linux-kernel - Re: [PATCH v2 2/8] riscv: Add PLATFORM_MAY_SUPPORT_RISCV_ISA

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Date: Fri, 10 May 2024 23:26:22 +0100
From: Conor Dooley <conor@...nel.org>
To: Charlie Jenkins <charlie@...osinc.com>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Conor Dooley <conor.dooley@...rochip.com>,
	Song Liu <song@...nel.org>, Xi Wang <xi.wang@...il.com>,
	Björn Töpel <bjorn@...osinc.com>,
	Clément Léger <cleger@...osinc.com>,
	Jessica Clarke <jrtc27@...c27.com>,
	Andy Chiu <andy.chiu@...ive.com>, linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/8] riscv: Add PLATFORM_MAY_SUPPORT_RISCV_ISA_V
 Kconfig option

On Fri, May 10, 2024 at 02:43:10PM -0700, Charlie Jenkins wrote:
> On Fri, May 10, 2024 at 09:43:33PM +0100, Conor Dooley wrote:
> > Hey Charlie,
> > 
> > On Tue, May 07, 2024 at 06:36:28PM -0700, Charlie Jenkins wrote:
> > > Current versions of the kernel add "v" to the march and then immeidately
> > > filter it out such that "v" is not passed to CFLAGS.  Instead of doing
> > > this filtering, code blocks in the kernel that want to use vector
> > > assembly have been changed to locally enable vector (using ".option
> > > arch, +v").
> > 
> > Other content in the series aside, since this is a change that could be
> > made independently of the main series objectives, I figured it was worth
> > pointing out that this is not a change without downsides: I think that
> > it would drop support for vector with most versions of LLVM as
> > .option arch support there is much more recent thing than it is for gcc.
> > Off the top of my head I don't know exactly the versions involved, but
> > it is something like LLVM-14 supports vector but only LLVM-17 and later
> > supports .option arch.
> 
> Toolchain incompatibilities are always fun. It does look like .option
> arch was introduced in LLVM-17. That would be a regression. We do use
> .option arch for every other extension, but vector was treated special
> when it was introduced unfortunately so maybe we have to live with the
> weird march parsing hack.

I wrote out a long message about the history of some of this, but right
at the end I was scrolling through my chat logs with Andy and realised
we actually did make it depend on AS_HAS_OPTION_ARCH, so it should be
safe to do without regressing anything. I didn't notice in the diff that
the AS_HAS_OPTION_ARCH was a movement, not an addition. Maybe Andy knows
why, despite the dependency on the assembler having it, we didn't use it
everywhere.


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