last 256MB of the linux 32bit addresses space on p4080ds arch

[yoav_rubin2]

I'm using 32bit linux 4.1.8 with Yocto environment on the P4080DS SoC and noticed that the last 256MB of the 32bit addresses space are reserved for some purpose.

from arch/powerpc/mm/init_32.c:

/*The amount of lowmem must be within 0xF0000000 - KERNELBASE. */

# if  (CONFIG_LOWMEM_SIZE > (0xF0000000 - PAGE_OFFSET))

# error "You must adjust CONFIG_LOWMEM_SIZE or CONFIG_KERNEL_START"

# endif

I wanted to use addresses 0XF0000000 - 0XFFFFFFFF for lowmemory but I can't.

What are they reserved for? maybe vmalloc area?  and is it possible to resize this reserved area (shrink it)?

thanks..

[yipingwang]

Linux can only access 768M bytes memory which is defined by CONFIG_LOWMEM_SIZE.

CONFIG_LOWMEM_SIZE=0x30000000

CONFIG_PAGE_OFFSET=0xc0000000

CONFIG_KERNEL_START=0xc0000000

CONFIG_PHYSICAL_START=0x00000000

CONFIG_TASK_SIZE=0xc000000

You could adjust CONFIG_KERNEL_START as 0xd0000000 to assign 0XF0000000 - 0XFFFFFFFF as low memory.

[yoav_rubin2]

thanks for your reply but my situation is different:

I have 1.5GB RAM and disabled the highmem support.

In order to utilize the full RAM I increased the LOWMEM size to 0X60000000 and reduced the task size to 0x90000000.

My configuration:

CONFIG_LOWMEM_SIZE=0x60000000

CONFIG_PAGE_OFFSET=0x90000000

CONFIG_KERNEL_START=0x90000000

CONFIG_PHYSICAL_START - disabled.

CONFIG_TASK_SIZE=0x90000000

The 4GB of virtual address space is divided:  2.25 - User space (tasks) , 1.75 - Kernel space (low memory + reserved memory)

My question is how can I decrease the kernel reserved memory region (from 256MB to128MB) and increase the TASK_SIZE accordingly without changing the LOWMEM_SIZE.

If I'll move kernel_start the way you suggested , I'll face the same problem - user space 2.25 , kernel reserved region 0.25 , kernel lowmem 1.5. It will just change the locations of kernel start and reserved area...

[yipingwang]

You could define CONFIG_KERNEL_START as 0x98000000.

[yoav_rubin2]

thanks for your reply!

just to completly understand:

4GB virtual space:

1. 0x0 - 0x9 : user apps

2. 0x9 - 0x98: ???

3. 0x98 - 0xE8 : kernels lowmem

4. 0xE8 - 0xF: ???

5. 0xF - 0xFFFFFFFF: ???

can you help me filling the blanks?