linux-kernel - Re: [PATCH 1/1] RISC-V: Add parameter to unaligned access speed

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Message-ID: <ZrEebH2wdjRgfYCB@ghost>
Date: Mon, 5 Aug 2024 11:48:12 -0700
From: Charlie Jenkins <charlie@...osinc.com>
To: Evan Green <evan@...osinc.com>
Cc: Jesse Taube <jesse@...osinc.com>, linux-riscv@...ts.infradead.org,
	Jonathan Corbet <corbet@....net>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Conor Dooley <conor@...nel.org>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Clément Léger <cleger@...osinc.com>,
	Andrew Jones <ajones@...tanamicro.com>,
	Xiao Wang <xiao.w.wang@...el.com>, Andy Chiu <andy.chiu@...ive.com>,
	Eric Biggers <ebiggers@...gle.com>,
	Greentime Hu <greentime.hu@...ive.com>,
	Björn Töpel <bjorn@...osinc.com>,
	Heiko Stuebner <heiko@...ech.de>,
	Costa Shulyupin <costa.shul@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Baoquan He <bhe@...hat.com>, Anup Patel <apatel@...tanamicro.com>,
	Zong Li <zong.li@...ive.com>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Ben Dooks <ben.dooks@...ethink.co.uk>,
	Alexandre Ghiti <alexghiti@...osinc.com>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Erick Archer <erick.archer@....com>,
	Joel Granados <j.granados@...sung.com>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH 1/1] RISC-V: Add parameter to unaligned access speed

On Mon, Aug 05, 2024 at 11:38:23AM -0700, Evan Green wrote:
> On Mon, Aug 5, 2024 at 10:38 AM Jesse Taube <jesse@...osinc.com> wrote:
> >
> > Add a kernel parameter to the unaligned access speed. This allows
> > skiping of the speed tests for unaligned accesses, which often is very
> > slow.
> >
> > Signed-off-by: Jesse Taube <jesse@...osinc.com>
> 
> How come this is a command line parameter rather than a Kconfig
> option? I could be wrong, so I'll lay out my rationale and people can
> pick it apart if I've got a bad assumption.
> 
> I think of commandline parameters as (mostly) something end users
> twiddle with, versus kconfig options as something system builders set
> up. I'd largely expect end users not to notice two ticks at boot time.
> I'd expect its system builders and fleet managers, who know their
> hardware and build their kernels optimized for it, are the ones who
> would want to shave off this time and go straight to the known answer.
> Anecdotally, at ChromeOS we had a strong preference for Kconfig
> options, as they were easier to compose and maintain than a loose pile
> of commandline arguments.
> 
> The commit text doesn't go into the rationale, intended audience, or
> expected usage, so maybe my guesses miss the mark on what you're
> thinking.
> -Evan

There was a brief discussion about this on Jesse's series about vector
unaligned support [1]. The original idea was to use Zicclsm to allow
people to set the unaligned access speed on pre-compiled distro kernels.
However Zicclsm isn't useful so the alternative route was to use a
kernel arg. There is already support for a Kconfig, the kernel arg is
just another option for users.

Link:
https://lore.kernel.org/lkml/af3152b6-adf7-40fa-b2a1-87e66eec45b0@rivosinc.com/
[1]

- Charlie