Re: [Xen-devel] [RFC V2] xen: interface: introduce pvclk interface

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 20 Jan 2016, Peng Fan wrote:
> To my use case, Dom0 and DomU both use device tree, I need to build
> the mapping table between id and name, since I use name to lookup
> the clk in backend, like this:
> "clk = __clk_loopkup(clkname); clk_prepare_enable(clk)". Maybe ACPI
> is another different case.

Theoretically on systems using ACPI there is no need to fiddle with
clocks, see

Documentation/arm64/arm-acpi.txt


"In DT, clocks need to be specified
and the drivers need to take them into account.  In ACPI, the assumption
is that UEFI will leave the device in a reasonable default state, including
any clock settings.  If for some reason the driver needs to change a clock
value, this can be done in an ACPI method; all the driver needs to do is
invoke the method and not concern itself with what the method needs to do
to change the clock.  Changing the hardware can then take place over time
by changing what the ACPI method does, and not the driver.

In DT, the parameters needed by the driver to set up clocks as in the example
above are known as "bindings"; in ACPI, these are known as "Device Properties"
and provided to a driver via the _DSD object."


However currently we don't have the ability to run ACPI in DomU guests
on ARM. Even if we had, there is no way to call native ACPI methods from
any guests other than Dom0, even on x86. We just have to hope that
clocks don't need to be reset on ACPI systems.

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