using /dev/mem to talk to peripherals on VIIP system

scott scotto2hot-green at
Sat Jul 29 05:54:01 EST 2006

Hi Dan, thanks for your quick response!  I was using a reference to mmap from this page:
which states that mmap will return MAP_FAILED if there is an error and the error will be located in errno.    please correct me if this is wrong.  As such i don't believe that the map is failing.

i did in any case make the size modulo with PAGE_SIZE (=4096) as found in asm-ppc/page.h which unfortunately did not fix the problem.  just for the heck of it I also tried every size from 1 up to 65536...

It's worth noting that I also tried treating /dev/mem as a file (as it was intended) by using functions lseek, read, and write.  read and write returned a value of zero upon completion, indicating that no bytes were read/written.  I get the feeling that file I/O functions failing on /dev/mem indicates something important, but I'm not sure what. especially since /dev/mem operations DO work on other parts of physical memory (such as RAM).


----- Original Message ----
From: Dan Malek <dan at>
To: scott <scotto2hot-green at>
Cc: linuxppc-embedded at
Sent: Friday, July 28, 2006 11:51:03 AM
Subject: Re: using /dev/mem to talk to peripherals on VIIP system

On Jul 28, 2006, at 1:35 PM, scott wrote:

>   ptr = MAP_FAILED; // Initialize to bad value
>   ptr = (int *) mmap(0, 256, PROT_READ|PROT_WRITE, MAP_SHARED, fd,  

Well, this isn't going to detect an error, since the error code
is returned instead of an address.

>   if(ptr==MAP_FAILED) {

    if ((int)ptr == -1) {
>     printf("Err: cannot access address!\n");
>     return -1;
>   }

> ....  Any other thoughts?

Your mmap is failing because the size is not modulo PAGE_SIZE.

    -- Dan

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