query regarding access of physical memory from user space

Vijay Padiyar vijay_padiyar at hotmail.com
Sat Feb 12 01:16:35 EST 2005


I am a bit new to Linux and therefore am unable to appreciate exactly the
use of various techniques used to map physical memory to user or kernel
space. I have come across functions like ioremap(), remap_page_range(), etc.
I am still a _bit_ confused as to exactly what to use for our purpose.

I have an MPC8260 target with 128 MB SDRAM (0x00000000 - 0x07FFFFFF), 32 MB
FLASH (0xFE000000 - 0xFFFFFFFF). In between, I have some CPLDs and FPGAs
mapped at various physical addresses. I wish to access these devices from a
user program or from a device driver inserted into the kernel.

Please tell me how do I go about it?

What I have done right now is to use ioremap() to remap a section of
physical memory to kernel virtual memory and try to use it.

For example, we have a CPLD whose registers we wish to manipulate. These
registers lie in the address range 0x30000000 - 0x300000FF. I map this range
as follows:

unsigned char *cpld_mem;

cpld_mem = (unsigned char *)ioremap(0x30000000, 0x100);

Is this correct? After I do this, can I write to and read from the physical
memory locations 0x30000000-0x300000FF using writeb() and readb()? Or do I
have to use outb() and inb()?

Please do clarify my doubts. Also, what is the *highest memory address* as
well as *maximum memory size* that I can map to kernel or user space using

In other words, can I map any address in the 32-bit address space to kernel
virtual space using ioremap()? Also, what chunk size of memory can I map in
one go?

Please give me some ideas regarding my doubts. I need to get finished with
the driver soon.


Vijay Padiyar


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