Re: [PATCH 02/19] riscv: cpufeature: Fix thead vector hwcap removal

Charlie Jenkins
2024-04-12T17:12:38Z

On Fri, Apr 12, 2024 at 11:25:47AM +0100, Conor Dooley wrote:
> On Thu, Apr 11, 2024 at 09:11:08PM -0700, Charlie Jenkins wrote:
> > The riscv_cpuinfo struct that contains mvendorid and marchid is not
> > populated until all harts are booted which happens after the DT parsing.
> > Use the vendorid/archid values from the DT if available or assume all
> > harts have the same values as the boot hart as a fallback.
> >
> > Fixes: d82f32202e0d ("RISC-V: Ignore V from the riscv,isa DT property on older T-Head CPUs")
>
> If this is our only use case for getting the mvendorid/marchid stuff
> from dt, then I don't think we should add it. None of the devicetrees
> that the commit you're fixing here addresses will have these properties
> and if they did have them, they'd then also be new enough to hopefully
> not have "v" either - the issue is they're using whatever crap the
> vendor shipped.

Yes, the DT those shipped with will not have the property in the DT so
will fall back on the boot hart. The addition of the DT properties allow
future heterogenous systems to be able to function.

> If we're gonna get the information from DT, we already have something
> that we can look at to perform the disable as the cpu compatibles give
> us enough information to make the decision.
>
> I also think that we could just cache the boot CPU's marchid/mvendorid,
> since we already have to look at it in riscv_fill_cpu_mfr_info(), avoid
> repeating these ecalls on all systems.

Yeah that is a minor optimization that can I can apply.

>
> Perhaps for now we could just look at the boot CPU alone? To my
> knowledge the systems that this targets all have homogeneous
> marchid/mvendorid values of 0x0.

They have an mvendorid of 0x5b7.

This is already falling back on the boot CPU, but that is not a solution
that scales. Even though all systems currently have homogenous
marchid/mvendorid I am hesitant to assert that all systems are
homogenous without providing an option to override this. The overhead is
looking for a field in the DT which does not seem to be impactful enough
to prevent the addition of this option.

>
> > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
>
> > @@ -514,12 +521,23 @@ static void __init riscv_fill_hwcap_from_isa_string(unsigned long *isa2hwcap)
> > pr_warn("Unable to find \"riscv,isa\" devicetree entry\n");
> > continue;
> > }
> > + if (of_property_read_u64(node, "riscv,vendorid", &this_vendorid) < 0) {
> > + pr_warn("Unable to find \"riscv,vendorid\" devicetree entry, using boot hart mvendorid instead\n");
>
> This should 100% not be a warning, it's not a required property in the
> binding.

Yes definitely, thank you.

- Charlie

>
> Cheers,
> Conor.
>
> > + this_vendorid = boot_vendorid;
> > + }