Re: [RFC PATCH 0/2] Boot time cpupools

[Julien Grall <julien@xxxxxxx>]

Hi Stefano,

On 19/11/2021 18:55, Stefano Stabellini wrote:
> On Fri, 19 Nov 2021, Julien Grall wrote:
> > I like better Juergen's version. But before agreeing on the command line , I
> > would like to understand how Linux is dealing with big.LITTLE today (see my
> > question above).
> I also like Juergen's version better :-)
>
> The device tree binding that covers big.LITTLE is the same that covers
> cpus: Documentation/devicetree/bindings/arm/cpus.yaml
Are you sure? I found 
Documentation/devicetree/bindings/arm/cpu-capacity.txt.
> So basically, you can tell it is a big.LITTLE system because the
> compatible strings differ between certain cpus, e.g.:
>
>        cpu@0 {
>          device_type = "cpu";
>          compatible = "arm,cortex-a15";
>          reg = <0x0>;
>        };
>
>        cpu@1 {
>          device_type = "cpu";
>          compatible = "arm,cortex-a15";
>          reg = <0x1>;
>        };
>
>        cpu@100 {
>          device_type = "cpu";
>          compatible = "arm,cortex-a7";
>          reg = <0x100>;
>        };
>
>        cpu@101 {
>          device_type = "cpu";
>          compatible = "arm,cortex-a7";
>          reg = <0x101>;
>        };
I have not found any code in Linux using the compatible. Instead, they 
all seem to use the cpu-map (see drivers/base/arch_topology.c).
Anyway, to me it would be better to parse the Device-Tree over using the 
MIDR. The advantage is we can cover a wide range of cases (you may have 
processor with the same MIDR but different frequency). For now, we could 
create a cpupool per cluster.
Cheers,

--
Julien Grall