linux-kernel - Re: [PATCH 2.6.29-rc7 2/2] x86: disallow DAC for MCP51 PCI bridge

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Date:	Wed, 04 Mar 2009 22:36:38 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	pgnet.trash@...il.com
Subject: Re: [PATCH 2.6.29-rc7 2/2] x86: disallow DAC for MCP51 PCI bridge

Alan Cox wrote:
>> x86 is blacklisting DAC for everything VIA. :-)
> 
> I'd assume that comes from published docs given VIA actually publish
> stuff if you ask nicely.

Dunno the history but it's from the time before VIA begins publishing.
It's probably worthwhile to refine the blacklisting.

>> * MCP51 is a very old chipset at this point (it's circa 2005).  Not
>>   many machine would be running with >4GB memory to begin with.
>>
>> * Given the above and scarcity of DAC on most end user machines
>>   (nothing on MCP51 does DAC by default), lack of reports isn't too
>>   surprising.
>>
>> * The board doesn't have a 64bit connector.  It can't be dodgy
>>   connector and sil24 is known to behave well with DAC.  The failure
>>   being specific to the particular machine doesn't seem likely.
>>
>> I'll ping nvidia about it but I think your bar is too high.
> 
> I don't think wanting to see *two* examples is a high bar.

I don't think it's an easy combination.  Circa 2005 desktop machine
with >4GB ram + DAC capable controller running Linux with owner who
will report seemingly random data corruption which will end up in the
correct hands and given that non-working DAC isn't too surprising of
the machines of that time and isn't very likely caused by random
component or connection failure, I think we'll be better off just
blacklisting it.  That said, I admit I'm more trigger happy with
blacklists than some people (I think it's generally better to have
working systems than chasing performance on not-so-common cases).

Anyways, I pinged Peer Chen on the issue.  Let's see whether he can
confirm it.

Thanks.

-- 
tejun
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