Re: Mapping full 1 GB of memory on a Freescale Book E part

On Jun 9, 2005, at 6:25 PM, Kylo Ginsberg wrote:

On 6/9/05, Kumar Gala [off-list ref] wrote:

quoted

On Jun 9, 2005, at 5:09 PM, Kylo Ginsberg wrote:

quoted

I've modified fsl_booke_mmu.c to use a 4th TLB, thus mapping the full
1 GB of memory.  However, I'm new to linux memory management, so I'd
appreciate any comments as to whether there are tradeoffs or dangers
here that I am missing.

There is a danger here in that you have left no virtual address space
for the kernel.  Since KERNEL_BASE is normally 0xc000_0000 + 1G ends 
up
a the end of memory.  There for any devices that need virtual memory
space would not have any to map out of.  I'm kinda amazed this works 
at
all.  I'd be interested in seeing what a boot log looks like for this
config.

Sorry, should have mentioned this in the first post.  I dropped
KERNEL_BASE to 0xa0000000 and upped LOWMEM_SIZE to 0x40000000.  So the
Advanced Setup portion of my .config reads:

#
# Advanced setup
#
CONFIG_ADVANCED_OPTIONS=y
CONFIG_HIGHMEM_START=0xfe000000
CONFIG_LOWMEM_SIZE_BOOL=y
CONFIG_LOWMEM_SIZE=0x40000000
CONFIG_KERNEL_START_BOOL=y
CONFIG_KERNEL_START=0xa0000000
# CONFIG_TASK_SIZE_BOOL is not set
CONFIG_TASK_SIZE=0x80000000
# CONFIG_BOOT_LOAD_BOOL is not set
CONFIG_BOOT_LOAD=0x00800000

and under Platform Options:

# CONFIG_HIGHMEM is not set

quoted

The way to handle this is by enabling HIGHMEM support in the kernel or
dropping the KERNEL_START in the config.  I'm haven't ever tried to
drop KERNEL_START so your milage may very on what if anything that 
will
break.

So, an alternative would be to leave fsl_booke_mmu.c untouched, revert
my KERNEL_START/LOWMEM mods, and instead enable HIGHMEM?  Setting
HIGHMEM_START to ...? Will that entail more TLB thrashing?

You have to set HIGHMEM_START to something. I think the default is 
fine.  It only means more TLB thrasing if the kernel has to touch one 
of the pages in HIGHMEM.  If its only application code that is using 
memory > 768M then you shouldn't really see any performance impact.

either solution should work, it just depends on what you want to 
accomplish at the end.

- kumar