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SubjectRe: consequences of lowering "MAX_LOW_MEM"?
On Wed, 4 Sep 2002, Matt Porter wrote:

> On Wed, Sep 04, 2002 at 08:32:15AM -0700, Martin J. Bligh wrote:
> >
> > >> In 2.4.x (currently using 2.4.18), for PPC, there is a value for
> > >> "MAX_LOW_MEM" defined in "arch/ppc/mm/pgtable.c" as 768MB RAM. Any
> > >> memory above 768MB is considered "high" memory. Now our problem is
> > >> that we have 1024MB of onboard RAM on our card. I do *NOT* wish to
> > >> compile with "CONFIG_HIGHMEM" set to true (see below for why), but i
> > >> do wish to have full use of the 1024MB of RAM onboard, or at least
> > >> 992MB which is the minimum for our app.
> > >> So what I did was just change "MAX_LOW_MEM" to be 0x3E000000
> > >> (0x30000000), ie. change it to 992 from 768. I recompiled and tested

> Correct. The solution, in the context of the linuxppc_2_4_devel tree,
> is to do the following:
> Enable "Prompt for advanced kernel configuration options"
> Enable "High memory support"
> Enable "Set maximum low memory" and set to 0x40000000
> Enable "Set custom kernel base address" and set to 0xa0000000
> Note that highmem support will not be used in his case (he didn't
> want to use it), because PAGE_OFFSET is at 0xa0000000 and
> MAX_LOW_MEM is at 1GB. With this configuration, VMALLOC_START
> will be at 0xe0000000 + VMALLOC_OFFSET leaving ample vmalloc
> space for most applications. All system memory is mapped as
> lowmem .

That looks exactly like what I'd need. Thanks.
I grabbed the "2.4.20-pre5" source of linuxppc_2_4_devel, and
I do see those options in there.
However, my problem is that I cannot move to a development kernel for
this application -> i have to stick on the stable tree. So i could
either wait for 2.4.20 to come out, OR could i just look at where
"CONFIG_LOWMEM_SIZE" and "CONFIG_KERNEL_START" are used in 2.4.20-pre5
and patch them back myself to 2.4.18?

Regardless, thanks very much to everyone who replied for the info.

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