* RE: Using more than 256 MB of memory on SB1250 in 32-bit mode
@ 2003-08-30 2:40 ` Finney, Steve
0 siblings, 0 replies; 5+ messages in thread
From: Finney, Steve @ 2003-08-30 2:40 UTC (permalink / raw)
To: Steve Madsen, linux-mips
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
>
>
^ permalink raw reply [flat|nested] 5+ messages in thread
* RE: Using more than 256 MB of memory on SB1250 in 32-bit mode
@ 2003-08-30 2:40 ` Finney, Steve
0 siblings, 0 replies; 5+ messages in thread
From: Finney, Steve @ 2003-08-30 2:40 UTC (permalink / raw)
To: Steve Madsen, linux-mips
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
>
>
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Using more than 256 MB of memory on SB1250 in 32-bit mode
2003-08-31 13:34 ` Ralf Baechle
@ 2004-05-20 20:27 ` Daniel Walton
0 siblings, 0 replies; 5+ messages in thread
From: Daniel Walton @ 2004-05-20 20:27 UTC (permalink / raw)
To: linux-mips
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
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Using more than 256 MB of memory on SB1250 in 32-bit mode
2003-08-29 21:59 Steve Madsen
@ 2003-08-31 13:34 ` Ralf Baechle
2004-05-20 20:27 ` Daniel Walton
0 siblings, 1 reply; 5+ messages in thread
From: Ralf Baechle @ 2003-08-31 13:34 UTC (permalink / raw)
To: Steve Madsen; +Cc: linux-mips
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
^ permalink raw reply [flat|nested] 5+ messages in thread
* Using more than 256 MB of memory on SB1250 in 32-bit mode
@ 2003-08-29 21:59 Steve Madsen
2003-08-31 13:34 ` Ralf Baechle
0 siblings, 1 reply; 5+ messages in thread
From: Steve Madsen @ 2003-08-29 21:59 UTC (permalink / raw)
To: linux-mips
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
^ permalink raw reply [flat|nested] 5+ messages in thread
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2003-08-30 2:40 Using more than 256 MB of memory on SB1250 in 32-bit mode Finney, Steve
2003-08-30 2:40 ` Finney, Steve
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2003-08-29 21:59 Steve Madsen
2003-08-31 13:34 ` Ralf Baechle
2004-05-20 20:27 ` Daniel Walton
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