linux-kernel - Re: [PATCH v2 0/1] arm: topology: parse the topology from the dt

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Message-ID: <87r1jald4u.mognet@arm.com>
Date:   Fri, 16 Apr 2021 11:47:29 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Ruifeng Zhang <ruifeng.zhang0110@...il.com>
Cc:     linux@...linux.org.uk, sudeep.holla@....com,
        Greg KH <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>, a.p.zijlstra@...llo.nl,
        mingo@...nel.org, ruifeng.zhang1@...soc.com, nianfu.bai@...soc.com,
        linux-arm-kernel@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/1] arm: topology: parse the topology from the dt

On 16/04/21 12:39, Dietmar Eggemann wrote:
> On 16/04/2021 11:32, Valentin Schneider wrote:
>> On 16/04/21 15:47, Ruifeng Zhang wrote:
>>> For more requirements, if all cores in one physical cluster, the
>>> {aff2} of all cores are the same value.
>>> i.e. the sc9863a,
>>> core0: 0000000081000000
>>> core1: 0000000081000100
>>> core2: 0000000081000200
>>> core3: 0000000081000300
>>> core4: 0000000081000400
>>> core5: 0000000081000500
>>> core6: 0000000081000600
>>> core7: 0000000081000700
>>>
>>> According to MPIDR all cores will parse to the one cluster, but it's
>>> the big.LITTLE system, it's need two logic cluster for schedule or
>>> cpufreq.
>>> So I think it's better to add the logic of parse topology from DT.
>>
>> Ah, so it's a slightly different issue, but still one that requires a
>> different means of specifying topology.
>
> I'm confused. Do you have the MT bit set to 1 then? So the issue that
> the mpidr handling in arm32's store_cpu_topology() is not correct does
> not exist?
>
> With DynamIQ you have only *one* cluster, you should also be able to run
> your big.LITTLE system with only an MC sched domain.
>
> # cat /proc/schedstat
> cpu0 ....
> domain0 ff ... <- MC
> ...
>

You're right, this is actually a DynamIQ system, not a (legacy) big.LITTLE
one, so all CPUs are under the same LLC (the DSU). I probably should have
checked this earlier on, but this is quite obvious from sc9863a.dtsi:

                cpu-map {
                        cluster0 {
                                core0 {
                                        cpu = <&CPU0>;
                                };
                                core1 {
                                        cpu = <&CPU1>;
                                };
                                core2 {
                                        cpu = <&CPU2>;
                                };
                                core3 {
                                        cpu = <&CPU3>;
                                };
                                core4 {
                                        cpu = <&CPU4>;
                                };
                                core5 {
                                        cpu = <&CPU5>;
                                };
                                core6 {
                                        cpu = <&CPU6>;
                                };
                                core7 {
                                        cpu = <&CPU7>;
                                };
                        };
                };

All CPUs are in the same cluster, and the MPIDR values actually match that.

> You can introduce a cpu-map to create what we called Phantom Domains in
> Android products.
>
> # cat /proc/schedstat
>
> cpu0 ....
> domain0 0f ... <- MC
> domain1 ff ... < DIE
>
> Is this what you need for your arm32 kernel system? Adding the
> possibility to parse cpu-map to create Phantom Domains?