Accessing BCSR's

Magnus Damm damm at
Mon Mar 4 10:57:31 EST 2002


If you have memory-mapped io (The (F)ADS8xx BCSR registers is a
good example) which is mapped to a certain physical address, it
is very simple to access it if the MMU is turned off.

The Linux kernel runs with the MMU on, and it needs a
address mapping to be able to access memory-mapped io.

So, my 2.2-kernel has a file called arch/ppc/mm/init.c.

Inside that file there's a function called "ioremap()" - that is what
you are looking for.

Make sure that your Linux kernel has a virtual page for your io-area,
and access the virtual address that is returned by "ioremap()".

If you use "ioremap()" before a certain moment when the Linux kernel
starts up, you will get a 1:1 mapping between virtual and physical
(This is true for 2.2 anyhow)

Look how the internal io block is ioremap():ped for a good example how
it is done.

Cheers /


Jeremy Rosen wrote:
> Hello everybody,
> I have 860T FADS board, and I am trying to access the (board specific)
> registers BCSRs.
> I have no problem accessing them in PPCBOOT, since they are accessed
> through CS1, at address FF01xxxx, but I can't access them in linux for
> the moment.
> My first attempt was by using the /dev/mem device, but it doesn't seem
> to work, the result read are not correct.
> so I have to question
> 1) why doesn't it work, I probably don't understand how /dev/mem works
> : does it map only memory and not the address space ? do I have to
> register the address range or the BCSR in the kernel ?
> 2) what is the proper way of doing this ? writing a driver ? or using
> /dev/mem properly ?
> thx for your help...
> Jeremy

** Sent via the linuxppc-embedded mail list. See

More information about the Linuxppc-embedded mailing list