CPU frequency on i.MX53 QSB?

[Stefan Kalkowski]

Hi David,

first off, cpu frequency scaling for the i.MX53 SoC isn't implemented in
Genode by now.

Controlling cpu frequency, power management, and the global clock module
directly from secure and normal world isn't possible without fatal
interferences.
All those functionality should be given as a whole either to the secure,
or non-secure side. They are assigned to one and the same slot within
the central security unit of the i.MX53 SoC (CSL slot 9). That means:
exclusive control can be given only as a whole.

If you want the secure world to stay active and alive, you can't give
control of that combined functionality to the normal world. Imagine the
normal world turns off power of devices that are under control of the
secure side. If the corresponding driver doesn't recognize, it will fail
to service. Moreover, if the cpu is turned into deep sleep, the secure
side can't do anything against it.

Nevertheless, within Genode's current _example_ setup we allow the
normal world to access those devices, so that Linux can use e.g. the
global clock module directly without the need to implement all that
drivers on the Genode side, and to support para-virtualized devices
within Linux.
Furthermore, to circumvent interferences between the Linux kernel, and
the secure side, certain power saving functions are disabled in the
modified Linux code, and cpu frequency scaling was disabled in the
example's Linux configuration.

If you need cpu frequency scaling, you can either implement a
corresponding driver on the Genode side, you might use the platform
driver of the Arndale board (repos/src/drivers/platform/arndale) as a
template, provide a hypercall interface to the normal world, and
implement a para-virtual cpu frequency driver in Linux.
Alternatively, if denial of service attacks doesn't matter in your
scenario, you can leave the assignment of the clocking, power, frequency
scaling to the normal world as it is, re-enable cpu frequency scaling
within Linux, and check whether it negatively interferences with the
devices on the secure side (e.g.: timers). If you encounter problems,
you might cooperatively inform the secure side about changed settings to
circumvent interferences. The same applies to changed power settings,
e.g. you can inform the secure world before disabling certain devices,
as well as after re-enabling them.

I hope that clarifies your questions.

Regards
Stefan

On 11/06/2014 12:18 PM, David Goltzsche wrote:
> Hi everyone,
> 
> I still use my setup based on the tz_vmm runscript.
> 
> I want to obtain the current cpu frequency in both secure and normal
> world.
> 
> In linux, the sysfs directories for the cpu are empty. How can i read
> the current CPU frequency in genode?
> 
> Maybe dynamic frequency scaling in unsupported and I can assume the CPU
> to always run at max frequency (1GHz in this case)?
> 
> thanks!
> 
> David
> 
> 
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-- 
Stefan Kalkowski
Genode Labs

http://www.genode-labs.com/ · http://genode.org/