Subject: Re: [PATCH] powerpc/64: Always build with 128-bit long double

From: Michael Ellerman <[email-masked]>
Date: Wed, 05 Apr 2023 15:32:21 +1000

Segher Boessenkool <[email-masked]> 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 <[email-masked]>

Thanks.

cheers