From: Michael Ellerman <mpe@ellerman•id.au>
To: Segher Boessenkool <segher@kernel•crashing.org>
Cc: dan@danny•cz, daniel@octaforge•org,
amd-gfx@lists•freedesktop.org, tpearson@raptorengineering•com,
alexdeucher@gmail•com, linuxppc-dev@lists•ozlabs.org
Subject: Re: [PATCH] powerpc/64: Always build with 128-bit long double
Date: Wed, 05 Apr 2023 15:32:21 +1000 [thread overview]
Message-ID: <87edoyki6y.fsf@mpe.ellerman.id.au> (raw)
In-Reply-To: <20230404151010.GC25951@gate.crashing.org>
Segher Boessenkool <segher@kernel•crashing.org> writes:
> On Tue, Apr 04, 2023 at 08:28:47PM +1000, Michael Ellerman wrote:
>> The amdgpu driver builds some of its code with hard-float enabled,
>> whereas the rest of the kernel is built with soft-float.
>>
>> When building with 64-bit long double, if soft-float and hard-float
>> objects are linked together, the build fails due to incompatible ABI
>> tags.
>
>> Currently those build errors are avoided because the amdgpu driver is
>> gated on 128-bit long double being enabled. But that's not a detail the
>> amdgpu driver should need to be aware of, and if another driver starts
>> using hard-float the same problem would occur.
>
> Well. The kernel driver either has no business using long double (or
> any other floating point even) at all, or it should know exactly what is
> used: double precision, double-double, or quadruple precision. Both of
> the latter two are 128 bits.
In a perfect world ... :)
>> All versions of the 64-bit ABI specify that long-double is 128-bits.
>> However some compilers, notably the kernel.org ones, are built to use
>> 64-bit long double by default.
>
> Mea culpa, I suppose? But builddall doesn't force 64 bit explicitly.
> I wonder how this happened? Is it maybe a problem in the powerpc64le
> config in GCC itself?
Not blaming anyone, just one of those things that happens. The
toolchains the distros (Ubuntu/Fedora) build all seem to use 128, but
possibly that's because someone told them to configure them that way at
some point.
> I have a patch from summer last year (Arnd's
> toolchains are built without it) that does
> + powerpc64le-*) TARGET_GCC_CONF=--with-long-double-128
> Unfortunately I don't remember why I did that, and I never investigated
> what the deeper problem is :-/
Last summer (aka winter) is when we first discovered this issue with the
long double size being implicated.
See:
https://git.kernel.org/torvalds/c/c653c591789b3acfa4bf6ae45d5af4f330e50a91
So I guess that's what prompted your patch?
> In either case, the kernel should always use specific types, not rely on
> the toolchain to pick a type that may or may not work. The correct size
> floating point type alone is not enough, but it is a step in the right
> direction certainly.
>
> Reviewed-by: Segher Boessenkool <segher@kernel•crashing.org>
Thanks.
cheers
next prev parent reply other threads:[~2023-04-05 5:33 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-04 10:28 [PATCH] powerpc/64: Always build with 128-bit long double Michael Ellerman
2023-04-04 15:10 ` Segher Boessenkool
2023-04-05 5:32 ` Michael Ellerman [this message]
2023-04-05 11:04 ` Segher Boessenkool
2023-04-06 17:12 ` Hamza Mahfooz
2023-04-12 12:34 ` Michael Ellerman
2023-04-26 12:01 ` Michael Ellerman
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