Re: [PATCH V4 1/2] dt-bindings: riscv: add MMU Standard Extensions support for Svpbmt

[Jessica Clarke]

On 30 Nov 2021, at 15:01, Philipp Tomsich <philipp.tomsich@xxxxxxxx> wrote:
> 
> We did touch on this in our coordination call a few weeks ago: the
> grouping under mmu and the bool-entries were chosen because of their
> similarity to other extensions (i.e. for Zb[abcs] there could/should
> be a bool-entry under each cpu-node — for some Zv* entries a subnode
> might be needed with further parameters).
> 
> The string-based approach (as in the originally proposed "mmu-type=")
> would like not scale with the proliferation of small & modular
> extensions.

I don’t see why the Sv* extensions need to be under an mmu node then,
unless the intent is that every extension be grouped under a sub-node
(which doesn’t seem viable due to extensions like Zbk*, unless you
group by Ss, Sv and Z)?

Also, what is going to happen to the current riscv,isa? Will that
continue to exist and duplicate the info, or will kernels be required
to reconstruct the string themselves if they want to display it to
users?

As a FreeBSD developer I’m obviously not a part of many of these
discussions, but what the Linux community imposes as the device tree
bindings has a real impact on us.

Jess

> On Tue, 30 Nov 2021 at 14:59, Jessica Clarke <jrtc27@xxxxxxxxxx> wrote:
>> 
>> On 30 Nov 2021, at 13:27, Heiko Stübner <heiko@xxxxxxxxx> wrote:
>>> 
>>> Hi,
>>> 
>>> Am Dienstag, 30. November 2021, 14:17:41 CET schrieb Jessica Clarke:
>>>> On 30 Nov 2021, at 12:07, Heiko Stübner <heiko@xxxxxxxxx> wrote:
>>>>> 
>>>>> Am Montag, 29. November 2021, 13:06:23 CET schrieb Heiko Stübner:
>>>>>> Am Montag, 29. November 2021, 09:54:39 CET schrieb Heinrich Schuchardt:
>>>>>>> On 11/29/21 02:40, wefu@xxxxxxxxxx wrote:
>>>>>>>> From: Wei Fu <wefu@xxxxxxxxxx>
>>>>>>>> 
>>>>>>>> Previous patch has added svpbmt in arch/riscv and add "riscv,svpmbt"
>>>>>>>> in the DT mmu node. Update dt-bindings related property here.
>>>>>>>> 
>>>>>>>> Signed-off-by: Wei Fu <wefu@xxxxxxxxxx>
>>>>>>>> Co-developed-by: Guo Ren <guoren@xxxxxxxxxx>
>>>>>>>> Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
>>>>>>>> Cc: Anup Patel <anup@xxxxxxxxxxxxxx>
>>>>>>>> Cc: Palmer Dabbelt <palmer@xxxxxxxxxxx>
>>>>>>>> Cc: Rob Herring <robh+dt@xxxxxxxxxx>
>>>>>>>> ---
>>>>>>>> Documentation/devicetree/bindings/riscv/cpus.yaml | 10 ++++++++++
>>>>>>>> 1 file changed, 10 insertions(+)
>>>>>>>> 
>>>>>>>> diff --git a/Documentation/devicetree/bindings/riscv/cpus.yaml b/Documentation/devicetree/bindings/riscv/cpus.yaml
>>>>>>>> index aa5fb64d57eb..9ff9cbdd8a85 100644
>>>>>>>> --- a/Documentation/devicetree/bindings/riscv/cpus.yaml
>>>>>>>> +++ b/Documentation/devicetree/bindings/riscv/cpus.yaml
>>>>>>>> @@ -63,6 +63,16 @@ properties:
>>>>>>>>      - riscv,sv48
>>>>>>>>      - riscv,none
>>>>>>>> 
>>>>>>>> +  mmu:
>>>>>>> 
>>>>>>> Shouldn't we keep the items be in alphabetic order, i.e. mmu before
>>>>>>> mmu-type?
>>>>>>> 
>>>>>>>> +    description:
>>>>>>>> +      Describes the CPU's MMU Standard Extensions support.
>>>>>>>> +      These values originate from the RISC-V Privileged
>>>>>>>> +      Specification document, available from
>>>>>>>> +      https://riscv.org/specifications/
>>>>>>>> +    $ref: '/schemas/types.yaml#/definitions/string'
>>>>>>>> +    enum:
>>>>>>>> +      - riscv,svpmbt
>>>>>>> 
>>>>>>> The privileged specification has multiple MMU related extensions:
>>>>>>> Svnapot, Svpbmt, Svinval. Shall they all be modeled in this enum?
>>>>>> 
>>>>>> I remember in some earlier version some way back there was the
>>>>>> suggestion of using a sub-node instead and then adding boolean
>>>>>> properties for the supported extensions.
>>>>>> 
>>>>>> Aka something like
>>>>>>   mmu {
>>>>>>           riscv,svpbmt;
>>>>>>   };
>>>>> 
>>>>> For the record, I'm talking about the mail from september
>>>>> https://lore.kernel.org/linux-riscv/CAAeLtUChjjzG+P8yg45GLZMJy5UR2K5RRBoLFVZhtOaZ5pPtEA@xxxxxxxxxxxxxx/
>>>>> 
>>>>> So having a sub-node would make adding future extensions
>>>>> way nicer.
>>>> 
>>>> Svpbmt is just an ISA extension, and should be treated like any other.
>>>> Let’s not invent two different ways of representing that in the device
>>>> tree.
>>> 
>>> Heinrich asked how the other extensions should be handled
>>> (Svnapot, Svpbmt, Svinval), so what do you suggest to do with these?
>> 
>> Whatever is done for Zb[abcs], Zk*, Zv*, Zicbo*, etc. There may not be
>> a concrete plan for that yet, but that means you should speak with the
>> people involved with such extensions and come up with something
>> appropriate together.
>> 
>> Jess
>>