Re: [PATCH v1 2/2] riscv: dts: starfive: add the missing monitor core

[<Conor.Dooley@microchip.com>]

Rob/Krzk,
Got a question about how to represent one of the cpu cores on
this SoC at the end of the mail

On 13/07/2022 16:26, Icenowy Zheng wrote:
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> 
> 在 2022-07-13星期三的 15:21 +0000，Conor.Dooley@microchip.com写道：
>> On 13/07/2022 16:12, Icenowy Zheng wrote:
>>> 在 2022-07-13星期三的 15:09 +0000，Conor.Dooley@microchip.com写道：
>>>>
>>>>
>>>> On 13/07/2022 16:02, Icenowy Zheng wrote:
>>>>> 在 2022-07-13星期三的 14:55 +0000，Conor.Dooley@microchip.com写道：
>>>>>> On 13/07/2022 15:26, Icenowy Zheng wrote:
>>>>>>> Sorry but I think this change makes the topology more
>>>>>>> inaccurate.
>>>>>>>
>>>>>>> The E24 core is very independent, just another CPU core
>>>>>>> connected
>>>>>>> the
>>>>>>> same bus -- even no coherency (E24 takes AHB, which is not
>>>>>>> coherency-
>>>>>>> sensible). Even the TAP of it is independent with the U74
>>>>>>> TAP.
>>>>>>>
>>>>>>> And by default it does not boot any proper code (if a
>>>>>>> debugger
>>>>>>> is
>>>>>>> attached, it will discover that the E24 is in consistently
>>>>>>> fault at
>>>>>>> 0x0
>>>>>>> (mtvec is 0x0 and when fault it jumps to 0x0 and fault
>>>>>>> again),
>>>>>>> until
>>>>>>> its clock is just shutdown by Linux cleaning up unused
>>>>>>> clocks.)
>>>>>>>
>>>>>>> Personally I think it should be implemented as a remoteproc
>>>>>>> instead.
>>>>>>
>>>>>> Maybe I am missing something, but I don't quite get what the
>>>>>> detail
>>>>>> of how we access this in code has to do with the devicetree?
>>>>>> It is added here in a disabled state, and will not be used by
>>>>>> Linux.
>>>>>> The various SiFive SoCs & SiFive corecomplex users that have
>>>>>> a
>>>>>> hart
>>>>>> not capable of running Linux also have that hart documented
>>>>>> in
>>>>>> the
>>>>>> devicetree.
>>>>>> To me, what we are choosing to do with this hart does not
>>>>>> really
>>>>>> matter very much, since this is a description of what the
>>>>>> hardware
>>>>>> actually looks like.
>>>>>
>>>>> The E24 is not in the core complex at all. It's just a dedicate
>>>>> CPU
>>>>> connected to another bus (well as I saw the document says the
>>>>> E24
>>>>> bus
>>>>> is maximum 2G, I doubt whether it's the same bus with the U74
>>>>> one).
>>>>>
>>>>> The U74 MC only allows S5 management cores to be part of it,
>>>>> not
>>>>> E24.
>>>>

---8<---

>>>
>>> Considering E24 seems to see a total different bus connected to it,
>>> I
>>> don't think it even proper to add it to cpus node.
>>
>> Well, it is a CPU is it not? How one is supposed to document that a
>> CPU is not attached to the same buses I do not know however.
> 
> I don't think this kind of CPUs should exist in /cpus, they should just
> be seen as peripherals as the main system. The speciality of FU[57]40's
> management core is that they're in the same core complex with the CPU
> cores that run Linux, just cores with a different capability that we
> could not expand Linux to them.

Maybe Rob or Krzysztof can shed some light on what would be the correct
way to depict this cpu.

> 
>>
>>>
>>> And I don't think it has a hart id of 2, as your node describes.
>>
>> Do you have any idea what it would be then?
> 
> As I asked one of my friend who has JTAG access to JH7110, the hart id
> is 0, the same with the first hart in U74-MC.

Hmm, my understanding is that the regs property needs to be unique, so
it'd have to stay as 2.

Thanks,
Conor.