Re: access memory mapped registers

In message [off-list ref] you wrote:

I can see if I mmap the internal 860 CPU registers, and screw up that, I
will
definitely screw up the system, and require it to be reboot.  BUt if the
hardware we are mapping is outside the CPU internal memory and outside the
RAM,
shouldn't the failure be limited to the user application only then?

No. For example, assume there is a port pin  which  is  meant  as  an
input,  and  the  signal  is  driven from external hardware. Now your
application program by mistake re-programs this pin  as  output.  You
may smoke your board this way.

only the memory regions that is mapped to the ethernet switching chip,
even if I screw up on the hardware setup, shouldn't my kernel still be
protected?  At most, the application faults and I should be able to restart
the application without rebooting the system?

There is a simple rule: if you open backdoors in a secure system  the
result  is  an insecure system. Write clean code.


There is many things you have to keep in mind:

* Functionality: you cannot process interrupts in user space

* Performance: how do you synchronize your accesses to the  hardware?

* Communication: How do you know when data has been processed, or has
  become available, or stable, or ...?

* Tricky things: What will the caches in  your  system  do  when  the
  application  accesses  the  hardware?  How will you address memory?
  Remember that there are systems where  a  32  bit  address  is  not
  sufficient any more...

etc. etc.

Write clean code.



Wolfgang Denk

--
Software Engineering:  Embedded and Realtime Systems,  Embedded Linux
Phone: (+49)-8142-4596-87  Fax: (+49)-8142-4596-88  Email: wd@denx.de
Life is a game. Money is how we keep score.              - Ted Turner

** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/