Subject: Re: The question about the high memory support on MPC8360?

From: vijay baskar <[email-masked]>
Date: Thu, 29 Nov 2007 12:59:28 +0530

郭劲 wrote:

Hi,friends,

I plug in 2GB DDR-1 in my MPC8360 board,there are two DIMM-184 slots,each
DIMM-184 slot hold 1GB.Could you tell me how to let the linux know about those
2GB?
In uboot, I set up each DDR CS to visit 512MB, total 4 CS signal. DDR window
range is 2GB. I think the uboot has passed those DDR parameter to linux.
I once did a test that config the bootargs with mem=512M, then the linux just
only find 512MB, but if I config the mem=2048M, the linux still find about 750MB.
How to make the linux find the total 2GB memory?

Hi Friend,

I hope u had tracked the conversion that has been going on. The max mem
that the kernel can show is only 768 MB because only that of memory is
directly mapped with your ram.. No matter how much ram > 768 MB (in
powerpc.. it varies in other architectures) u have, the kernel will
show only 768 MB.. This is called low mem..

How t o use high mem??

scott can probably guide us on this.. I also have lot of confusions
about high mem.. Also i would suggest you a bit of reading on virtual
memory management on linux to get a grasp about how things are done

Rest for scott

regards,

vijai

From: Scott Wood <[email-masked]>
Reply-To:
To: vijay baskar <[email-masked]>
Subject: Re: The question about the high memory support on MPC8360?
Date:Wed, 28 Nov 2007 10:57:38 -0600

vijay baskar wrote:

Hi, "The kernel also allows hardcoded mapping of IO regions into its
virtual address space through the io_block_mapping interface."

Can u tell me how this is in current arch/powerpc.

Everything is explicitly ioremapped.

Also does it mean that whatever be the size of the ram > 768 MB there
is not going to be much improvement in performance in kernel space
irrespective of invoking CONFIG_HIGHMEM or not?

Well, the kernel can use highmem for cache... I'm not sure what you
mean by "in kernel space".

Also do you think this low mem be enough if i have lots of kernel
space processes each invoking lots of kmallocs.

That depends on what you mean by "lots". :-)

You'll have 768MB of lowmem, and kmallocs can only use lowmem.

Will there be bottle necks?? Also what alternative do we have if low
mem of 768 MB is not enough??

You'll need to change the user/kernel split, and deal with anything that
breaks in the process.

Or get a 64-bit chip. :-)

-Scott