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

[Juraj Linkeš]

On Thu, Feb 22, 2024 at 1:45 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 | 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. 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.

>                  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
>