linux-kernel - Re: [PATCH v2 10/10] RISC-V: provide a Kconfig option to disable parsing "riscv,isa"

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Message-ID: <20230629-deceit-macarena-2a744ac70148@wendy>
Date:   Thu, 29 Jun 2023 12:39:51 +0100
From:   Conor Dooley <conor.dooley@...rochip.com>
To:     Andrew Jones <ajones@...tanamicro.com>
CC:     <palmer@...belt.com>, <conor@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Heiko Stuebner <heiko.stuebner@...ll.eu>,
        Evan Green <evan@...osinc.com>,
        Sunil V L <sunilvl@...tanamicro.com>,
        <linux-riscv@...ts.infradead.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Palmer Dabbelt <palmer@...osinc.com>
Subject: Re: [PATCH v2 10/10] RISC-V: provide a Kconfig option to disable
 parsing "riscv,isa"

On Thu, Jun 29, 2023 at 11:31:33AM +0200, Andrew Jones wrote:
> On Thu, Jun 29, 2023 at 09:28:56AM +0100, Conor Dooley wrote:
> > As it says on the tin, provide a Kconfig option to disabling parsing the
> > "riscv,isa" devicetree property. Hide the option behind NONPORTABLE so
> > that only those willing to keep the pieces enable it, and make sure the
> > default kernel contains the fallback code.
> > 
> > Suggested-by: Palmer Dabbelt <palmer@...osinc.com>
> > Signed-off-by: Conor Dooley <conor.dooley@...rochip.com>
> > ---
> >  arch/riscv/Kconfig             | 16 ++++++++++++++++
> >  arch/riscv/kernel/cpu.c        |  3 +++
> >  arch/riscv/kernel/cpufeature.c |  2 +-
> >  3 files changed, 20 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index 1d39efe2b940..0e1909ac5947 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -291,6 +291,22 @@ config NONPORTABLE
> >  
> >  	  If unsure, say N.
> >  
> > +config NO_RISCV_ISA_FALLBACK
> > +	bool "Permit falling back to parsing riscv,isa for extension support"
> > +	depends on NONPORTABLE
> > +	help
> > +	  Parsing the "riscv,isa" devicetree property has been deprecated and
> > +	  replaced by a list of explicitly defined strings. For compatibility
> > +	  with existing platforms, the kernel will fall back to parsing the
> > +	  "riscv,isa" property if the replacements are not found.
> > +
> > +	  Selecting Y here will result in a kernel without this fallback, and
> > +	  will not work on platforms where the devicetree does not contain the
> > +	  replacement 	  properties of "riscv,isa-base" and
>                      ^ spacing issue

Huh, weird. Given the tab followed by spaces, it must have snuck in
during reflow of the text after some rewording.
Wonder how I missed it, given that...

> Should we also have a kernel command line option, 'isa_fallback', where
> without this config the command line option is not necessary to fallback,
> but, with this config, no fallback will be done unless 'isa_fallback' is
> provided?

I don't know, maybe I have the wrong end of the stick but it feels a bit
premature for something that may never not be hidden behind NONPORTABLE?
Perhaps that could be left for a point in time where the default value
of the symbol changes, or the dependency on NONPORTABLE is removed?

Cheers,
Conor.

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