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

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Date:   Fri, 9 Sep 2022 09:57:39 -0500
From:   George Pee <georgepee@...il.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Russell King <linux@...linux.org.uk>,
        "Russell King (Oracle)" <rmk+kernel@...linux.org.uk>,
        "Kirill A. Shutemov" <kirill.shtuemov@...ux.intel.com>,
        Austin Kim <austindh.kim@...il.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Mike Rapoport <rppt@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Report support for optional ARMv8.2 half-precision
 floating point extension

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?

On Fri, Sep 9, 2022 at 9:07 AM Catalin Marinas <catalin.marinas@....com> wrote:
>
> On Fri, Sep 09, 2022 at 08:34:26AM -0500, George Pee wrote:
> > 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.
>
> Ah, I wasn't aware that the feature doesn't work on arm32. I don't think
> it makes sense to expose a hwcap bit to user in this case.
>
> --
> Catalin