[EXT] Re: [PATCH v4 2/3] config/arm: add support for fallback march

[Pavan Nikhilesh Bhagavatula]

> On Wed, Feb 21, 2024 at 9:20 PM <pbhagavatula at marvell.com> wrote:
> >
> > From: Pavan Nikhilesh <pbhagavatula at 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 at marvell.com>
> > ---
> >  config/arm/meson.build | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/config/arm/meson.build b/config/arm/meson.build
> > index e77b696d8e..f6521653c8 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"'],
> > @@ -716,6 +717,7 @@ if update_flags
> >
> >      # probe supported archs and their features
> >      candidate_march = ''
> > +    fallback_march = ''
> >      if part_number_config.has_key('march') and candidate_mcpu == ''
> >          if part_number_config.get('force_march', false)
> >              if cc.has_argument('-march=' +  part_number_config['march'])
> > @@ -736,10 +738,18 @@ if update_flags
> >                      # 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
> > +                endif
> 
> I'm trying to wrap my head around this. If I understand this
> correctly, fallback_march is going to be set only if fallback_march >
> part_number_config['march'] (fallback_march must be higher than the
> one in part_number_config, otherwise the loop will end with break
> before setting fallback_march).
> 

Fallback_march will always be < part_number_config['march'].
If part_number_config['march'] is supported by the compiler then we don’t need
fallback march.

> The added fallback_march configuration above is the opposite
> (fallback_march < part_number_config['march']) so I'm not sure what
> we're trying to accomplish here. Is the intention to specify a
> fallback_march that's lower than the one the original fallback
> mechanism would find? If so, we'll need to remove the break

I missed the break part, will add that in along with candidate_march == '' check.

> (and
> probably set check_compiler_support to false) so that the loop could
> continue.
> 
> >              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
> >