Re: [PATCH v1] dt-bindings: riscv: deprecate riscv,isa

[Sean Anderson <sean.anderson@seco.com>]

On 5/18/23 10:06, Conor Dooley wrote:
> On Thu, May 18, 2023 at 07:13:15PM +0530, Anup Patel wrote:
>> On Thu, May 18, 2023 at 4:02 PM Andrew Jones <ajones@ventanamicro.com> wrote:
>> > On Thu, May 18, 2023 at 09:58:30AM +0100, Conor Dooley wrote:
> 
>> > > -  riscv,isa:
>> > > -    description:
>> > > -      Identifies the specific RISC-V instruction set architecture
>> > > -      supported by the hart.  These are documented in the RISC-V
>> > > -      User-Level ISA document, available from
>> > > -      https://riscv.org/specifications/
>> > > -
>> > > -      Due to revisions of the ISA specification, some deviations
>> > > -      have arisen over time.
>> > > -      Notably, riscv,isa was defined prior to the creation of the
>> > > -      Zicsr and Zifencei extensions and thus "i" implies
>> > > -      "zicsr_zifencei".
>> > > -
>> > > -      While the isa strings in ISA specification are case
>> > > -      insensitive, letters in the riscv,isa string must be all
>> > > -      lowercase to simplify parsing.
>> > > -    $ref: "/schemas/types.yaml#/definitions/string"
>> > > -    pattern: ^rv(?:64|32)imaf?d?q?c?b?k?j?p?v?h?(?:[hsxz](?:[a-z])+)?(?:_[hsxz](?:[a-z])+)*$
>> > > -
>> > >    # RISC-V requires 'timebase-frequency' in /cpus, so disallow it here
>> > >    timebase-frequency: false
>> > >
>> > > @@ -133,8 +117,13 @@ properties:
>> > >        DMIPS/MHz, relative to highest capacity-dmips-mhz
>> > >        in the system.
>> > >
>> > > +oneOf:
>> > > +  - required:
>> > > +      - riscv,isa
>> >
>> > This is the part Anup keeps reminding me about. We can create better ways
>> > to handle extensions in DT and ACPI, but we'll still need to parse ISA
>> > strings to handle legacy DTs and holdouts that keep creating ISA strings,
>> > at least during the deprecation period, since ISA strings are still "the
>> > way to do it" according to the spec.
>> 
>> Coming up with an alternate way in DT is fine but we can't deprecate
>> ISA strings since ISA strings are widely used:
>> 1) Various bootloaders
> 
> Aye, for the reason, as I mentioned earlier and in the RFC thread,
> removing existing parsers isn't a good idea.
> 
>> 2) It is part of /proc/cpuinfo
> 
> That is irrelevant.
> 
>> 3) Hypervisors use it to communicate HW features to Guest/VM.
>> Hypervisors can't get away from generating ISA strings because
>> Hypervisors don't know what is running inside Guest/VM.
> 
> Generate both :) As things stand, your guests could interpret what you
> communicate to them via riscv,isa differently!
> 
>> In the case of ACPI, it is a very different situation. Like Sunil mentioned,
>> ACPI will always follow mechanisms defined by RVI (such as ISA string).
>> Other ACPI approaches such as GUID for ISA extension are simply not
>> scalable and will take a lot more memory for ACPI tables compared to
>> ISA strings.
> 
> My proposal should actually suit ACPI, at least for Linux, as it would
> be a chance to align currently misaligned definitions. I won't speak to
> GUIDs or whatever as that's someone else's problem :)
> 
>> > Also, if we assume the wording in the spec does get shored up, then,
>> > unless I'm missing something, the list of advantages for this boolean
>> > proposal from your commit message would be
>> 
>> IMO, we should try our best to have the wordings changed in RVI spec.
> 
> Yes, doing so is beneficial for all of us regardless of what happens
> here. I do think that it is partially orthogonal - it allows us to not
> design an interface that needs to be capable of communicating a wide
> variety of versions, but I don't think it solves some of the issues
> that riscv,isa has. If I thought it did, I would not have gone to the
> trouble of respinning this patch out of the other approach.
> 
>> > * More character choices for name -- probably not a huge gain for ratified
>> >   extensions, since the boolean properties will likely still use the same
>> >   name as the ISA string (riscv,isa-extension-<name>). But, for vendor
>> >   extensions, this is indeed a major improvement, since vendor extension
>> >   boolean property names may need to be extended in unambiguous ways to
>> >   handle changes in the extension.
>> >
>> > * Simpler, more complete DT validation (but we still need a best effort
>> >   for legacy ISA strings)
>> >
>> > * Simpler DT parsing (but we still need the current parser for legacy ISA
>> >   strings)
>> >
>> > > +  - required:
>> > > +      - riscv,isa-base
>> > > +
>> > >  required:
>> > > -  - riscv,isa
>> > >    - interrupt-controller
>> > >
>> > >  additionalProperties: true
>> > > @@ -177,7 +166,13 @@ examples:
>> > >                  i-tlb-size = <32>;
>> > >                  mmu-type = "riscv,sv39";
>> > >                  reg = <1>;
>> > > -                riscv,isa = "rv64imafdc";
>> > > +                riscv,isa-base = "rv64i";
>> > > +                riscv,isa-extension-i;
>> > > +                riscv,isa-extension-m;
>> > > +                riscv,isa-extension-a;
>> > > +                riscv,isa-extension-f;
>> > > +                riscv,isa-extension-d;
>> > > +                riscv,isa-extension-c;
>> 
>> One downside of this new approach is it will increase the size of DTB.
>> Imaging 50 such DT properties in 46 CPU DT nodes.
> 
> I should do a comparison between 50 extensions in riscv,isa and doing
> this 50 times and see what the sizes are.

Why not just have something like

mycpu {
	...
	riscv,isa {
		i;
		m;
		a;
		zicsr;
		...
	};
};

?

--Sean

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