linux-kernel - Re: [PATCH 14/19] riscv: hwprobe: Disambiguate vector and xtheadvector in hwprobe

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Message-ID: <Zhm3nflMIuue2/RS@ghost>
Date: Fri, 12 Apr 2024 15:37:17 -0700
From: Charlie Jenkins <charlie@...osinc.com>
To: Evan Green <evan@...osinc.com>
Cc: Conor Dooley <conor.dooley@...rochip.com>,
	Conor Dooley <conor@...nel.org>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Guo Ren <guoren@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, Chen-Yu Tsai <wens@...e.org>,
	Jernej Skrabec <jernej.skrabec@...il.com>,
	Samuel Holland <samuel@...lland.org>,
	Clément Léger <cleger@...osinc.com>,
	Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>,
	linux-riscv@...ts.infradead.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, Palmer Dabbelt <palmer@...osinc.com>,
	linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev,
	linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 14/19] riscv: hwprobe: Disambiguate vector and
 xtheadvector in hwprobe

On Fri, Apr 12, 2024 at 03:08:31PM -0700, Evan Green wrote:
> On Fri, Apr 12, 2024 at 11:22 AM Charlie Jenkins <charlie@...osinc.com> wrote:
> >
> > On Fri, Apr 12, 2024 at 10:04:42AM -0700, Evan Green wrote:
> > > On Fri, Apr 12, 2024 at 4:35 AM Conor Dooley <conor.dooley@...rochip.com> wrote:
> > > >
> > > > On Thu, Apr 11, 2024 at 09:11:20PM -0700, Charlie Jenkins wrote:
> > > > > Ensure that hwprobe does not flag "v" when xtheadvector is present.
> > > > >
> > > > > Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
> > > > > ---
> > > > >  arch/riscv/kernel/sys_hwprobe.c | 4 ++--
> > > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/arch/riscv/kernel/sys_hwprobe.c b/arch/riscv/kernel/sys_hwprobe.c
> > > > > index 8cae41a502dd..e0a42c851511 100644
> > > > > --- a/arch/riscv/kernel/sys_hwprobe.c
> > > > > +++ b/arch/riscv/kernel/sys_hwprobe.c
> > > > > @@ -69,7 +69,7 @@ static void hwprobe_isa_ext0(struct riscv_hwprobe *pair,
> > > > >       if (riscv_isa_extension_available(NULL, c))
> > > > >               pair->value |= RISCV_HWPROBE_IMA_C;
> > > > >
> > > > > -     if (has_vector())
> > > > > +     if (has_vector() && !riscv_has_vendor_extension_unlikely(RISCV_ISA_VENDOR_EXT_XTHEADVECTOR))
> > > >
> > > > Hmm, I think this is "dangerous". has_vector() is used across the kernel
> > > > now in several places for the in-kernel vector. I don't think that
> > > > has_vector() should return true for the T-Head stuff given that &
> > > > has_vector() should represent the ratified spec. I'll have to think
> > > > about this one and how nasty this makes any of the save/restore code
> > > > etc.
> > >
> > > Yeah, my nose crinkled here as well. If you're having to do a
> > > vendorish thing in this generic spot, then others may too, suggesting
> > > perhaps this isn't the cleanest way to go about it. Ideally extensions
> > > are all additive, rather than subtractive, I guess?
> >
> > This was the "easiest" way to support this but I agree this is not
> > ideal. The vector code is naturally coupled with having support for
> > "v" and I wanted to leverage that. The other concern is all of the
> > ifdefs for having V enabled. I can make all of those V or XTHEADVECTOR;
> > that will increase the surface area of xtheadvector but it is probably
> > the right(?) way to go.
> 
> For the ifdefs, if you've got a Kconfig somewhere for THEAD_VECTOR,
> can't that just depend on the V config? We'd end up with the
> limitation that you can't add V 0.7 support without also dragging in
> V1.0 support, but that's probably fine, right?

That's a great idea, thank you for the suggestion.

- Charlie

> 
> -Evan