Re: [PATCH v3] dt-bindings: riscv: deprecate riscv,isa

[Jessica Clarke]

On 26 Jun 2023, at 11:10, Conor Dooley <conor.dooley@xxxxxxxxxxxxx> 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 versioned documents awaiting the reader
> on the "recently ratified extensions" page:
> https://wiki.riscv.org/display/HOME/Recently+Ratified+Extensions
> 
> As an aside, and this is reflected in the patch too, since many
> extensions have yet to appear in a release of the ISA specs,
> they 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~ string array property
> ==========================================
> 
> 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 properties that are defined on a per extension basis,
> 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 individual properties, rather than elements in a single
> string, will also permit validation that the properties have a meaning,
> as well as potentially reject mutually exclusive combinations, or
> enforce dependencies between extensions. That would not have 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 (for
> the brave, or the foolish).
> 
> acpi
> ====
> 
> The current ACPI ECR is based on having a single ISA string unfortunately,
> but ideally ACPI will move to another method, perhaps GUIDs, that give
> explicit meaning to extensions.

Who’s assigning GUIDs to extensions in that world? And I really don’t
want to see DT and ACPI diverge in basics like _describing what ISA
your processor has_.

As a non-Linux OS developer I am unhappy with this churn.

Jess

> parser simplicity
> =================
> 
> Many systems that parse DT at runtime already implement an function that
> can check for the presence of a string in an array of string, as it is
> similar to the process for parsing a list of compatible strings, so a
> bunch of new, custom, DT parsing should not be needed.
> Getting rid of "riscv,isa" parsing would be a nice simplification, but
> unfortunately for backwards compatibility with old dtbs, existing
> parsers may not be removable - which may greatly simplify
> dt parsing code. In Linux, for example, checking for whether a hart
> supports an extension becomes as simple as:
> of_property_match_string(node, "riscv,isa-extensions", "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.
> The new 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 and a new
> riscv,isa-base property to replace the aspect of riscv,isa that is
> not represented by the new property - the base ISA implemented by a hart.
> 
> As a starting point, add properties for extensions currently used in
> Linux.
> 
> Finally, mark riscv,isa as deprecated, as removing support for it in
> existing programs would be an ABI break.
> 
> CC: Palmer Dabbelt <palmer@xxxxxxxxxxx>
> CC: Paul Walmsley <paul.walmsley@xxxxxxxxxx>
> CC: Rob Herring <robh+dt@xxxxxxxxxx>
> CC: Krzysztof Kozlowski <krzysztof.kozlowski+dt@xxxxxxxxxx>
> CC: Alistair Francis <alistair.francis@xxxxxxx>
> CC: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>
> CC: Anup Patel <apatel@xxxxxxxxxxxxxxxx>
> CC: Atish Patra <atishp@xxxxxxxxxxxxxx>
> CC: Jessica Clarke <jrtc27@xxxxxxxxxx>
> CC: Rick Chen <rick@xxxxxxxxxxxxx>
> CC: Leo <ycliang@xxxxxxxxxxxxx>
> CC: Oleksii <oleksii.kurochko@xxxxxxxxx>
> CC: linux-riscv@xxxxxxxxxxxxxxxxxxx
> CC: qemu-riscv@xxxxxxxxxx
> CC: u-boot@xxxxxxxxxxxxx
> CC: devicetree@xxxxxxxxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx
> Reviewed-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
> Acked-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
> Signed-off-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
> ---
> Changes in v3:
> - Per Rob's suggestion, switch to an array of strings. Cuts down on the
>  size, compared to booleans. It has a standard mechanism for parsing
>  (you need to parse arrays of strings for compatibles). It still allows
>  for having a limited set of explicitly defined properties - so the
>  advantages over a free-form string still apply.
> - Pick up Palmer's Ack and Review (although I expect that he will be the
>  one to apply this).
> ---
> .../devicetree/bindings/riscv/cpus.yaml       |  43 ++-
> .../devicetree/bindings/riscv/extensions.yaml | 245 ++++++++++++++++++
> 2 files changed, 265 insertions(+), 23 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 67bd239ead0b..74bc92591086 100644
> --- a/Documentation/devicetree/bindings/riscv/cpus.yaml
> +++ b/Documentation/devicetree/bindings/riscv/cpus.yaml
> @@ -25,6 +25,7 @@ description: |
> 
> allOf:
>   - $ref: /schemas/cpu.yaml#
> +  - $ref: extensions.yaml
> 
> properties:
>   compatible:
> @@ -82,25 +83,6 @@ 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
> -      Zicntr, Zicsr, Zifencei and Zihpm extensions and thus "i"
> -      implies "zicntr_zicsr_zifencei_zihpm".
> -
> -      While the isa strings in ISA specification are case
> -      insensitive, letters in the riscv,isa string must be all
> -      lowercase.
> -    $ref: /schemas/types.yaml#/definitions/string
> -    pattern: ^rv(?:64|32)imaf?d?q?c?b?k?j?p?v?h?(?:[hsxz](?:[a-z])+)?(?:_[hsxz](?:[a-z])+)*$
> -
>   # RISC-V has multiple properties for cache op block sizes as the sizes
>   # differ between individual CBO extensions
>   cache-op-block-size: false
> @@ -139,8 +121,17 @@ properties:
>       DMIPS/MHz, relative to highest capacity-dmips-mhz
>       in the system.
> 
> +oneOf:
> +  - required:
> +      - riscv,isa
> +  - required:
> +      - riscv,isa-base
> +
> +dependencies:
> +  riscv,isa-base: [ "riscv,isa-extensions" ]
> +  riscv,isa-extensions: [ "riscv,isa-base" ]
> +
> required:
> -  - riscv,isa
>   - interrupt-controller
> 
> unevaluatedProperties: false
> @@ -160,7 +151,9 @@ examples:
>                 i-cache-sets = <128>;
>                 i-cache-size = <16384>;
>                 reg = <0>;
> -                riscv,isa = "rv64imac";
> +                riscv,isa-base = "rv64i";
> +                riscv,isa-extensions = "i", "m", "a", "c";
> +
>                 cpu_intc0: interrupt-controller {
>                         #interrupt-cells = <1>;
>                         compatible = "riscv,cpu-intc";
> @@ -183,8 +176,10 @@ examples:
>                 i-tlb-size = <32>;
>                 mmu-type = "riscv,sv39";
>                 reg = <1>;
> -                riscv,isa = "rv64imafdc";
>                 tlb-split;
> +                riscv,isa-base = "rv64i";
> +                riscv,isa-extensions = "i", "m", "a", "f", "d", "c";
> +
>                 cpu_intc1: interrupt-controller {
>                         #interrupt-cells = <1>;
>                         compatible = "riscv,cpu-intc";
> @@ -202,8 +197,10 @@ examples:
>                 device_type = "cpu";
>                 reg = <0>;
>                 compatible = "riscv";
> -                riscv,isa = "rv64imafdc";
>                 mmu-type = "riscv,sv48";
> +                riscv,isa-base = "rv64i";
> +                riscv,isa-extensions = "i", "m", "a", "f", "d", "c";
> +
>                 interrupt-controller {
>                         #interrupt-cells = <1>;
>                         interrupt-controller;
> diff --git a/Documentation/devicetree/bindings/riscv/extensions.yaml b/Documentation/devicetree/bindings/riscv/extensions.yaml
> new file mode 100644
> index 000000000000..af98307f2c2c
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/riscv/extensions.yaml
> @@ -0,0 +1,245 @@
> +# 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@xxxxxxxxxx>
> +  - Palmer Dabbelt <palmer@xxxxxxxxxx>
> +  - Conor Dooley <conor@xxxxxxxxxx>
> +
> +description: |
> +  RISC-V has a large number of extensions, some of which are "standard"
> +  extensions, 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 extension.
> +
> +  Once a standard extension has been ratified, no changes in behaviour can be
> +  made without the creation of a new extension.
> +  The properties for standard extensions therefore map to their originally
> +  ratified states, with the exception of the I, Zicntr & Zihpm extensions.
> +  See the "i" property for more information.
> +
> +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
> +      Zicntr, Zicsr, Zifencei and Zihpm extensions and thus "i"
> +      implies "zicntr_zicsr_zifencei_zihpm".
> +
> +      While the isa strings in ISA specification are case
> +      insensitive, letters in the riscv,isa string must be all
> +      lowercase.
> +    $ref: /schemas/types.yaml#/definitions/string
> +    pattern: ^rv(?:64|32)imaf?d?q?c?b?k?j?p?v?h?(?:[hsxz](?:[a-z])+)?(?:_[hsxz](?:[a-z])+)*$
> +    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-extensions:
> +    $ref: /schemas/types.yaml#/definitions/string-array
> +    minItems: 1
> +    description: Extensions supported by the hart.
> +    items:
> +      anyOf:
> +        # single letter extensions, in canonical order
> +        - const: i
> +          description: |
> +            The base integer instruction set, as ratified in the 20191213
> +            version of the unprivileged ISA specification, with the exception of
> +            counter access.
> +            Counter access was removed after the ratification of the 20191213
> +            version of the unprivileged specification and shunted into the
> +            Zicntr and Zihpm extensions.
> +
> +        - const: m
> +          description:
> +            The standard M extension for integer multiplication and division, as
> +            ratified in the 20191213 version of the unprivileged ISA
> +            specification.
> +
> +        - const: a
> +          description:
> +            The standard A extension for atomic instructions, as ratified in the
> +            20191213 version of the unprivileged ISA specification.
> +
> +        - const: f
> +          description:
> +            The standard F extension for single-precision floating point, as
> +            ratified in the 20191213 version of the unprivileged ISA
> +            specification.
> +
> +        - const: d
> +          description:
> +            The standard D extension for double-precision floating-point, as
> +            ratified in the 20191213 version of the unprivileged ISA
> +            specification.
> +
> +        - const: q
> +          description:
> +            The standard Q extension for quad-precision floating-point, as
> +            ratified in the 20191213 version of the unprivileged ISA
> +            specification.
> +
> +        - const: c
> +          description:
> +            The standard C extension for compressed instructions, as ratified in
> +            the 20191213 version of the unprivileged ISA specification.
> +
> +        - const: v
> +          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.
> +
> +        - const: h
> +          description:
> +            The standard H extension for hypervisors as ratified in the 20191213
> +            version of the privileged ISA specification.
> +
> +        # multi-letter extensions, sorted alphanumerically
> +        - const: smaia
> +          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.
> +
> +        - const: ssaia
> +          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.
> +
> +        - const: sscofpmf
> +          description: |
> +            The standard Sscofpmf supervisor-level extension for count overflow
> +            and mode-based filtering as ratified at commit 01d1df0 ("Add ability
> +            to manually trigger workflow. (#2)") of riscv-count-overflow.
> +
> +        - const: sstc
> +          description: |
> +            The standard Sstc supervisor-level extension for time compare as
> +            ratified at commit 3f9ed34 ("Add ability to manually trigger
> +            workflow. (#2)") of riscv-time-compare.
> +
> +        - const: svinval
> +          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.
> +
> +        - const: svnapot
> +          description:
> +            The standard Svnapot supervisor-level extensions for napot
> +            translation contiguity as ratified in the 20191213 version of the
> +            privileged ISA specification.
> +
> +        - const: svpbmt
> +          description:
> +            The standard Svpbmt supervisor-level extensions for page-based
> +            memory types as ratified in the 20191213 version of the privileged
> +            ISA specification.
> +
> +        - const: zba
> +          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.
> +
> +        - const: zbb
> +          description: |
> +            The standard Zbb bit-manipulation extension for basic bit-manipulation
> +            as ratified at commit 6d33919 ("Merge pull request #158 from
> +            hirooih/clmul-fix-loop-end-condition") of riscv-bitmanip.
> +
> +        - const: zbc
> +          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.
> +
> +        - const: zbs
> +          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.
> +
> +        - const: zicbom
> +          description:
> +            The standard Zicbom extension for base cache management operations as
> +            ratified in commit 3dd606f ("Create cmobase-v1.0.pdf") of riscv-CMOs.
> +
> +        - const: zicbop
> +          description:
> +            The standard Zicbop extension for cache-block prefetch instructions
> +            as ratified in commit 3dd606f ("Create cmobase-v1.0.pdf") of
> +            riscv-CMOs.
> +
> +        - const: zicboz
> +          description:
> +            The standard Zicboz extension for cache-block zeroing as ratified
> +            in commit 3dd606f ("Create cmobase-v1.0.pdf") of riscv-CMOs.
> +
> +        - const: zicntr
> +          description:
> +            The standard Zicntr extension for base counters and timers, as
> +            ratified in the 20191213 version of the unprivileged ISA
> +            specification.
> +
> +        - const: zicsr
> +          description:
> +            The standard Zicsr extension for control and status register
> +            instructions, as ratified in the 20191213 version of the
> +            unprivileged ISA specification.
> +
> +        - const: zifencei
> +          description:
> +            The standard Zifencei extension for instruction-fetch fence, as
> +            ratified in the 20191213 version of the unprivileged ISA
> +            specification.
> +
> +        - const: zihintpause
> +          description:
> +            The standard Zihintpause extension for pause hints, as ratified in
> +            commit d8ab5c7 ("Zihintpause is ratified") of the riscv-isa-manual.
> +
> +        - const: zihpm
> +          description:
> +            The standard Zihpm extension for hardware performance counters, as
> +            ratified in the 20191213 version of the unprivileged ISA
> +            specification.
> +
> +        - const: ztso
> +          description:
> +            The standard Ztso extension for total store ordering, as ratified
> +            in commit 2e5236 ("Ztso is now ratified.") of the
> +            riscv-isa-manual.
> +
> +additionalProperties: true
> +...
> -- 
> 2.40.1
>