linux-kernel - Re: AGP aperture beyond 4GB not valid?

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Message-ID: <48A278AE.1070800@zytor.com>
Date:	Tue, 12 Aug 2008 23:01:18 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Chris Adams <cmadams@...aay.net>
CC:	linux-kernel@...r.kernel.org
Subject: Re: AGP aperture beyond 4GB not valid?

Chris Adams wrote:
> 
> Checking aperture...
> Node 0: aperture @ 1006000000 size 32 MB
> Aperture beyond 4GB. Ignoring.
> No AGP bridge found
> Your BIOS doesn't leave a aperture memory hole
> Please enable the IOMMU option in the BIOS setup
> This costs you 64 MB of RAM
> Mapping aperture over 65536 KB of RAM @ c000000
> PM: Registered nosave memory: 000000000c000000 - 0000000010000000
> 
> 
> My BIOS doesn't have an IOMMU option (latest BIOS available for the
> mboard).  However, it appears that an aperture is allocated but the
> kernel then ignores it due to where it is placed.  Why?
> 

You can't use it as an IOMMU to handle 32-bit devices, since it's above 
the 32-bit addressible region.  Hence the kernel doesn't use it.

	-hpa
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