Messages in this thread | | | Date | Mon, 5 Jan 2004 19:40:11 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: PCI memory allocation bug with CONFIG_HIGHMEM |
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On Tue, 6 Jan 2004, Andi Kleen wrote: > > IMHO the only reliable way to get physical bus space for mappings > is to allocate some memory and map the mapping over that.
You literally can't do that: the RAM addresses are decoded by the northbridge before they ever hit the PCI bus, so it's impossible to "map over" RAM in general.
Normally, the way this works is that there are magic northbridge mapping registers that remap part of the memory, so that the memory that is physically in the upper 4GB of RAM shows up somewhere else (or just possibly disappears entirely - once you have more than 4GB of RAM, you might not care too much about a few tens of megs missing).
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