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From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: George Pee <georgepee@gmail.com>,
	Robin Murphy <robin.murphy@arm.com>,
	"Kirill A. Shutemov" <kirill.shtuemov@linux.intel.com>,
	Austin Kim <austindh.kim@gmail.com>,
	Ard Biesheuvel <ardb@kernel.org>, Mike Rapoport <rppt@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Report support for optional ARMv8.2 half-precision floating point extension
Date: Mon, 12 Sep 2022 14:05:41 +0100	[thread overview]
Message-ID: <Yx8upeabh9p6gGtY@shell.armlinux.org.uk> (raw)
In-Reply-To: <YxtWUasndYl8syE1@arm.com>

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/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!

WARNING: multiple messages have this Message-ID (diff)
From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: George Pee <georgepee@gmail.com>,
	Robin Murphy <robin.murphy@arm.com>,
	"Kirill A. Shutemov" <kirill.shtuemov@linux.intel.com>,
	Austin Kim <austindh.kim@gmail.com>,
	Ard Biesheuvel <ardb@kernel.org>, Mike Rapoport <rppt@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Report support for optional ARMv8.2 half-precision floating point extension
Date: Mon, 12 Sep 2022 14:05:41 +0100	[thread overview]
Message-ID: <Yx8upeabh9p6gGtY@shell.armlinux.org.uk> (raw)
In-Reply-To: <YxtWUasndYl8syE1@arm.com>

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/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

  reply	other threads:[~2022-09-12 13:05 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-01 14:13 [PATCH] Report support for optional ARMv8.2 half-precision floating point extension george pee
2022-09-01 14:13 ` george pee
2022-09-09 11:39 ` Catalin Marinas
2022-09-09 11:39   ` Catalin Marinas
2022-09-09 13:35   ` George Pee
2022-09-09 13:35     ` George Pee
2022-09-09 12:46 ` Robin Murphy
2022-09-09 12:46   ` Robin Murphy
2022-09-09 13:34   ` George Pee
2022-09-09 13:34     ` George Pee
2022-09-09 14:07     ` Catalin Marinas
2022-09-09 14:07       ` Catalin Marinas
2022-09-09 14:57       ` George Pee
2022-09-09 14:57         ` George Pee
2022-09-09 15:05         ` Catalin Marinas
2022-09-09 15:05           ` Catalin Marinas
2022-09-12 13:05           ` Russell King (Oracle) [this message]
2022-09-12 13:05             ` Russell King (Oracle)
2022-09-12 18:09             ` George Pee
2022-09-12 18:09               ` George Pee
2022-09-09 14:17     ` Robin Murphy
2022-09-09 14:17       ` Robin Murphy
2022-09-09 14:54       ` George Pee
2022-09-09 14:54         ` George Pee

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