Re: [Xen-devel] cpufreq implementation for OMAP under xen hypervisor.

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2014-09-10 at 19:35 +0100, Stefano Stabellini wrote:
> > > One concern I have is that i2c busses also tend to have other things on
> > > them which dom0 might legitimately access (e.g. rtc), I'm not sure what
> > > to suggest here.
> > 
> > I would try to avoid i2c transactions in Xen. I2C driver is quite
> > complicated in Linux kernel. It consists of several parts - common
> > core + platform specific. I'm pretty sure Xen should not handle this.
> > I think that establishing of event channel for frequency changing is a
> > good idea. It would be good to try to implement this. In process of
> > implementation we will see what is need to be resolved.
> 
> OK, that's reasonable.

I don't think a single purpose in-Xen i2c would need quite so much
infrastructure as what Linux provides, so it could be a whole lot
simpler, I think, so I wouldn't necessarily rule it out without trying
it.

> > The only question here is how to pass physical cpu to dom0.
> 
> We can use a device tree based interface to pass the information to
> dom0, but requiring a number of dom0 vcpus equal to the number of
> physical cpus and in addition to that having to pin the vcpus each to a
> different pcpu is quite a stringent limitation. However I don't know the
> frequency changing interfaces in Linux well enough to know how hard
> would be to lift it.
> 
> 
> > Regarding x86.
> > I'm not sure but maybe ACPI interface encapsulate voltage changing as well?
> 
> I think so (but I am not an expert on that).

Likewise.

Ian.


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