Re: [PATCH] Report support for optional ARMv8.2 half-precision floating point extension

[George Pee <georgepee@gmail.com>]

Adding the hwcap was part of the diagnosis process-- I added it just
to make sure that the cpu in question supported the optional
extension.
It seems like it could be useful to be able to check for support in
/proc/cpuinfo.

On Fri, Sep 9, 2022 at 7:46 AM Robin Murphy <robin.murphy@arm.com> wrote:
>
> On 2022-09-01 15:13, george pee wrote:
> > Report as fphp to be consistent with arm64
>
> Wasn't the original problem that the VFP support code doesn't understand
> the new FP16 instruction encodings, so in practice they don't actually
> work reliably? Exposing a hwcap to say they're functional doesn't
> inherently make them functional - if there is already another patch
> somewhere for that, it should be made clear that this depends on it.
>
> Robin.
>
> > Signed-off-by: george pee <georgepee@gmail.com>
> > ---
> >   arch/arm/include/uapi/asm/hwcap.h | 1 +
> >   arch/arm/kernel/setup.c           | 1 +
> >   arch/arm/vfp/vfpmodule.c          | 2 ++
> >   3 files changed, 4 insertions(+)
> >
> > diff --git a/arch/arm/include/uapi/asm/hwcap.h b/arch/arm/include/uapi/asm/hwcap.h
> > index 990199d8b7c6..f975845ce5d3 100644
> > --- a/arch/arm/include/uapi/asm/hwcap.h
> > +++ b/arch/arm/include/uapi/asm/hwcap.h
> > @@ -28,6 +28,7 @@
> >   #define HWCAP_IDIV  (HWCAP_IDIVA | HWCAP_IDIVT)
> >   #define HWCAP_LPAE  (1 << 20)
> >   #define HWCAP_EVTSTRM       (1 << 21)
> > +#define HWCAP_FPHP   (1 << 22)
> >
> >   /*
> >    * HWCAP2 flags - for elf_hwcap2 (in kernel) and AT_HWCAP2
> > diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
> > index 1e8a50a97edf..6694ced0552a 100644
> > --- a/arch/arm/kernel/setup.c
> > +++ b/arch/arm/kernel/setup.c
> > @@ -1249,6 +1249,7 @@ static const char *hwcap_str[] = {
> >       "vfpd32",
> >       "lpae",
> >       "evtstrm",
> > +     "fphp",
> >       NULL
> >   };
> >
> > diff --git a/arch/arm/vfp/vfpmodule.c b/arch/arm/vfp/vfpmodule.c
> > index 2cb355c1b5b7..cef8c64ce8bd 100644
> > --- a/arch/arm/vfp/vfpmodule.c
> > +++ b/arch/arm/vfp/vfpmodule.c
> > @@ -831,6 +831,8 @@ static int __init vfp_init(void)
> >
> >                       if ((fmrx(MVFR1) & 0xf0000000) == 0x10000000)
> >                               elf_hwcap |= HWCAP_VFPv4;
> > +                     if ((fmrx(MVFR1) & 0x0f000000) == 0x03000000)
> > +                             elf_hwcap |= HWCAP_FPHP;
> >               }
> >       /* Extract the architecture version on pre-cpuid scheme */
> >       } else {