linux-kernel - Re: [PATCH 3/4] arm64/fp: Clarify effect of setting an unsupported system VL

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Date: Wed, 24 Jan 2024 14:09:51 +0000
From: Dave Martin <Dave.Martin@....com>
To: Mark Brown <broonie@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Jonathan Corbet <corbet@....net>,
	linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Edmund Grimley-Evans <edmund.grimley-evans@....com>
Subject: Re: [PATCH 3/4] arm64/fp: Clarify effect of setting an unsupported
 system VL

On Tue, Jan 23, 2024 at 06:42:03PM +0000, Mark Brown wrote:
> On Tue, Jan 23, 2024 at 03:49:27PM +0000, Dave Martin wrote:
> > On Mon, Jan 22, 2024 at 08:41:53PM +0000, Mark Brown wrote:
> 
> > >  /proc/sys/abi/sme_default_vector_length
> > >  
> > >      Writing the text representation of an integer to this file sets the system
> > > -    default vector length to the specified value, unless the value is greater
> > > -    than the maximum vector length supported by the system in which case the
> > > -    default vector length is set to that maximum.
> > > +    default vector length to the specified value rounded to a supported value
> > > +    using the same rules as for setting vector length via prctl().
> 
> > Do parallel changes need to be made in sve.rst?
> 
> They are, in this very patch?

Duh, yes.  My brain seems to have auto-ignored the second hunk, since it
was clearly a duplicate :P

> > (There seems to be so much duplication and copy-paste between these
> > files that I wonder whether it would make sense to merge them...  but
> > that's probably a separate discussion.)
> 
> Indeed, thanks for volunteering.  Note that there are differences
> resulting from specification differences.

Thanks for agreeing to an unspecfied deadline ;)

I might have a go at some point though, just to familiarise myself with
the differences...

> > Nit: is it better to name the prctl here than just to say prctl()?
> > That would be easier for the reader to cross-reference.
> 
> I guess, though it doesn't seem entirely idiomatic.

I expect counterexamples can be found, but I guess the reader can figure
it out either way.

Cheers
---Dave