Re: [PATCH v2 03/12] dt-bindings: riscv: add Zc* extension rules implied by C extension

["Clément Léger" <cleger@rivosinc.com>]

On 22/04/2024 13:19, Conor Dooley wrote:
> On Mon, Apr 22, 2024 at 10:53:04AM +0200, Clément Léger wrote:
>> On 19/04/2024 17:49, Conor Dooley wrote:
>>> On Thu, Apr 18, 2024 at 02:42:26PM +0200, Clément Léger wrote:
>>>> As stated by Zc* spec:
>>>>
>>>> "As C defines the same instructions as Zca, Zcf and Zcd, the rule is that:
>>>>  - C always implies Zca
>>>>  - C+F implies Zcf (RV32 only)
>>>>  - C+D implies Zcd"
>>>>
>>>> Add additionnal validation rules to enforce this in dts.
>>>
>>> I'll get it out of the way: NAK, and the dts patch is the perfect
>>> example of why. I don't want us to have to continually update
>>> devicetrees. If these are implied due to being subsets of other
>>> extensions, then software should be able to enable them when that
>>> other extension is present.
>>
>> Acked.
>>
>>>
>>> My fear is that, and a quick look at the "add probing" commit seemed to
>>> confirm it, new subsets would require updates to the dts, even though
>>> the existing extension is perfectly sufficient to determine presence.
>>>
>>> I definitely want to avoid continual updates to the devicetree for churn
>>> reasons whenever subsets are added, but not turning on the likes of Zca
>>> when C is present because "the bindings were updated to enforce this"
>>> is a complete blocker. I do concede that having two parents makes that
>>> more difficult and will likely require some changes to how we probe - do
>>> we need to have a "second round" type thing?
>>
>> Yeah, I understand. At first, I actually did the modifications in the
>> ISA probing loop with some dependency probing (ie loop while we don't
>> have a stable extension state). But I thought that it was not actually
>> our problem but rather the ISA string provider. For instance, Qemu
>> provides them.
> 
> 
> A newer version of QEMU might, but not all do, so I'm not sure that using
> it is a good example. My expectations is that a devicetree will be written
> to the standards of the day and not be updated as subsets are released.
> 
> If this were the first instance of a superset/bundle I'd be prepared to
> accept an argument that we should not infer anything - but it's not and
> we'd be introducing inconsistency with the crypto stuff. I know that both
> scenarios are different in terms of extension history given that this is
> splitting things into a subset and that was a superset/bundle created at
> the same time, but they're not really that different in terms of the
> DT/ACPI to user "interface".
> 
>>> Taking Zcf as an example, maybe something like making both of C and F into
>>> "standard" supersets and adding a case to riscv_isa_extension_check()
>>> that would mandate that Zca and F are enabled before enabling it, and we
>>> would ensure that C implies Zca before it implies Zcf?
>>
>> I'm afraid that riscv_isa_extension_check() will become a rat nest so
>> rather than going that way, I would be in favor of adding a validation
>> callback for the extensions if needed.
> 
> IOW, extension check split out per extension moving to be a callback?
> 
>>> Given we'd be relying on ordering, we have to perform the same implication
>>> for both F and C and make sure that the "implies" struct has Zca before Zcf.
>>> I don't really like that suggestion, hopefully there's a nicer way of doing
>>> that, but I don't like the dt stuff here.
>>
>> I guess the "cleanest" way would be to have some "defered-like"
>> mechanism in ISA probing which would allow to handle ordering as well as
>> dependencies/implies for extensions. For Zca, Zcf, we actually do not
>> have ordering problems but I think it would be a bit broken not to
>> support that as well.
> 
> We could, I suppose, enable all detected extensions on a CPU and run the
> aforemention callback, disabling them if conditions are not met?
> 
> Is that something like what you're suggesting?

Yep, exactly. First parse the ISA blindly in a bitmap, (either from
riscv,isa string, riscv,isa-extensions, or ACPI). Then in a second time,
verify the ISA extensions by validating extension and looping until we
reach a stable set.

Clément