From: Conor Dooley <conor.dooley@microchip.com>
To: "Clément Léger" <cleger@rivosinc.com>
Cc: Conor Dooley <conor@kernel.org>, Jonathan Corbet <corbet@lwn.net>,
Paul Walmsley <paul.walmsley@sifive.com>,
Palmer Dabbelt <palmer@dabbelt.com>,
Albert Ou <aou@eecs.berkeley.edu>, Rob Herring <robh@kernel.org>,
Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
Anup Patel <anup@brainfault.org>, Shuah Khan <shuah@kernel.org>,
Atish Patra <atishp@atishpatra.org>, <linux-doc@vger.kernel.org>,
<linux-riscv@lists.infradead.org>, <linux-kernel@vger.kernel.org>,
<devicetree@vger.kernel.org>, <kvm@vger.kernel.org>,
<kvm-riscv@lists.infradead.org>,
<linux-kselftest@vger.kernel.org>
Subject: Re: [PATCH v2 03/12] dt-bindings: riscv: add Zc* extension rules implied by C extension
Date: Mon, 22 Apr 2024 12:19:36 +0100 [thread overview]
Message-ID: <20240422-stumbling-aliens-b408eebe1f32@wendy> (raw)
In-Reply-To: <f89c79f7-a09e-4fcf-8e16-0875202ade4a@rivosinc.com>
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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?
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next prev parent reply other threads:[~2024-04-22 11:20 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-18 12:42 [PATCH v2 00/12] Add support for a few Zc* extensions as well as Zcmop Clément Léger
2024-04-18 12:42 ` [PATCH v2 01/12] dt-bindings: riscv: add Zca, Zcf, Zcd and Zcb ISA extension description Clément Léger
2024-04-19 15:24 ` Conor Dooley
2024-04-18 12:42 ` [PATCH v2 02/12] riscv: dts: enable Zc* extensions when needed Clément Léger
2024-04-19 15:55 ` Conor Dooley
2024-04-18 12:42 ` [PATCH v2 03/12] dt-bindings: riscv: add Zc* extension rules implied by C extension Clément Léger
2024-04-19 15:49 ` Conor Dooley
2024-04-22 8:53 ` Clément Léger
2024-04-22 11:19 ` Conor Dooley [this message]
2024-04-22 11:40 ` Clément Léger
2024-04-18 12:42 ` [PATCH v2 04/12] riscv: add ISA parsing for Zca, Zcf, Zcd and Zcb Clément Léger
2024-04-19 15:51 ` Conor Dooley
2024-04-22 8:53 ` Clément Léger
2024-04-22 9:35 ` Conor Dooley
2024-04-22 11:14 ` Clément Léger
2024-04-22 11:36 ` Conor Dooley
2024-04-22 11:41 ` Clément Léger
2024-04-18 12:42 ` [PATCH v2 05/12] riscv: hwprobe: export Zca, Zcf, Zcd and Zcb ISA extensions Clément Léger
2024-04-18 12:42 ` [PATCH v2 06/12] RISC-V: KVM: Allow Zca, Zcf, Zcd and Zcb extensions for Guest/VM Clément Léger
2024-04-18 13:20 ` Anup Patel
2024-04-18 12:42 ` [PATCH v2 07/12] KVM: riscv: selftests: Add some Zc* extensions to get-reg-list test Clément Léger
2024-04-18 13:20 ` Anup Patel
2024-04-18 12:42 ` [PATCH v2 08/12] dt-bindings: riscv: add Zcmop ISA extension description Clément Léger
2024-04-19 15:50 ` Conor Dooley
2024-04-18 12:42 ` [PATCH v2 09/12] riscv: add ISA extension parsing for Zcmop Clément Léger
2024-04-18 12:42 ` [PATCH v2 10/12] riscv: hwprobe: export Zcmop ISA extension Clément Léger
2024-04-18 12:42 ` [PATCH v2 11/12] RISC-V: KVM: Allow Zcmop extension for Guest/VM Clément Léger
2024-04-18 13:21 ` Anup Patel
2024-04-18 12:42 ` [PATCH v2 12/12] KVM: riscv: selftests: Add Zcmop extension to get-reg-list test Clément Léger
2024-04-18 13:21 ` Anup Patel
2024-04-18 13:28 ` [PATCH v2 00/12] Add support for a few Zc* extensions as well as Zcmop Anup Patel
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