Re: [RFC PATCH 2/8] Documentation: arm: define DT cpu capacity bindings

[Rob Herring]

On Mon, Nov 23, 2015 at 02:28:35PM +0000, Juri Lelli wrote:
> ARM systems may be configured to have cpus with different power/performance
> characteristics within the same chip. In this case, additional information
> has to be made available to the kernel (the scheduler in particular) for it
> to be aware of such differences and take decisions accordingly.
> 
> Therefore, this patch aims at standardizing cpu capacities device tree
> bindings for ARM platforms. Bindings define cpu capacity parameter, to
> allow operating systems to retrieve such information from the device tree
> and initialize related kernel structures, paving the way for common code in
> the kernel to deal with heterogeneity.
> 
> Cc: Rob Herring <robh+dt@xxxxxxxxxx>
> Cc: Pawel Moll <pawel.moll@xxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Ian Campbell <ijc+devicetree@xxxxxxxxxxxxxx>
> Cc: Kumar Gala <galak@xxxxxxxxxxxxxx>
> Cc: Maxime Ripard <maxime.ripard@xxxxxxxxxxxxxxxxxx>
> Cc: Olof Johansson <olof@xxxxxxxxx>
> Cc: Gregory CLEMENT <gregory.clement@xxxxxxxxxxxxxxxxxx>
> Cc: Paul Walmsley <paul@xxxxxxxxx>
> Cc: Linus Walleij <linus.walleij@xxxxxxxxxx>
> Cc: Chen-Yu Tsai <wens@xxxxxxxx>
> Cc: Thomas Petazzoni <thomas.petazzoni@xxxxxxxxxxxxxxxxxx>
> Cc: devicetree@xxxxxxxxxxxxxxx
> Signed-off-by: Juri Lelli <juri.lelli@xxxxxxx>
> ---
>  .../devicetree/bindings/arm/cpu-capacity.txt       | 227 +++++++++++++++++++++
>  Documentation/devicetree/bindings/arm/cpus.txt     |  17 ++
>  2 files changed, 244 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/arm/cpu-capacity.txt
> 
> diff --git a/Documentation/devicetree/bindings/arm/cpu-capacity.txt b/Documentation/devicetree/bindings/arm/cpu-capacity.txt
> new file mode 100644
> index 0000000..2a00af0
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/arm/cpu-capacity.txt
> @@ -0,0 +1,227 @@
> +==========================================
> +ARM CPUs capacity bindings
> +==========================================
> +
> +==========================================
> +1 - Introduction
> +==========================================
> +
> +ARM systems may be configured to have cpus with different power/performance
> +characteristics within the same chip. In this case, additional information
> +has to be made available to the kernel (the scheduler in particular) for
> +it to be aware of such differences and take decisions accordingly.
> +
> +==========================================
> +2 - CPU capacity definition
> +==========================================
> +
> +CPU capacity is a number that provides the scheduler information about CPUs
> +heterogeneity. Such heterogeneity can come from micro-architectural differences
> +(e.g., ARM big.LITTLE systems) or maximum frequency at which CPUs can run
> +(e.g., SMP systems with multiple frequency domains). Heterogeneity in this
> +context is about differing performance characteristics; this binding tries to
> +capture a first-order approximation of the relative performance of CPUs.
> +
> +One simple way to estimate CPU capacities is to iteratively run a well-known
> +CPU user space benchmark (e.g, sysbench, dhrystone, etc.) on each CPU at
> +maximum frequency and then normalize values w.r.t.  the best performing CPU.
> +One can also do a statistically significant study of a wide collection of
> +benchmarks, but pros of such an approach are not really evident at the time of
> +writing.
> +
> +==========================================
> +3 - capacity-scale
> +==========================================
> +
> +CPUs capacities are defined with respect to capacity-scale property in the cpus
> +node [1]. The property is optional; if not defined a 1024 capacity-scale is
> +assumed. This property defines both the highest CPU capacity present in the
> +system and granularity of CPU capacity values.

I don't really see the point of this vs. having an absolute scale.

> +
> +==========================================
> +4 - capacity
> +==========================================
> +
> +capacity is an optional cpu node [1] property: u32 value representing CPU
> +capacity, relative to capacity-scale. It is required and enforced that capacity
> +<= capacity-scale.

I think you need something absolute and probably per MHz (like 
dynamic-power-coefficient property). Perhaps the IPC (instructions per 
clock) value?

In other words, I want to see these numbers have a defined method 
of determining them and don't want to see random values from every 
vendor. ARM, Ltd. says core X has a value of Y would be good enough for 
me. Vendor X's A57 having a value of 2 and Vendor Y's A57 having a 
value of 1024 is not what I want to see. Of course things like cache 
sizes can vary the performance, but is a baseline value good enough? 

However, no vendor will want to publish their values if these are 
absolute values relative to other vendors.

If you expect these to need frequent tuning, then don't put them in DT.

Rob

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