Using more than 256 MB of memory on SB1250 in 32-bit mode

Using more than 256 MB of memory on SB1250 in 32-bit mode

[Steve Madsen]

Is it possible to use more than 256 MB of system memory with the Broadcom 
SB1250 in 32-bit mode?  The memory map I'm looking at shows me that the 
second 256 MB of memory is at physical address 0x80000000.  I suspect that 
due to the 2G/2G split in the kernel, I can't use memory this high without 
moving to the 64-bit kernel.

Would someone confirm this for me?

-- 
Steve Madsen <madsen@tadpole.com>
Tadpole Computer, Inc.  http://www.tadpole.com

Re: Using more than 256 MB of memory on SB1250 in 32-bit mode

[Ralf Baechle]

On Fri, Aug 29, 2003 at 02:59:49PM -0700, Steve Madsen wrote:

> Is it possible to use more than 256 MB of system memory with the Broadcom 
> SB1250 in 32-bit mode?  The memory map I'm looking at shows me that the 
> second 256 MB of memory is at physical address 0x80000000.  I suspect that 
> due to the 2G/2G split in the kernel, I can't use memory this high without 
> moving to the 64-bit kernel.

Steve Finney's answer was correct; I'd like to add a few details though.

The explanation you gave isn't exactly right.  A 2GB/2GB split would normally
support 2GB of low memory.  We don't on MIPS due to the very inconvenient and
unchangable mappings of KSEG0/KSEG1 - something that may have been sweet
in '85 when the address map was designed but not today when 32-bit address
spaces are beginning to be fairly tight.

Highmem works ok in 2.4 as long as you have a reasonably low ratio of
highmem to lowmem.  For typical loads that means going beyond 4:1 isn't
sensible but the actual number may vary much based on exact system
configuration or workload.

  Ralf

Re: Using more than 256 MB of memory on SB1250 in 32-bit mode

[Daniel Walton]

On Sun, Aug 31, 2003 at 03:34:34PM +0200, Ralf Baechle wrote:
> The explanation you gave isn't exactly right.  A 2GB/2GB split would normally
> support 2GB of low memory.  We don't on MIPS due to the very inconvenient and
> unchangable mappings of KSEG0/KSEG1 - something that may have been sweet
> in '85 when the address map was designed but not today when 32-bit address
> spaces are beginning to be fairly tight.
> 
> Highmem works ok in 2.4 as long as you have a reasonably low ratio of
> highmem to lowmem.  For typical loads that means going beyond 4:1 isn't
> sensible but the actual number may vary much based on exact system
> configuration or workload.
> 
>   Ralf
> 

So what is a practical limit to total memory.

Daniel

RE: Using more than 256 MB of memory on SB1250 in 32-bit mode

[Finney, Steve]

Not sure what you mean by "system memory". You can turn on CONFIG_HIGHMEM and get access to 512 MB physical memory  or more on the BCM with a 32-bit kernel, but you can't access memory above the low 256 MB  directly through KSEG0/1, so there are some things that the kernel can't use it for (though it can be allocated to user processes). Also, since CONFIG_DISCONTIGMEM isn't supported, you end up wasting a bunch of RAM on useless page table space (36MB for 512 MB physical), and there's also some caching weirdness if you try and mmap() /dev/mem to get user access to the >0x10000000 I/O registers. Also, your startup will report "1792 MB HIGHMEM available".

sf

> -----Original Message-----
> From: Steve Madsen [mailto:madsen@tadpole.com]
> Sent: Friday, August 29, 2003 12:00 PM
> To: linux-mips@linux-mips.org
> Subject: Using more than 256 MB of memory on SB1250 in 32-bit mode
> 
> 
> Is it possible to use more than 256 MB of system memory with 
> the Broadcom 
> SB1250 in 32-bit mode?  The memory map I'm looking at shows 
> me that the 
> second 256 MB of memory is at physical address 0x80000000.  I 
> suspect that 
> due to the 2G/2G split in the kernel, I can't use memory this 
> high without 
> moving to the 64-bit kernel.
> 
> Would someone confirm this for me?
> 
> -- 
> Steve Madsen <madsen@tadpole.com>
> Tadpole Computer, Inc.  http://www.tadpole.com
> 
> 

RE: Using more than 256 MB of memory on SB1250 in 32-bit mode

[Finney, Steve]

Not sure what you mean by "system memory". You can turn on CONFIG_HIGHMEM and get access to 512 MB physical memory  or more on the BCM with a 32-bit kernel, but you can't access memory above the low 256 MB  directly through KSEG0/1, so there are some things that the kernel can't use it for (though it can be allocated to user processes). Also, since CONFIG_DISCONTIGMEM isn't supported, you end up wasting a bunch of RAM on useless page table space (36MB for 512 MB physical), and there's also some caching weirdness if you try and mmap() /dev/mem to get user access to the >0x10000000 I/O registers. Also, your startup will report "1792 MB HIGHMEM available".

sf

> -----Original Message-----
> From: Steve Madsen [mailto:madsen@tadpole.com]
> Sent: Friday, August 29, 2003 12:00 PM
> To: linux-mips@linux-mips.org
> Subject: Using more than 256 MB of memory on SB1250 in 32-bit mode
> 
> 
> Is it possible to use more than 256 MB of system memory with 
> the Broadcom 
> SB1250 in 32-bit mode?  The memory map I'm looking at shows 
> me that the 
> second 256 MB of memory is at physical address 0x80000000.  I 
> suspect that 
> due to the 2G/2G split in the kernel, I can't use memory this 
> high without 
> moving to the 64-bit kernel.
> 
> Would someone confirm this for me?
> 
> -- 
> Steve Madsen <madsen@tadpole.com>
> Tadpole Computer, Inc.  http://www.tadpole.com
> 
>