accessing physical memory from linux kernel

Question

Can we access any physical memory via some kernel code.? Because, i wrote a device driver which only had init_module and exit_module.. the code is following.

int init_module(void) {
    unsigned char *p = (unsigned char*)(0x10);
    printk( KERN_INFO  "I got %u \n", *p);
    return 0;
}

and a dummy exit_module.. the problem is the computer gets hung when i do lsmod.. What happens? Should i get some kinda permission to access the mem location?

kindly explain.. I'm a beginner!

This is a very complicated programming area and you're not likely to get the answer you need in such limited space. Have you read any documentation in this area? — trojanfoe
yeah, i know the basics.. the question is i know i can access physical memory as a device driver. Bu why does my PC freeze.. after all i'm just reading ; not writing. ? — raj
It's likely that the address 0x10 is a special register that you can't just read. You're treading in dangerous waters here. Arbitrary memory accesses can be hazardous to your health. — Chriszuma
Even the kernel has no direct access to physical memory in the way your are thinking. Basically the kind of memory you seem to imagine (linear memory with flat addressing) is something that is constructed by the kernel (from physical memory banks) and exported, it doesn't "exist". To know about these things and on how to access the different kind of addresses the kernel can handle you really would have to look deeper into the kernel documentation. (Also a value that is not divisible by 4 is certainly a bad choice to expect an unsigned). — Jens Gustedt
This code is not attempting to access physical memory. It's attempting to access an invalid (unmapped) address in virtual memory. This is rather bad. :-) — R.. GitHub STOP HELPING ICE

flolo flolo · Accepted Answer · 2011-10-25T22:38:35

To access real physical memory you should use phys_to_virt function. In case it is io memory (e.g. PCI memory) you should have a closer look at ioremap.

This whole topic is very complex, if you are a beginner I would suggest some kernel/driver development books/doc.

accessing physical memory from linux kernel

3 Answers