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

[Charlie Jenkins <charlie@rivosinc.com>]

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@rivosinc.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@rivosinc.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