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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