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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Fri, 13 May 2022 08:56:33 +0200
Message-ID: <CAMuHMdUb=c5WYviGqJXSVtWe4rob28oOQsPE2V==V3P7hDWwyg@mail.gmail.com>
Subject: Re: [PATCH] m68k: math-emu: Fix dependencies of math emulation support
To:     Greg Ungerer <gerg@uclinux.org>
Cc:     linux-m68k <linux-m68k@lists.linux-m68k.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        kernel test robot <lkp@intel.com>
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Hi Greg,

On Fri, May 13, 2022 at 1:23 AM Greg Ungerer <gerg@uclinux.org> wrote:
> On 11/5/22 22:54, Geert Uytterhoeven wrote:
> > On Wed, May 11, 2022 at 11:48 AM Geert Uytterhoeven
> > <geert@linux-m68k.org> wrote:
> >> If CONFIG_M54xx=y, CONFIG_MMU=y, and CONFIG_M68KFPU_EMU=y:
> >>
> >>      {standard input}:272: Error: invalid instruction for this architecture; needs 68000 or higher (68000 [68ec000, 68hc000, 68hc001, 68008, 68302, 68306, 68307, 68322, 68356], 68010, 68020 [68k, 68ec020], 68030 [68ec030], 68040 [68ec040], 68060 [68ec060], cpu32 [68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349, 68360], fidoa [fido]) -- statement `sub.b %d1,%d3' ignored
> >>      {standard input}:609: Error: invalid instruction for this architecture; needs 68020 or higher (68020 [68k, 68ec020], 68030 [68ec030], 68040 [68ec040], 68060 [68ec060]) -- statement `bfextu 4(%a1){%d0,#8},%d0' ignored
> >>      {standard input}:752: Error: operands mismatch -- statement `mulu.l 4(%a0),%d3:%d0' ignored
> >>      {standard input}:1155: Error: operands mismatch -- statement `divu.l %d0,%d3:%d7' ignored
> >>
> >> The math emulation support code is intended for 68020 and higher, and
> >> uses several instructions or instruction modes not available on coldfire
> >> or 68000.
> >>
> >> Originally, the dependency of M68KFPU_EMU on MMU was fine, as MMU
> >> support was only available on 68020 or higher.  But this assumption
> >> was broken by the introduction of MMU support for M547x and M548x.
> >>
> >> Drop the dependency on MMU, as the code should work fine on 68020 and up
> >> without MMU (which are not yet supported by Linux, though).
> >> Add dependencies on M68K_CLASSIC (to rule out Coldfire) and FPU (to rule
> >> out 68xxx below 68020).
> >>
> >> Fixes: 1f7034b9616e6f14 ("m68k: allow ColdFire 547x and 548x CPUs to be built with MMU enabled")
> >> Reported-by: kernel test robot <lkp@intel.com>
> >> Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
> >> ---
> >> To be queued in the m68k for-v5.19 branch.
> >>
> >>   arch/m68k/Kconfig.cpu | 2 +-
> >>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/arch/m68k/Kconfig.cpu b/arch/m68k/Kconfig.cpu
> >> index 16ea9a67723c09dc..9abddbbee819f1d3 100644
> >> --- a/arch/m68k/Kconfig.cpu
> >> +++ b/arch/m68k/Kconfig.cpu
> >> @@ -327,7 +327,7 @@ comment "Processor Specific Options"
> >>
> >>   config M68KFPU_EMU
> >>          bool "Math emulation support"
> >> -       depends on MMU
> >> +       depends on M68K_CLASSIC && FPU
> >                        ^^^^^^^^^^^^
> > Whoops, that's a silly typo...
>
> Are you going to resend, or are you thinking of changing the way this is done?

I was going to resend, with the above fixed, and an improved patch
description w.r.t. the dependency on FPU.

> I had thought that CONFIG_FPU enabled meant you had some type of
> floating point support in the kernel - be it hardware or software emulated.
> So I don't have a problem with this "as is".

That matches my understanding, too.

BTW, I'm not familiar with Coldfire FPU support. Does it rely on any
emulation (e.g. for transcendental functions, like '040/'060)?
Would there be any point in making the math emulation code
Coldfire-proof?

Thanks!

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds