• Locked Locked
  • Replies 6 replies
  • Subscribers 119 subscribers
  • Views 6181 views
  • Users 0 members are here
2025 survey

We are running a survey to help us improve the experience for all of our members. If you see the survey appear, please take the time to tell us about your experience if you can.

Options
Related
How was your experience today?
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

Accessing CP14 from user mode

Ackbar Ackbar
Note: This was originally posted on 9th July 2009 at http://forums.arm.com

Hello,

   I'm currently trying to change the frequency of a PXA270 processor, the system I'm working on is a Linux  OS, and I would like to write a code that allows an user to change the frequency, to do so I need to write in a register of coprocessor 14. According to ARM documentation i need to use MRC/MCR instructions, I tried but I got an error : "Illegal instruction".

   So I think I need to switch to supervisor mode, it seems that I could do it using SWI but even after reading some documentation I don't know how I should implement this.
I'm not even sure that what I want to do is possible, but if it is, does someone have any idea of the code I should write to switch to supervisor mode and use MRC or MCR instruction?

Thanks,
Parents
  • Arm Employee Badge
    Note: This was originally posted on 9th July 2009 at http://forums.arm.com

    When in User mode the only way to switch to a different (priviledged) mode is by taking an exception.  For example by excuting a SWI/SVC instruction to trigger a Supervisor call exception.  On a bare metal system you would have to provide a vector table and exception handlers.  The handlers could deal with the accesses to CP14.  But...

    As you are running under an OS, the OS will handle the exceptions.  I suspect you will have to write a Linux kernel driver to control the frequency changing.  Then your application uses the driver as an interface.  Not a Linux developer, so can't say how you'd do this.
Reply
  • Arm Employee Badge
    Note: This was originally posted on 9th July 2009 at http://forums.arm.com

    When in User mode the only way to switch to a different (priviledged) mode is by taking an exception.  For example by excuting a SWI/SVC instruction to trigger a Supervisor call exception.  On a bare metal system you would have to provide a vector table and exception handlers.  The handlers could deal with the accesses to CP14.  But...

    As you are running under an OS, the OS will handle the exceptions.  I suspect you will have to write a Linux kernel driver to control the frequency changing.  Then your application uses the driver as an interface.  Not a Linux developer, so can't say how you'd do this.
Children
No data