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

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Message-ID: <Yx8upeabh9p6gGtY@shell.armlinux.org.uk>
Date:   Mon, 12 Sep 2022 14:05:41 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     George Pee <georgepee@...il.com>,
        Robin Murphy <robin.murphy@....com>,
        "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 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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