dt-bindings: riscv: deprecate riscv,isa

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 elements, rather than 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).

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@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: Oleksii <oleksii.kurochko@gmail.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
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230702-eats-scorebook-c951f170d29f@spud
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

0 files changed, 0 insertions, 0 deletions