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Date:   Thu, 18 May 2023 12:31:51 +0200
From:   Andrew Jones <ajones@ventanamicro.com>
To:     Conor Dooley <conor.dooley@microchip.com>
Cc:     palmer@dabbelt.com, conor@kernel.org,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Alistair Francis <alistair.francis@wdc.com>,
        Anup Patel <apatel@ventanamicro.com>,
        Atish Patra <atishp@atishpatra.org>,
        Jessica Clarke <jrtc27@jrtc27.com>,
        Rick Chen <rick@andestech.com>, Leo <ycliang@andestech.com>,
        linux-riscv@lists.infradead.org, qemu-riscv@nongnu.org,
        u-boot@lists.denx.de, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1] dt-bindings: riscv: deprecate riscv,isa
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On Thu, May 18, 2023 at 09:58:30AM +0100, Conor Dooley wrote:
> intro
> =====
> 
> When the RISC-V dt-bindings were accepted upstream in Linux, the base
> ISA etc had yet to be ratified. By the ratification of the base ISA,
> incompatible changes had snuck into the specifications - for example the
> Zicsr and Zifencei extensions were spun out of the base ISA.
> 
> Fast forward to today, and the reason for this patch.
> Currently the riscv,isa dt property permits only a specific subset of
> the ISA string - in particular it excludes version numbering.
> With the current constraints, it is not possible to discern whether
> "rv64i" means that the hart supports the fence.i instruction, for
> example.
> Future systems may choose to implement their own instruction fencing,
> perhaps using a vendor extension, or they may not implement the optional
> counter extensions. Software needs a way to determine this.
> 
> versioning schemes
> ==================
> 
> "Use the extension versions that are described in the ISA manual" you
> may say, and it's not like this has not been considered.
> Firstly, software that parses the riscv,isa property at runtime will
> need to contain a lookup table of some sort that maps arbitrary versions
> to versions it understands. There is not a consistent application of
> version number applied to extensions, with a higgledy-piggledy
> collection of tags, "bare" and version documents awaiting the reader on
> the "recently ratified extensions" page:
> https://wiki.riscv.org/display/HOME/Recently+Ratified+Extensions
> 
> 	As an aside, this is reflected in the patch too, since many
> 	extensions have yet to appear in a release of the ISA specs,
> 	and are defined by commits in their respective "working draft"
> 	repositories.
> 
> Secondly, there is an issue of backwards compatibility, whereby allowing
> numbers in the ISA string, some parsers may be broken. This would
> require an additional property to be created to even use the versions in
> this manner.
> 
> boolean properties
> ==================
> 
> If a new property is needed, the whole approach may as well be looked at
> from the bottom up. A string with limited character choices etc is
> hardly the best approach for communicating extension information to
> software.
> 
> Switching to using boolean properties, one per extension, allows us to
> define explicit meanings for the DT representation of each extension -
> rather than the current situation where different operating systems or
> other bits of software may impart different meanings to characters in
> the string. Clearly the best source of meanings is the specifications
> themselves, this just provides us the ability to choose at what point
> in time the meaning is set. If an extension changes incompatibility in
> the future, a new property will be required.
> 
> Off-list, some of the RVI folks have committed to shoring up the wording
> in either the ISA specifications, the riscv-isa-manual or
> so that in the future, modifications to and additions or removals of
> features will require a new extension. Codifying that assertion
> somewhere would make it quite unlikely that compatibility would be
> broken, but we have the tools required to deal with it, if & when it
> crops up.
> It is in our collective interest, as consumers of extension meanings, to
> define a scheme that enforces compatibility.
> 
> The use of boolean properties, rather than elements in a string, will
> also permit validation that the strings have a meaning, as well as
> potentially reject mutually exclusive combinations, or enforce
> dependencies between instructions. That would not be possible with the
> current dt-schema infrastructure for arbitrary strings, as we would need
> to add a riscv,isa parser to dt-validate!
> 	That's not implemented in this patch, but rather left as
> 	future work!
> 
> acpi
> ====
> 
> The current ACPI ECR is based on having a string unfortunately, but
> ideally ACPI will move to another method, perhaps GUIDs, that give
> explicit meaning to extensions.
> 
> parser simplicity
> =================
> 
> Many systems that parse DT at runtime already implement an function that
> can check for the presence of boolean properties, rather than having to
> implement - although unfortunately for backwards compatibility with old
> dtbs, existing parsers may not be removable - which may greatly simplify
> dt parsing code. For example, in Linux, checking for an extension
> becomes as simple as:
> 	of_property_present(node, "riscv,isa-extension-zicbom")
> 
> vendor extensions
> =================
> 
> Compared to riscv,isa, this proposed scheme promotes vendor extensions,
> oft touted as the strength of RISC-V, to first-class citizens.
> At present, extensions are defined as meaning what the RISC-V ISA
> specifications say they do. There is no realistic way of using that
> interface to provide cross-platform definitions for what vendor
> extensions mean. Vendor extensions may also have even less consistency
> than RVI do in terms of versioning, or no care about backwards
> compatibility.
> A boolean property allows us to assign explicit meanings on a per vendor
> extension basis, backed up by a description of their meanings.
> 
> fin
> ===
> 
> Create a new file to store the extension meanings, each in the form
> riscv,isa-extension-<foo> and a new riscv,isa-base property to replace
> the missing aspect of riscv,isa - the base ISA implemented by a hart.
> As a starting point, properties were added for extensions currently used
> in Linux.
> 
> Finally, mark riscv,isa as deprecated. o7.
> 
> CC: Palmer Dabbelt <palmer@dabbelt.com>
> CC: Paul Walmsley <paul.walmsley@sifive.com>
> CC: Rob Herring <robh+dt@kernel.org>
> CC: Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>
> CC: Alistair Francis <alistair.francis@wdc.com>
> CC: Andrew Jones <ajones@ventanamicro.com>
> CC: Anup Patel <apatel@ventanamicro.com>
> CC: Atish Patra <atishp@atishpatra.org>
> CC: Jessica Clarke <jrtc27@jrtc27.com>
> CC: Rick Chen <rick@andestech.com>
> CC: Leo <ycliang@andestech.com>
> CC: linux-riscv@lists.infradead.org
> CC: qemu-riscv@nongnu.org
> CC: u-boot@lists.denx.de
> CC: devicetree@vger.kernel.org
> CC: linux-kernel@vger.kernel.org
> Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
> ---
> I've tried to CC a few folks here that would care about this, but I am
> sure there are more. I'll go cross-post it to sw-dev, if it allows me to
> post there...
> ---
>  .../devicetree/bindings/riscv/cpus.yaml       |  45 +--
>  .../devicetree/bindings/riscv/extensions.yaml | 259 ++++++++++++++++++
>  2 files changed, 282 insertions(+), 22 deletions(-)
>  create mode 100644 Documentation/devicetree/bindings/riscv/extensions.yaml
> 
> diff --git a/Documentation/devicetree/bindings/riscv/cpus.yaml b/Documentation/devicetree/bindings/riscv/cpus.yaml
> index 3d2934b15e80..446801fb7495 100644
> --- a/Documentation/devicetree/bindings/riscv/cpus.yaml
> +++ b/Documentation/devicetree/bindings/riscv/cpus.yaml
> @@ -23,6 +23,9 @@ description: |
>    two cores, each of which has two hyperthreads, could be described as
>    having four harts.
>  
> +allOf:
> +  - $ref: extensions.yaml
> +
>  properties:
>    compatible:
>      oneOf:
> @@ -79,25 +82,6 @@ properties:
>      description:
>        The blocksize in bytes for the Zicboz cache operations.
>  
> -  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.

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

* 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;
>                  tlb-split;
>                  cpu_intc1: interrupt-controller {
>                          #interrupt-cells = <1>;
> @@ -196,7 +191,13 @@ examples:
>                  device_type = "cpu";
>                  reg = <0>;
>                  compatible = "riscv";
> -                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;
>                  mmu-type = "riscv,sv48";
>                  interrupt-controller {
>                          #interrupt-cells = <1>;
> diff --git a/Documentation/devicetree/bindings/riscv/extensions.yaml b/Documentation/devicetree/bindings/riscv/extensions.yaml
> new file mode 100644
> index 000000000000..1b4d726f7174
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/riscv/extensions.yaml
> @@ -0,0 +1,259 @@
> +# SPDX-License-Identifier: (GPL-2.0 OR MIT)
> +%YAML 1.2
> +---
> +$id: http://devicetree.org/schemas/riscv/extensions.yaml#
> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> +
> +title: RISC-V ISA extensions
> +
> +maintainers:
> +  - Paul Walmsley <paul.walmsley@sifive.com>
> +  - Palmer Dabbelt <palmer@sifive.com>
> +  - Conor Dooley <conor@kernel.org>
> +
> +description: |
> +  RISC-V has large number of extensions, some of which "standard" extensions,
               ^ a                                       ^ are

> +  meaning they are ratified by RISC-V International, and others are "vendor"
> +  extensions.  This document defines properties that indicate whether a hart
> +  supports a given extensions.

drop 'a' or depluralize 'extensions'

> +
> +  Once a standard extension has been ratified, no features can be added or

I'd change 'features' to 'changes in behavior', and then...

> +  removed without the creation of a new extension for that sub- or super-set.

...drop 'for that sub- or super-set'

> +  The properties for standard extensions therefore map to their originally
> +  ratified states, with the exception of the I, Zicntr & Zihpm extensions.

Can you elaborate on the exceptions? Or, if the exceptions are described
below, maybe a '(see below)' here would help ease the reader's
insecurities about their lack of knowledge about these exceptions, as
they'll see that the education is coming :-)

> +
> +select:
> +  properties:
> +    compatible:
> +      contains:
> +        const: riscv
> +
> +properties:
> +  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
                                 ^ the

				 (but I see this was a faithful move
				 of the current text, so maybe better
				 to fix it up separately)

> +      insensitive, letters in the riscv,isa string must be all
> +      lowercase to simplify parsing.
> +
> +      This property has been deprecated due to disparity between the
> +      extension at the time of its creation and ratification of the
> +      base ISA.
> +
> +    $ref: /schemas/types.yaml#/definitions/string
> +    pattern: ^rv(?:64|32)imaf?d?q?c?b?k?j?p?v?h?(?:[hsxz](?:[a-z])+)?(?:_[hsxz](?:[a-z])+)*$
> +    deprecated: true
> +
> +  riscv,isa-base:
> +    description:
> +      The base ISA implemented by this hart, as described by the 20191213
> +      version of the unprivileged ISA specification.
> +    enum:
> +      - rv32i
> +      - rv64i
> +
> +  riscv,isa-extension-i:
> +    type: boolean
> +    description:
> +      The base integer instruction set, as ratified in the 20191213 version of the
> +      unprivileged ISA specification.
> +
> +  riscv,isa-extension-m:
> +    type: boolean
> +    description:
> +      The standard M extension for integer multiplication and division, as
> +      ratified in the 20191213 version of the unprivileged ISA specification.
> +
> +  riscv,isa-extension-a:
> +    type: boolean
> +    description:
> +      The standard A extension for atomic instructions, as ratified in the
> +      20191213 version of the unprivileged ISA specification.
> +
> +  riscv,isa-extension-f:
> +    type: boolean
> +    description:
> +      The standard M extension for single-precision floating point, as
                      ^ F

> +      ratified in the 20191213 version of the unprivileged ISA specification.
> +
> +  riscv,isa-extension-d:
> +    type: boolean
> +    description:
> +      The standard M extension for double-precision floating-point, as
                      ^ D

> +      ratified in the 20191213 version of the unprivileged ISA specification.
> +
> +  riscv,isa-extension-q:
> +    type: boolean
> +    description:
> +      The standard M extension for quad-precision floating-point, as ratified in
                      ^ Q

> +      the 20191213 version of the unprivileged ISA specification.
> +
> +  riscv,isa-extension-c:
> +    type: boolean
> +    description:
> +      The standard M extension for compressed instructions, as ratified in the
                      ^ C

> +      20191213 version of the unprivileged ISA specification.
> +
> +  riscv,isa-extension-v:
> +    type: boolean
> +    description:
> +      The standard V extension for vector operations, as ratified in-and-around
> +      commit 7a6c8ae ("Fix text that describes vfmv.v.f encoding") of the
> +      riscv-v-spec.
> +
> +  riscv,isa-extension-h:
> +    type: boolean
> +    description:
> +      The standard h extension for hypervisors as ratified in the 20191213
                      ^ H (might as well keep the case consistent)

> +      version of the privileged ISA specification.
> +
> +  # Additional Standard Extensions, sorted by category then alphabetically

Can we just do pure alphabetically? And the single-letter extensions above
don't have a "sorted by" comment above them. I guess they need one, or
maybe they can also be alphabetical?

> +
> +  riscv,isa-extension-zicntr:
> +    type: boolean
> +    description:
> +      The standard Zicntr extension for base counters and timers, as ratified
> +      in the 20191213 version of the unprivileged ISA specification.
> +
> +  riscv,isa-extension-zicsr:
> +    type: boolean
> +    description:
> +      The standard Zicsr extension for control and status register instructions,
> +      as ratified in the 20191213 version of the unprivileged ISA specification.
> +
> +  riscv,isa-extension-zifencei:
> +    type: boolean
> +    description:
> +      The standard Zifencei extension for instruction-fetch fence, as ratified
> +      in the 20191213 version of the unprivileged ISA specification.
> +
> +  riscv,isa-extension-zihpm:
> +    type: boolean
> +    description:
> +      The standard Zihpm extension for hardware performance counters, as
> +      ratified in the 20191213 version of the unprivileged ISA specification.
> +
> +  riscv,isa-extension-zicbom:
> +    type: boolean
> +    description:
> +      The standard Zicbom extension for base cache management operations as
> +      ratified in commit 3dd606f ("Create cmobase-v1.0.pdf") of riscv-CMOs.
> +
> +  riscv,isa-extension-zicbop:
> +    type: boolean
> +    description:
> +      The standard Zicbop extension for cache-block prefetch instructions as
> +      ratified in commit 3dd606f ("Create cmobase-v1.0.pdf") of riscv-CMOs.
> +
> +  riscv,isa-extension-zicboz:
> +    type: boolean
> +    description:
> +      The standard  Zicbomz extension for cache-block zeroing as ratified in
                     ^ ^Zicboz
                     ^ extra space

(The repetition is making my vision blur, so I'm feeling like I should
write a script to compare $a and $b, where $a is riscv,isa-extension-$a
and $b is 'The standard $b' to make sure they match :-) But I probably
won't...

> +      commit 3dd606f ("Create cmobase-v1.0.pdf") of riscv-CMOs.
> +
> +  riscv,isa-extension-zihintpause:
> +    type: boolean
> +    description: |
> +      The standard Zihintpause extension for pause hints, as ratified in
> +      commit d8ab5c7 ("Zihintpause is ratified") of the riscv-isa-manual.
> +
> +  riscv,isa-extension-zba:
> +    type: boolean
> +    description: |
> +      The standard Zba bit-manipulation extension for address generation
> +      acceleration instructions as ratified at commit 6d33919 ("Merge pull
> +      request #158 from hirooih/clmul-fix-loop-end-condition") of
> +      riscv-bitmanip.
> +
> +  riscv,isa-extension-zbb:
> +    type: boolean
> +    description: |
> +      The standard Zbb bit-manipulation extension for basic bit-manipulation as
> +      atified at commit 6d33919 ("Merge pull request #158 from
         ^ ratified

> +      hirooih/clmul-fix-loop-end-condition") of riscv-bitmanip.
> +
> +  riscv,isa-extension-zbc:
> +    type: boolean
> +    description: |
> +      The standard Zbc bit-manipulation extension for carry-less multiplication
> +      as ratified at commit 6d33919 ("Merge pull request #158 from
> +      hirooih/clmul-fix-loop-end-condition") of riscv-bitmanip.
> +
> +  riscv,isa-extension-zbs:
> +    type: boolean
> +    description: |
> +      The standard Zbs bit-manipulation extension for single-bit instructions
> +      as ratified at commit 6d33919 ("Merge pull request #158 from
> +      hirooih/clmul-fix-loop-end-condition") of riscv-bitmanip.
> +
> +  riscv,isa-extension-ztso:
> +    type: boolean
> +    description:
> +      The standard Ztso extension for total store ordering, as ratified in
> +      commit 2e5236 ("Ztso is now ratified.") of the riscv-isa-manual.
> +
> + # Standard Supervisor-level Extensions, sorted by category then alphabetically

The spec only says alphabetical sorting for supervisor-level extensions,
no category.

> +
> +  'riscv,isa-extension-smaia':
> +    type: boolean
> +    description: |
> +      The standard Smaia supervisor-level extension for the advanced interrupt
> +      architecture for machine-mode-visible csr and behavioural changes to
> +      interrupts as frozen at commit ccbddab ("Merge pull request #42 from
> +      riscv/jhauser-2023-RC4") of riscv-aia.
> +
> +  riscv,isa-extension-ssaia:
> +    type: boolean
> +    description: |
> +      The standard Ssaia supervisor-level extension for the advanced interrupt
> +      architecture for supervisor-mode-visible csr and behavioural changes to
> +      interrupts as frozen at commit ccbddab ("Merge pull request #42 from
> +      riscv/jhauser-2023-RC4") of riscv-aia.
> +
> +  riscv,isa-extension-sscofpmf:
> +    type: boolean
> +    description: |
> +      The standard supervisor-level extension for count overflow and mode-based
                     ^ Sscofpmf

> +      filtering as ratified at commit 01d1df0 ("Add ability to manually trigger
> +      workflow. (#2)") of riscv-count-overflow.
> +
> +  riscv,isa-extension-sstc:
> +    type: boolean
> +    description:
> +      The standard supervisor-level extension for time compare
                     ^ Sstc

> +      as ratified at commit 3f9ed34 ("Add ability to manually trigger
> +      workflow. (#2)") of riscv-time-compare.
> +
> +  riscv,isa-extension-svinval:
> +    type: boolean
> +    description:
> +      The standard Svinval supervisor-level extension for fine-grained
> +      address-translation cache invalidation as ratified in the 20191213 version
> +      of the privileged ISA specification.
> +
> +  riscv,isa-extension-svnapot:
> +    type: boolean
> +    description:
> +      The standard Svnapot supervisor-level extensions for napot translation
> +      contiguity as ratified in the 20191213 version of the privileged ISA
> +      specification.
> +
> +  riscv,isa-extension-svpbmt:
> +    type: boolean
> +    description:
> +      The standard Svpbmt supervisor-level extensions for page-based memory
> +      types as ratified in the 20191213 version of the privileged ISA
> +      specification.
> +
> +additionalProperties: true
> +...
> -- 
> 2.39.2
>

I'll take your word for it on the versions/dates/commit hashes referenced,
at least until we get closer to actually merging this.

Thanks,
drew