Subject: [PATCH] ARM: cpu: Document and tweak clock-frequency property

From: [email-masked] (Rob Herring)
Date: Sat, 07 Dec 2013 12:36:35 -0600

On 12/06/2013 05:57 AM, Mark Brown wrote:
> From: Mark Brown <[email-masked]>
> 
> The ARMv7 topology code uses the ePAPR specified mandatory clock-frequency
> property to determine the relative performances of the CPUs along with the
> CPU type. However with FDT we don't update to take account of the current
> speed and if the cores are not running at full speed on boot then a device
> tree which is accurate on boot can provide incorrect information about the
> relative performances of the cores.
> 
> Document the current usage both to override ePAPR and to make the binding
> within the kernel more complete. Ideally the kernel would use information
> from the CPU frequency scaling drivers here but they may in turn consider
> this property and such changes are likely to be part of the energy aware
> scheduling work so not immediately available.
> 
> Signed-off-by: Mark Brown <[email-masked]>
> ---
>  Documentation/devicetree/bindings/arm/cpus.txt | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/arm/cpus.txt b/Documentation/devicetree/bindings/arm/cpus.txt
> index 91304353eea4..e3726f6bca92 100644
> --- a/Documentation/devicetree/bindings/arm/cpus.txt
> +++ b/Documentation/devicetree/bindings/arm/cpus.txt
> @@ -191,6 +191,15 @@ nodes to be present and contain the properties described below.
>  			  property identifying a 64-bit zero-initialised
>  			  memory location.
>  
> +	- clock-frequency
> +		Usage: required

This breaks compatibility. It may be required for a feature in the
kernel to work, but should not be required in general. Perhaps we need
"optional/recommended" or "optional/required for new designs". Or we
could say required only with heterogeneous cores.

> +		Value type: <u32> or <u64>

How do I determine the size? I think generally this property which is
used in multiple bindings is always u32. Of course, that won't work for
our 5GHz parts next year.

> +		Definition:
> +			This is specified in ePAPR as the current clock
> +			frequency of the CPU.  When used with these
> +			extensions it should reflect the maximum clock
> +			frequency for the CPU.

What does extensions mean? cpu topology nodes?

It is useful to have a standard way to determine the current cpu
frequency. I've been asked for this several times on highbank. This
could be cpufreq, but there is not always a driver loaded. lshw already
has support for reading the frequency using this property. So I'm not
real sure deviating from the ePAPR is a good idea.

If a cpu only supports 1 frequency, then clock-frequency will always
reflect the current and max freq. If a cpu supports multiple
frequencies, then it should have an OPP table with those frequencies. We
should then get max frequency from the OPP table rather than
clock-frequency. It is clear that clock-frequency is insufficient to
describe everything we need.

Rob