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

Russell King (Oracle)
2022-09-12T13:05:58Z

On Fri, Sep 09, 2022 at 04:05:53PM +0100, Catalin Marinas wrote:
> On Fri, Sep 09, 2022 at 09:57:39AM -0500, George Pee wrote:
> > The details are here. I originally thought it was a compiler bug
> > because it first showed up after a toolchain update.
> >
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106763
> >
> > Since FP16 is an optional extension, wouldn't it be beneficial to a
> > user who compiled some userspace float16 code using gcc
> > -mcpu=cortex-a55 which ran on a cortex-a55 with FP16 extensions but
> > SIGILL'd on a cortex-a55 w/o FP16?
>
> (please don't top-post)
>
> My point is that if the kernel doesn't have full support for FP16, it
> shouldn't advertise it to user even if the hardware supports it. If you
> fix the kernel to properly handle FP16 on supporting hardware, then the
> HWCAP part is fine by me.

Presumably, the only CPUs that are going to support FP16 will have
non-trapping floating point, so the support code shouldn't be entered
at any time to emulate a half-precision instruction, but only to
handle the lazy restore of the thread's floating point registers?

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