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Date:      Wed, 27 Jun 2012 07:47:37 -0400
From:      John Baldwin <jhb@freebsd.org>
To:        Marius Strobl <marius@alchemy.franken.de>
Cc:        svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org
Subject:   Re: svn commit: r237008 - head/sys/dev/pci
Message-ID:  <201206270747.37859.jhb@freebsd.org>
In-Reply-To: <20120626211445.GB63893@alchemy.franken.de>
References:  <201206131504.q5DF4opt031336@svn.freebsd.org> <201206251424.24621.jhb@freebsd.org> <20120626211445.GB63893@alchemy.franken.de>

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On Tuesday, June 26, 2012 5:14:45 pm Marius Strobl wrote:
> Yes, it does but switching to positive decoding nevertheless is
> problematic in this case. According to the PCI 2.3 specification,
> a child wanting its addresses to be decoded subtractively needs
> to provide special DEVSEL# behavior. It seems that it should work
> for a bridge to actually positively decoding such transactions,
> but I'm not familiar with the physical layer of PCI and thus not
> sure. On the other hand, both the PCI-ISA and PCI-PCI bridges
> in question are ALi/ULi crap and based on fun I had with their
> chips in the past, I also wouldn't be surprised their again being
> a magic bit in the configuration space needing to be set to make
> this work. Unfortunately, I don't have a datasheet for this
> M5249 PCI-PCI bridge. If we'd implement a tunable for changing
> the decoding of subtractive bridges, the way to go would seem to
> be to have it off by default in order to preserve pre-NEW_PCIB/
> r237008 as at this time it's unknown how many bridges are
> affected. If you'd want to keep the new/current behavior by
> default, introducing something like a PCIB_SUBTRACTIVE_FORCED
> flag along a PCI_QUIRK_SUB_FORCED quirk in addition to a
> tuneable defaulting to on probably would be a good idea.

Hmm.  I'm fine with your original change going in then.  I am not really sure 
what the right way to handle this is, and just leaving the windows alone on 
subtractive bridges is probably safest.

-- 
John Baldwin



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