Re: [Xen-devel] [PATCH] xen: arm: load FDT below 0.5G

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Wed, 2013-07-24 at 14:44 +0100, Julien Grall wrote:
> On 07/23/2013 06:12 PM, Ian Campbell wrote:
> > From: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > 
> > The 32-bit Linux kernel uses its lowmem direct mapping to access the FDT. 
> > The
> > lowmem mapping is around 3/4GiB but varies depending on the kernel's 
> > .config.
> > Use a limit of 1/2G to be safe.
> > 
> > Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> 
> I'm curious, how did you find this value? I can't easily find a specific
> range for the lowmem direct mapping.

There isn't one...

With a standard 3:1 user:kernel split there is ~1G of kernel address
space, but some of that is taken up by vmalloc areas, fixmap areas etc.
My experience is that there is generally around 700-800M of actual 1:1
mapping of RAM, but the precise value depends on the kernel .config
(e.g. how many fixmaps are implied by the settings etc). 512MB is
therefore just a conservative guess...

BTW I asked for confirmation on linux-arm-kernel but I don't see that in
my l-a-k folder or the archives -- did you get the copy I CCd to you?
Subject is "Requirements for FDT load address on ARM".

Ian.


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