access memory mapped registers

[Wolfgang Denk]

In message <885489B3B89FB6449F93E525DF78777F064542 at srvnt506.ALLOPTIC.COM> 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 at denx.de
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