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Message-ID: <4CD7E925.6020608@ladisch.de> Date: Mon, 08 Nov 2010 13:12:21 +0100 From: Clemens Ladisch <clemens@...isch.de> To: Nico <segfault2@...mail.com> CC: linux-kernel <linux-kernel@...r.kernel.org> Subject: Re: AGP Aperture question Nico wrote: > 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 This message is a little bit too optimistic. I don't know of any motherboard not specifically designed for Linux that has such an option or that just silently reserves address space for the IOMMU. > So, is the above something to be concerned about? Does it have any > negative impact on the system? There are three options for using the GART as an IOMMU: 1) Put the GART into some free address space below 4 GB. 2) Put the GART over some memory below 4 GB. 3) Don't use the GART. The first one would be optimal, but requires that there is some free address space below 4 GB. The third one makes the computer slower because any DMA from/to memory above the 4 GB limit requires copying buffers to/from low memory. Option 2) wastes 64 MB of memory, compared to 1), but is very much superior to 3). (AFAIK Windows uses 3), and this is why no BIOS bothers to set this.) Regards, Clemens -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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