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

[George Pee]

On Mon, Sep 12, 2022 at 8:05 AM Russell King (Oracle)
<linux@xxxxxxxxxxxxxxx> wrote:
>
> 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?
>
> --
> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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I didn't see this until after I submitted v2 of the patch.  Let me
take a look at the fp emulation code path.
I had assumed that CP9 handling would work just like CP10/CP11 does in
entry-armv.S and wouldn't need any special handling.