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

[Conor Dooley <conor@kernel.org>]

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On Thu, May 18, 2023 at 02:30:53PM -0400, Sean Anderson wrote:
> 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;
> 		...
> 	};
> };

Naming of the node aside (perhaps that could be riscv,isa-extensions)
there's not something hitting me immediately as to why that is a no-no.
If the size is a concern, this would certainly be more efficient & not
like the probing would be anything other than trivial more difficult
what I have in my proposal.

Rob's AFK at the moment, and I was hoping that he would take a look at
the idea, so I won't respin til he is back, but I'll give this a go in
the interim.

Cheers,
Conor.

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