linux-kernel - Re: [PATCH] quiet down swiotlb warnings

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Date:	Fri, 1 Jun 2007 22:26:01 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Andrew Vasquez <andrew.vasquez@...gic.com>,
	Christoph Hellwig <hch@...radead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] quiet down swiotlb warnings

On Fri, Jun 01, 2007 at 04:20:38PM -0400, Rik van Riel wrote:
> Andi Kleen wrote:
> >On Fri, Jun 01, 2007 at 01:00:49PM -0700, Andrew Vasquez wrote:
> >>yes, the card can support 64bit DMA transfers. but in this case the
> >>'required' DMA mask returned from dma_get_required_mask() states that a
> >>32bit mask would suffice.
> >
> >That's a bug in the kernel then that needs to be fixed. Find out
> >why it does that and change it. Don't fix the symptoms, fix the root cause.
> 
> The system in question only has 2GB of memory.
> 
> Why would it need to do 64 bit DMA?

It doesn't. But it definitely doesn't need swiotlb either.

Normally swiotlb doesn't even try to bounce when dma mask is <= end_pfn
so something must be very wrong in your kernel. It definitely isn't
a mainline kernel. If this happens in Xen then Xen just needs fixing --
it should not try to bounce when the normal kernel wouldn't.

-Andi
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