Date: Wed, 4 Sep 2002 09:12:31 -0400 (EDT)
From: Craig Arsenault <penguin@wombat.ca>
To: linux-kernel@vger.kernel.org
Subject: consequences of lowering "MAX_LOW_MEM"?
Message-ID: <Pine.LNX.4.44L.0209040744170.6536-100000@tabmow.ca.nortel.com>
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Hi all,
  Now I'll explain "why" i want to do what I'm asking below, but if
anyone has any reasons/explanations why it won't work, I'd love to
hear it.

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
our application.  Things seemed to be running normal with a max of
992MB of RAM.

Is this a potential problem, or will this cause some lurking bug that
anyone can think of?  (ie. I'm sure "MAX_LOW_MEM" was set to 768MB for
a reason, but what is that reason).   We don't want to move higher
than 1Gig RAM for now, so are we going to be okay doing what I
describe above?  Any suggestions or comments as to why that's a very
bad idea would be greatly appreciated.  Again, this is for a
PPC-specific board, I'm not sure what the x86 architecture's low
memory max is.


REASON for asking:
Currently, one piece of hardware on our card (a PMC card) is using a
closed-source driver, and they have less-than stellar linux drivers
and support.  Their driver has problems with CONFIG_HIGHMEM turned on
(they are using kiobuf's and they are getting messed up), so as a hack
until they fix their driver, we were contemplating moving MAX_LOW_MEM.
Yes, I know closed-source drivers are bad in some cases, but we had
little choice in this product, and our goal is to move away from it
and use something else.

Thanks for any info/help.

--
Craig.
+------------------------------------------------------+
http://www.wombat.ca/rpmon.html    RP Music Monitor
http://www.washington.edu/pine/    Pine @ the U of Wash.
+-------------=*sent via Pine4.44*=--------------------+


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