atif_shabir

Hello

I want to write to some specific physical address in the system memory space. I am writing a driver for it. I am using following functions

in module_init:
check_region() to check the availability of that specific physical address.
request_mem_region() to request the memory resource from the kernel in case of successful check_region(). The function request_mem_region() returns success which means (as far as I assume) that the resource has been allocated to my driver.

in write function
If the request_region() returns success then I must be able to write to that memory address just freely.
For example: *(char *)myphysicaladdr = some value

But when I open the driver file from /dev and write a charater to it, segmentation fault occurs. Kindly somebody help me what should I do?

outb() is used to write to an i/o port address. Kindly tell me is there any function to write to physical address?

regards,
Atif Shabir

coopstah13

debug your code to find out why you are getting a seg fault

the most common cause of seg fault is dereferencing null pointer and accessing uninitialized memory

atif_shabir

Thank you coopstah13 for your reply.

A piece of my source code is

#define PHYSICAL_ADDR 0x00000100
#define MEM_SIZE 2

int memory_init(void)
{
int result;

/*Registering device*/
result = register_chrdev(memory_major, "my_module", &memory_fops);
if(result < 0) {
printk("<1> my_module: cannot obtain major number %d\n", memory_major);
return result;
}

/*Allocating memory byte for the port*/
my_port = check_region(PHYSICAL_ADDR, MEM_SIZE); //check_region returns 0 (success)
if(my_port) {
printk("<1>my_module: cannot reserve 0x%X\n",PHYSICAL_ADDR);
result = my_port;
goto fail;
}

alloc_result = request_mem_region(PHYSICAL_ADDR, MEM_SIZE, "my_module");
//request_mem_region() returns 0 (success)
if(alloc_result) {
printk("<1>my_partport2: Requested memory region not allocated\n");
goto fail;
}
printk("<1>my_module: Requested memory region allocated successfully....Inserting Module\n");
return 0;

fail:
memory_exit();
return result;
}

ssize_t memory_read(struct file *filp, char *buf,
size_t count, loff_t *f_pos) {

char my_parport_buf;

////////////////////////////////////////////
my_parport_buf = *((char *)PHYSICAL_ADDR); //This statement gives the segmentation fault
///////////////////////////////////////////////

copy_to_user(buf, &my_parport_buf, 1);
if(*f_pos == 0) {
*f_pos+=1;
return 1;
} else {
return 0;
}
}


When I run a C program that opens and reads one character from the file /dev/my_module, segmentation fault occurs. dmesg shows following

BUG: unable to handle kernel NULL pointer dereference at 00000100
IP: [<f8b6f080>] :my_parport2:memory_read+0x1c/0x66
*pdpt = 00000000243e6001 *pde = 0000000000000000
Oops: 0000 [#2] SMP


Kindly tell me where my the problem may lie. Am I on the right path for writing on physical memory? or should I adopt a new path?

regards,
Atif Shabir

coopstah13

you are dereferencing a null pointer, you need to initialize PHYSICAL_ADDR

atif_shabir

I have #defined PHYSICAL_ADDR to be 0x00000100

and how will I initiallize this pointer? I mean if I use kalloc(), the kernel will allocate me an address from any where in the memory. But If want to allocate specifically 0x00000100 physical address then what should I do?

coopstah13

Is there any reason you need to allocate at a specific memory address? What if something else is already using it? If you are just looking to write a driver that you can write something to memory from and read it from anywhere, then any address should be fine...