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Date:      Thu, 18 Sep 2003 18:45:38 +1000
From:      Peter Jeremy <PeterJeremy@optushome.com.au>
To:        Andrew Gallatin <gallatin@cs.duke.edu>
Cc:        freebsd-hackers@freebsd.org
Subject:   Re: PCI interrupts passing DMA
Message-ID:  <20030918084538.GB19197@cirb503493.alcatel.com.au>
In-Reply-To: <16232.56412.745069.248974@grasshopper.cs.duke.edu>
References:  <16232.56412.745069.248974@grasshopper.cs.duke.edu>

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On Wed, Sep 17, 2003 at 06:12:44PM -0400, Andrew Gallatin wrote:
>My question is: What the heck could the SMP kernel be doing which
>causes the DMA to "complete" faster?   

My guess is that this is a coherency issue rather than a timing issue.
The SMP kernels are far more careful about ensuring consistency
between CPUs and this could be fixing/masking the problem

That said, any decent disk controller uses DMA and if the UP kernel
really did have PCI-bus coherency problems (or massive delays), a
significant part of the user-base would be screaming.

The other possibility: Is your PCI card doing something wierd?  Are
you bending one of the PCI specs a bit far?

Peter



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