RE: [EXT] Re: [PATCH v5 2/3] config/arm: add support for fallback march

[Pavan Nikhilesh Bhagavatula <pbhagavatula@marvell.com>]

> On Thu, Feb 22, 2024 at 1:45 PM <pbhagavatula@marvell.com> wrote:
> >
> > From: Pavan Nikhilesh <pbhagavatula@marvell.com>
> >
> > Some ARM CPUs have specific march requirements and
> > are not compatible with the supported march list.
> > Add fallback march in case the mcpu and the march
> > advertised in the part_number_config are not supported
> > by the compiler.
> >
> > Example
> >         mcpu = neoverse-n2
> >         march = armv9-a
> >         fallback_march = armv8.5-a
> >
> >         mcpu, march not supported
> >         machine_args = ['-march=armv8.5-a']
> >
> >         mcpu, march, fallback_march not supported
> >         least march supported = armv8-a
> >
> >         machine_args = ['-march=armv8-a']
> >
> > Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
> > ---
> >  config/arm/meson.build | 14 +++++++++++---
> >  1 file changed, 11 insertions(+), 3 deletions(-)
> >
> > diff --git a/config/arm/meson.build b/config/arm/meson.build
> > index d05d54b564..87ff5039f6 100644
> > --- a/config/arm/meson.build
> > +++ b/config/arm/meson.build
> > @@ -94,6 +94,7 @@ part_number_config_arm = {
> >      '0xd49': {
> >          'march': 'armv9-a',
> >          'march_features': ['sve2'],
> > +        'fallback_march': 'armv8.5-a',
> >          'mcpu': 'neoverse-n2',
> >          'flags': [
> >              ['RTE_MACHINE', '"neoverse-n2"'],
> > @@ -708,6 +709,7 @@ if update_flags
> >
> >      candidate_mcpu = ''
> >      candidate_march = ''
> > +    fallback_march = ''
> >
> >      if part_number_config.has_key('mcpu') and
> >         cc.has_argument('-mcpu=' + part_number_config['mcpu'])
> > @@ -736,16 +738,22 @@ if update_flags
> >                      # start checking from this version downwards
> >                      check_compiler_support = true
> >                  endif
> > -                if (check_compiler_support and
> > +                if (check_compiler_support and candidate_march == '' and
> >                      cc.has_argument('-march=' + supported_march))
> >                      candidate_march = supported_march
> > -                    # highest supported march version found
> > -                    break
> > +                endif
> > +                if (part_number_config.has_key('fallback_march') and
> > +                    supported_march == part_number_config['fallback_march'] and
> > +                    cc.has_argument('-march=' + supported_march))
> > +                    fallback_march = supported_march
> 
> If both fallback_march and march are supported, fallback_march is
> going to be chosen over march.

The current flow still chooses the march

Message: Arm implementer: Arm
Message: Arm part number: 0xd49
Compiler for C supports arguments -march=armv9-a: YES 
Compiler for C supports arguments -march=armv8.5-a: YES 
Compiler for C supports arguments -march=armv9-a+sve2: YES 
Compiler for C supports arguments -march=armv9-a+sve2+crypto: YES 
Message: Using machine args: ['-march=armv9-a+sve2+crypto']


> I think this is what we want instead:
> Use march if supported,
> then use fallback_march if supported,
> then use the other fallback marchs.
> 
> If the above is indeed what we want, we could just put this after
> endforeach (in the original version):
> 
> # at this point, candidate march is either
> part_number_config['fallback_march'] or some other lower version
> if (part_number_config.has_key('fallback_march') and
>     candidate_march != part_number_config['march'] and
>     cc.has_argument('-march=' + part_number_config['fallback_march']))
>     # this overwrites only the lower version with preferred
> fallback_march, if supported
>     candidate_march = part_number_config['fallback_march']
> endif
> 
> This way we won't even need the fallback_march variable.

Sure, I will change remove fallback_march variable, Thanks.

> 
> >                  endif
> >              endforeach
> >          endif
> >
> >          if candidate_march != part_number_config['march']
> > +            if fallback_march != ''
> > +                candidate_march = fallback_march
> > +            endif
> >              warning('Configuration march version is @0@, not supported.'
> >                      .format(part_number_config['march']))
> >              if candidate_march != ''
> > --
> > 2.25.1
> >