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

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Date:   Fri, 9 Sep 2022 16:05:53 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     George Pee <georgepee@...il.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

On Fri, Sep 09, 2022 at 09:57:39AM -0500, George Pee wrote:
> 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.
> 
> 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.

-- 
Catalin