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Date: Fri, 12 Apr 2024 19:47:48 +0100
From: Conor Dooley <conor@kernel.org>
To: Charlie Jenkins <charlie@rivosinc.com>
Cc: Conor Dooley <conor.dooley@microchip.com>,
	Rob Herring <robh@kernel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>, Guo Ren <guoren@kernel.org>,
	Conor Dooley <conor+dt@kernel.org>, Chen-Yu Tsai <wens@csie.org>,
	Jernej Skrabec <jernej.skrabec@gmail.com>,
	Samuel Holland <samuel@sholland.org>,
	Evan Green <evan@rivosinc.com>,
	=?iso-8859-1?Q?Cl=E9ment_L=E9ger?= <cleger@rivosinc.com>,
	Jonathan Corbet <corbet@lwn.net>, Shuah Khan <shuah@kernel.org>,
	linux-riscv@lists.infradead.org, devicetree@vger.kernel.org,
	linux-kernel@vger.kernel.org, Palmer Dabbelt <palmer@rivosinc.com>,
	linux-arm-kernel@lists.infradead.org, linux-sunxi@lists.linux.dev,
	linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org
Subject: Re: [PATCH 02/19] riscv: cpufeature: Fix thead vector hwcap removal
Message-ID: <20240412-eastcoast-disparity-9c9e7d178df5@spud>
References: <20240411-dev-charlie-support_thead_vector_6_9-v1-0-4af9815ec746@rivosinc.com>
 <20240411-dev-charlie-support_thead_vector_6_9-v1-2-4af9815ec746@rivosinc.com>
 <20240412-tuesday-resident-d9d07e75463c@wendy>
 <ZhlrdGXfSushUNTp@ghost>
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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 parsi=
ng.
> > > 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.
> > >=20
> > > Fixes: d82f32202e0d ("RISC-V: Ignore V from the riscv,isa DT property=
 on older T-Head CPUs")
> >=20
> > 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.
>=20
> 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.
> >=20
> > 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.
>=20
> Yeah that is a minor optimization that can I can apply.
>=20
> >=20
> > 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.
>=20
> 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.
>=20
> >=20
> > > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> >=20
> > > @@ -514,12 +521,23 @@ static void __init riscv_fill_hwcap_from_isa_st=
ring(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, usi=
ng boot hart mvendorid instead\n");
> >=20
> > This should 100% not be a warning, it's not a required property in the
> > binding.
>=20
> Yes definitely, thank you.
>=20
> - Charlie
>=20
> >=20
> > Cheers,
> > Conor.
> >=20
> > > +				this_vendorid =3D boot_vendorid;
> > > +			}
> >=20
>=20
>=20

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