen: if FRAME# = '0' and IRDY# changes while
> > TRDY# = '1' and insists FRAME# = '0'. It only change clock time on which
> > clock to transfer data, not whether or not the data will be transfered.
> >
> > And I don't see any harm will happen if FRAME# = '0' and IRDY# = '1' and
> > TRDY# changes its state
> >
> > In other words, when Either Master or Target is inserting wait cycles, NOT
> > FOR LAST DATA (FRAME# = '0'), the opposite partner can change its state
> > (IRDY# or TRDY#) without any harm!!! That's all.
> >
> > When entering last data cycle, FRAME# = '1' and IRDY# = '0', there is no
> > room for either Master or Target to change mind.
> >
> > Weng Tianxiang
> >
> > wtx@umem.com
> > wengtianxiang@yahoo.com
> >
> > Micro Memory Inc.
> > 9540 Vassar Avenue
> > Chartsworth, CA 91311
> > Tel: 818-998-0070
> > Fax: 818-998-4459
> >
> > ----- Original Message -----
> > From: Gérard Roudier <groudier@club-internet.fr>
> > To: Weng <wtx@umem.com>
> > Cc: <pci-sig@znyx.com>
> > Sent: Friday, April 13, 2001 10:42 AM
> > Subject: Re: Why cannot Target change its mind
> >
> >
> >
> >
> > On Fri, 13 Apr 2001, Weng wrote:
> >
> > > Hi,
> > > PCI 2.2 p.53: Once a target has asserted TRDY# or STOP#, it cannot
> change
> > DEVSEL#, TRDY#, or STOP# until the current data phase completes.
> > >
> > >  I have two questions about the above definition.
> > >
> > > The reason that DEVSEL# and STOP# cannot be changed is clear, I don't
> see
> > any merit why TRDY# cannot be changed if Target found Master is asserting
> > wait cycles and it wants to change mind too.
> > >
> > > 1. On what senerio the changing TRDY# signal will have permanent damage
> to
> > any transactions?
> >
> > I imagine that this allows the other part (the master) to anticipate the
> > data phase, if such anticipation can optimize the data transfer, and avoid
> > side effects without too much complexity.
> > Anyway, such behaviour by the target might lead to bus cycles wastage or,
> > for example, let the master (some kind of bridge for example) prefetch
> > data and throw them away...
> >
> > > When I have been designing Master module, I don't see any difference in
> > design whether or not PCI-Target is changing its mind. The only logic that
> > go into my design is IRDY# = '0' and (TRDY# = '0' or STOP# = '0'), that
> > means only when both Master and Target agree with a transaction, the deal
> is
> > made, otherwise wait until the above conditions happen.
> > >
> > > 2. While not allowing Target to change its mind, why there is no
> > symmetrical limit on Master side, that is, on IRDY#?
> >
> > In my opinion you misread the specs here. The assertion of IRDY# must also
> > be considered as a commitment for a data phase by the master as seen by
> > the target. In this situation, catastroph may happen if the master wants
> > to read to non-prefetchable and then changes its mind, since the target
> > may have read the data (with side effect) and then will not be able to
> > deliver it to the master.
> >
> > > My opinion is: (a relaxed condition that is good enough to match
> original
> > definition)
> > > Once a target has asserted DEVSEL# or STOP#, it must be asserted until a
> > transaction is finished. Data is transfered only on conditions that IRDY#
> =
> > '0' and TRDY# = '0'. Target can change TRDY# state if no data is
> transfered.
> >
> > You should forget about such suggestion, in my opinion.
> >
> > > Weng Tianxiang
> > >
> > > wtx@umem.com
> > > wengtianxiang@yahoo.com
> > >
> > > Micro Memory Inc.
> > > 9540 Vassar Avenue
> > > Chartsworth, CA 91311
> > > Tel: 818-998-0070
> > > Fax: 818-998-4459
> >
> >   Gérard.
> >
> >
>
>

-- 
-- Neal Palmer

The Dini Group
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La Jolla, CA 92037
(858) 454-3419 x16
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