cpu specific kernel call

Michael Grunditz michael.grunditz at gmail.com
Fri Mar 5 12:54:14 CET 2021

In message <20210305110844.dl5rao6mntiugjc5 at genode-labs.com>
          Stefan Kalkowski <stefan.kalkowski at genode-labs.com> wrote:

> Hello Michael,

> On Wed, Mar 03, 2021 at 11:22:33PM +0100, Michael Grunditz wrote:
>> Hi,
>> I have a kernel call that starts RISC OS, and uses psci. I wonder , where
>> should I put that very aarch64 dependent code. I somehow need to be able
>> to call it from userspace.
>> Regarding kernel calls in general , what is the best way of calling them?
>> Right now I do "MOV x0,#number ; SVC" , probably not very Genode style! :)

> Actually, there is already support for what you are looking for, I
> think. As far as I understood it correctly, you want to wake up the
> Cortex-M4 (side-) processor, after you've prepared RiscOS to run on top of
> it?

RISC OS doesn't run on a M4, it is a desktop OS. Even if it is very light 
and no overhead, it needs a proper cpu

> Of course, you should not just add a system call that can be executed
> by every component to do something like this ;-). But we already had
> similar needs, when doing power control inside of the platform driver
> for i.MX 8M. There are some GPC controls that are done by the ARM
> Trusted firmware. Therefore we extended a previously existent
> privilege to the so called "managing_system" privilege that can be
> given when creating a new PD resp. component. This privilege is
> typically given only to the platform driver. With it a component can
> do a "managing_system(State&)" call on its PD session. This call transfers
> registers (State == Cpu_state) to core, which on will do a "secure
> monitor call" with the given registers (at least on ARMv8).
> For further understanding, please grep for "managing_system" inside
> base and base-hw.

I think that the startup call , and possible restart needs to be in a 
driver. I think that what you did wrote is about that, but I need to 
explore the sources to fully understand. I also need a interrupthandler 
for the MU. I just poll the Message Unit now ,which is fine for serial 
output from RISC OS. But I have started with a filing system, which 
probably would be good if it was interrupt driven. Yet a lot to decide ...

> To sum it up, you could define a specific "Power_domain" for the M4 in
> the platform driver of your board, and then do the corresponding SMC
> call therein using the managing_system call. Your application doing
> the RiscOS loading just needs to acquire the device with the M4
> Power_domain from the platform driver at the point where you want to
> power it, and release it again when you want to power it off.

> This way, you will have a secure and sane integration of the M4
> handling.

Thanks! Can this Power_domain thing also work for A53?


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