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

[James King]

Hi Lorenzo,

On 7 June 2013 11:23, Lorenzo Pieralisi <lorenzo.pieralisi at arm.com> 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 at 0 {
                                                device_type = "cpu";
                                                compatible = "arm,cortex-a7";
                                                reg = <0>;
                                                cluster = <&cluster0>;
                                                core = <&core0>;
                                };

                                cpu1: cpu at 1 {
                                                device_type = "cpu";
                                                compatible = "arm,cortex-a7";
                                                reg = <1>;
                                                cluster = <&cluster0>;
                                                core = <&core1>;
                                };

                                cpu2: cpu at 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 at 2 {
                                                status = "disabled";
                                };
                };

> 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?

Thanks,
James
>
> I am just asking, since it is something I thought about while writing
> code that parses the DT cpu map, basically we do not have a way to
> "disable" a cpu in the DT and that's what you are doing, I just would like
> to understand the best way to put it into DT bindings.
>
> Thanks,
> Lorenzo
>