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

[Conor Dooley <conor.dooley@microchip.com>]

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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@rivosinc.com>
> > Signed-off-by: Conor Dooley <conor.dooley@microchip.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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