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

[Conor Dooley <conor@kernel.org>]

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On Fri, Apr 12, 2024 at 10:12:20AM -0700, Charlie Jenkins wrote:
> 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.

I think you've kinda missed the point about what the original code was
actually doing here. Really the kernel should not be doing validation of
the devicetree at all, but I was trying to avoid people shooting
themselves in the foot by doing something simple that would work for
their (incorrect) vendor dtbs.
Future heterogenous systems should be using riscv,isa-extensions, which
is totally unaffected by this codepath (and setting actual values for
mimpid/marchid too ideally!).

> > 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.

That was a braino, clearly I should have typed "mimpid".

> 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.

There are already is an option. Use the non-deprecated property in your
new system for describing what extesions you support. We don't need to
add any more properties (for now at least).

> 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;
> > > +			}
> > 
> 
> 

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