[RFC 0/6] Deprecate riscv,isa DT property?

[Palmer Dabbelt]

On Fri, 12 May 2023 15:05:24 PDT (-0700), Conor Dooley wrote:
> +CC Greg, Mark, Krste, Philipp, Andrew,
>
> (this is LKML now, no top posting or html replies)
>
> On Fri, May 12, 2023 at 08:40:10PM +0100, Conor Dooley wrote:
>> On Fri, May 12, 2023 at 11:01:09AM -0700, Palmer Dabbelt wrote:
>> > On Thu, 11 May 2023 15:38:10 PDT (-0700), Conor Dooley wrote:
>> > > On Thu, May 11, 2023 at 03:34:24PM -0700, Atish Patra wrote:
>> > > > On Thu, May 11, 2023 at 2:47 PM Conor Dooley <conor at kernel.org> wrote:
>> > > > > On Thu, May 11, 2023 at 02:27:44PM -0700, Atish Patra wrote:
>> > > 
>> > > > > That's more than last year at this point, and nothing has changed in the
>> > > > > documentation! Talk's cheap, ehh?
>> > > > >
>> > > > 
>> > > > Yup. I will poke RVI folks to check if it still is the plan or changed !!
>> > > 
>> > > Sounds good, thanks!
>> 
>> There has been some movement on that front, shall see where it goes
>> :upsidedown_smile:
>
> There's been some off-list discussion prompted by Atish with some of the
> RVI spec folk, from which the upshot __appears__ to be an understanding
> that using version numbering to indicate removal of ISA features is a bad
> idea.
> I'm hoping that this results in the enshrinement of this in the ISA
> specs, so that we have something concrete to point to as the basis for
> not needing to handle version numbering. 
> Certainly that'd be great for ACPI and remove concerns there.
>
>> > > > We will likely have a vendor specific string parsing logic.
>> > > 
>> > > Complicating the parsing logic is the exact sort of crap that I want
>> > > to avoid.
>> > 
>> > Ya, I think we're reallly overcomplicating things with the ISA strings.
>> > Let's just deprecate them and move to something that doesn't need all the
>> > bespoke string parsing.
>> 
>> Versioning aside, although that removes a large part of the motivation,
>> the interface becomes quite nice:
>> of_property_present(node, "riscv,isa-extension-zicbom")
>
> My current feeling is that, rather than introducing a key-value type of
> property, adding boolean properties for extensions is the way to go
> here. The "riscv,isa" part of the DT ABI pre-dates even the ratification
> of the base extensions (and thus the removal of some features...).
> Starting again with a new property would allow us to define extensions
> as per their ratified state, rather than the intermediate & incompatible
> states that we have currently got with "riscv,isa".
> Such a model does rely on the strengthening of the wording in the
> specification.

IMO the important part is that we encode an exact version (or commit if 
they're going to stop doing versions) of the spec in the DT.  We've 
gotten burned enough times by just trying to point at the latest spec 
and trusting that compatibility will be handled in the specs, in 
practice that doesn't work.

Given how inconsistent the RISC-V version schemes have been, we really 
can't assign any semantic meaning to version numbers.  So I don't think 
it matters all that much if we encode this as

    riscv,$SPEC = ["v1.0", "v1.1"]

or

    riscv,$SPEC = true // with the binding saying v1.0 or $SHA...
    riscv,$SPEC-1.1 = true // with the binding saying v1.1 or $SHA...

Since we can't ascribe any meaning to those version numbers there's 
nothing to parse in them, so it's just going to plumb into some lookup 
table in the kernel either way.  The important part is just that we 
document exactly what spec version we're encoding, as that way we can 
avoid getting burned by these changes again in the future.

> This had the advantage of being, as I mention above, even easier to
> parse in software than the key-value pair business - but also is
> trivially implemented in a dt-binding. What I have been trying to do
> with the validation of the key-value stuff does not appear to be readily
> doable!

IMO that's the most important deciding factor on how these should be 
encoded.  We're not tracking the ISA string any more, so it doesn't 
matter how closely those line up.  We do have a chance to actually 
validate the interface, though, which was a big problem with the ISA 
strings.

>From talking it sounds like the form you're proposing is easier to 
encode in dt-schema than what I'd proposed.  I didn't look at dt-schema 
at all before thinking up the interface, so you're probably right ;).

Assuming that's the case it seems like the way to go as for as I'm 
concerned.

> (Another drawback that has come to mind is that we have no way to
> validate whether mutually exclusive extensions have been added with
> "riscv,isa")
>
>> That also gives us the ability to define what supported vendor
>> extensions actually mean in a dt-binding, which to me is a big win in
>> terms of the aforementioned "wild west".
>
> Vendor extensions etc are oft quoted as one of the strengths of RISC-V,
> and my feeling is that "riscv,isa" is not a mechanism where we can
> easily handle meanings - especially for vendor stuff where there is
> otherwise no centralised location for _xfoo -> feature mappings.

IMO there's not any fundamental difference: it's not like the standard 
extensions have any meaningful naming/versioning scheme, so it's still 
all just lookup tables.

We do get the same benefits from schema validation that we'd get for 
standard extensions, though.  That's probably a way bigger win for 
vendor extensions, as it'll close a big loophole in our DT validation 
right now where users can cram arbitrary stuff into "riscv,isa" and then 
we have to just deal with it.

> Cheers,
> Conor.