Re: [PATCH] arm/dt: Don't add disabled CPUs to system topology

[Lorenzo Pieralisi <lorenzo.pieralisi-5wv7dgnIgG8@public.gmane.org>]

On Fri, Jun 07, 2013 at 12:48:58PM +0100, James King wrote:
> Hi Lorenzo,
> 
> On 7 June 2013 11:23, Lorenzo Pieralisi <lorenzo.pieralisi-5wv7dgnIgG8@public.gmane.org> wrote:
> > Hi James,
> >
> > On Thu, Jun 06, 2013 at 06:11:25PM +0100, James King wrote:
> >> If CPUs are marked as disabled in the devicetree, make sure they do
> >> not exist in the system CPU information and CPU topology information.
> >> In this case these CPUs will not be able to be added to the system later
> >> using hot-plug. This allows a single chip with many CPUs to be easily
> >> used in a variety of hardware devices where they may have different
> >> actual processing requirements (eg for thermal/cost reasons).
> >>
> >> - Change devicetree.c to ignore any cpu nodes marked as disabled,
> >>   this effectively limits the number of active cpu cores so no need
> >>   for the max_cpus=x in the chosen node.
> >> - Change topology.c to ignore any cpu nodes marked as disabled, this
> >>   is where the scheduler would learn about big/LITTLE cores so this
> >>   effectively keeps the scheduler in sync.
> >>
> >
> > I have two questions:
> >
> > 1) Since with this approach the DT should change anyway if on different
> >    hardware devices based on the same chip you want to allow booting a
> >    different number of CPUs, why do not we remove the cpu nodes instead of
> >    disabling them ? Put it another way: cpu nodes define a cpu as
> >    possible (currently), we can simply remove the node if we do not want
> >    that cpu to be seen by the kernel.
> 
> The reason we want disabled status rather than just remove the nodes
> is to use a common soc.dtsi file which is included in many board.dts
> files - eg:
> 
> file soc.dtsi contains:
> 
>                 cpus {
>                                 cpu0: cpu@0 {
>                                                 device_type = "cpu";
>                                                 compatible = "arm,cortex-a7";
>                                                 reg = <0>;
>                                                 cluster = <&cluster0>;
>                                                 core = <&core0>;

Minor nit, "cluster" and "core" phandles are not part of cpu the bindings
that will be merged this cycle, I know it is just an example.

>                                 };
> 
>                                 cpu1: cpu@1 {
>                                                 device_type = "cpu";
>                                                 compatible = "arm,cortex-a7";
>                                                 reg = <1>;
>                                                 cluster = <&cluster0>;
>                                                 core = <&core1>;
>                                 };
> 
>                                 cpu2: cpu@2 {
>                                                 device_type = "cpu";
>                                                 compatible = "arm,cortex-a15";
>                                                 reg = <2>;
>                                                 cluster = <&cluster0>;
>                                                 core = <&core2>;
>                                 };
>                 };
> 
> file board1.dts where we want the A15 disabled contains:
> 
> /include/ "soc.dtsi"
> 
>                 cpus {
>                                 cpu2: cpu@2 {
>                                                 status = "disabled";
>                                 };
>                 };

Understood, see the other reply as far as the status property is concerned.

> > 2) If we go for the "status" property, why do not we use it to set present
> >    mask ? That way the cpu is possible but not present, you cannot
> >    hotplug it in. It is a bit of a stretch, granted, the cpu _is_ present,
> >    we just want to disable it, do not know how this is handled in x86
> >    and other archs though.
> 
> I have been struggling to find any equivalent example for another
> arch, so just tried to solve our problem. I guess in the x86 world it
> is less likely to want to disable processors in a SoC for heat/battery
> issues so this has just never arisen. In this case the cpu is
> physically present but not possible, but I am not sure it should be in
> a present mask (giving the impression it can be used). Perhaps you
> could elaborate with an example what you are thinking about here?

I was thinking about using status == "disabled" to mark a cpu as
possible but not present; that is a bad idea for the reason you mentioned
and also for the point Rob raised related to the ePAPR.

I am not sure how we can solve this issue, as I mentioned the easiest
solution consists in not defining cpu nodes in the DT or probably add
an additional property != status with proper bindings attached to it.

Lorenzo