Build issue "Error: operands mismatch -- statement `movec %d2,%caar' ignored" on m68k

Build issue "Error: operands mismatch -- statement `movec %d2,%caar' ignored" on m68k

[Thomas Petazzoni]

Hello,

While trying to build the multi_defconfig, from Linux 6.12.5, for a
68040 system, the build fails with:

{standard input}: Assembler messages:
{standard input}:104: Error: operands mismatch -- statement `movec %d2,%caar' ignored
make[5]: *** [scripts/Makefile.build:229: arch/m68k/kernel/sys_m68k.o] Error 1

It seems like the 68020/68030 code:

        if (CPU_IS_020_OR_030) {
                if (scope == FLUSH_SCOPE_LINE && len < 256) {
                        unsigned long cacr;
                        __asm__ ("movec %%cacr, %0" : "=r" (cacr));
                        if (cache & FLUSH_CACHE_INSN)
                                cacr |= 4;
                        if (cache & FLUSH_CACHE_DATA)
                                cacr |= 0x400;
                        len >>= 2;
                        while (len--) {
                                __asm__ __volatile__ ("movec %1, %%caar\n\t"
                                                      "movec %0, %%cacr"
                                                      : /* no outputs */
                                                      : "r" (cacr), "r" (addr));
                                addr += 4;
                        }
                } else {
                        /* Flush the whole cache, even if page granularity requested. */
                        unsigned long cacr;
                        __asm__ ("movec %%cacr, %0" : "=r" (cacr));
                        if (cache & FLUSH_CACHE_INSN)
                                cacr |= 8;
                        if (cache & FLUSH_CACHE_DATA)
                                cacr |= 0x800;
                        __asm__ __volatile__ ("movec %0, %%cacr" : : "r" (cacr));
                }
                ret = 0;
                goto out_unlock;

gets compiled in, and binutils isn't happy about it.

The issue can trivially be reproduced by building the following
Buildroot configuration:

BR2_m68k=y
BR2_LINUX_KERNEL=y
BR2_LINUX_KERNEL_USE_ARCH_DEFAULT_CONFIG=y

Quick recipe:

$ git clone https://gitlab.com/buildroot.org/buildroot.git
$ cd buildroot/
$ cat >.config <<EOF
BR2_m68k=y
BR2_LINUX_KERNEL=y
BR2_LINUX_KERNEL_USE_ARCH_DEFAULT_CONFIG=y
EOF
$ make olddefconfig
$ make

Is this expected?

Best regards,

Thomas
-- 
Thomas Petazzoni, co-owner and CEO, Bootlin
Embedded Linux and Kernel engineering and training
https://bootlin.com

Re: Build issue "Error: operands mismatch -- statement `movec %d2,%caar' ignored" on m68k

[Geert Uytterhoeven]

Hi Thomas,

On Sun, Dec 29, 2024 at 1:48 PM Thomas Petazzoni
<thomas.petazzoni@bootlin.com> wrote:
> While trying to build the multi_defconfig, from Linux 6.12.5, for a
> 68040 system, the build fails with:
>
> {standard input}: Assembler messages:
> {standard input}:104: Error: operands mismatch -- statement `movec %d2,%caar' ignored
> make[5]: *** [scripts/Makefile.build:229: arch/m68k/kernel/sys_m68k.o] Error 1

That's not the only error:

{standard input}: Assembler messages:
{standard input}:17: Error: invalid instruction for this architecture;
needs M68K mmu (68020 [68k, 68ec020], 68030 [68ec030], 68040
[68ec040], 68060 [68ec060]) -- statement `pflusha' ignored
make[5]: *** [scripts/Makefile.build:229: arch/m68k/mm/kmap.o] Error 1

There are more like the above, but at least these tell you what's the
underlying problem: the cross-compiler does not seem to support real
m68k CPUs. Interestingly, it was built with --with-cpu=68040...

Also:

    cat > /tmp/a.s <<EOF
    movec %d2,%caar
    EOF
    output/host/bin/m68k-buildroot-linux-gnu-as /tmp/a.s

does build.

    output/host/bin/m68k-buildroot-linux-gnu-gcc -c /tmp/a.s

does not.

With plain m68k-linux-gnu-{as,gcc} (Ubuntu 13.3.0-6ubuntu2~24.04,
which is also 13.3.0, just like the buildroot toolchain), both work.

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

Re: Build issue "Error: operands mismatch -- statement `movec %d2,%caar' ignored" on m68k

[Finn Thain]

On Sun, 29 Dec 2024, Geert Uytterhoeven wrote:

>     output/host/bin/m68k-buildroot-linux-gnu-as /tmp/a.s
> 
> does build.
> 
>     output/host/bin/m68k-buildroot-linux-gnu-gcc -c /tmp/a.s
> 
> does not.
> 

You may want to try,

$ output/host/bin/m68k-buildroot-linux-gnu-gcc -v -c /tmp/a.s

As that will print the full assembler command line, among other things. 
E.g.

...
/usr/libexec/gcc/m68k-unknown-linux-musl/as -v -mcpu=68020 --pcrel -o a.o a.s

Re: Build issue "Error: operands mismatch -- statement `movec %d2,%caar' ignored" on m68k

[Geert Uytterhoeven]

Hi Finn,

On Mon, Dec 30, 2024 at 9:30 AM Finn Thain <fthain@linux-m68k.org> wrote:
> You may want to try,
>
> $ output/host/bin/m68k-buildroot-linux-gnu-gcc -v -c /tmp/a.s
>
> As that will print the full assembler command line, among other things.
> E.g.
>
> ...
> /usr/libexec/gcc/m68k-unknown-linux-musl/as -v -mcpu=68020 --pcrel -o a.o a.s

Thanks, so it fails when -mcpu=68040 is passed to the assembler.

Oh, I misread "caar" as "cacr". The latter exists on MC68040, the
former does not. However, that doesn't explain the failures for pflusha,
which is supported on 68040.

After disabling CONFIG_680[236]0 in the kernel config, I can build
it fine using the buildroot toolchain.

Conclusion: make sure your toolchain supports (a superset of) the
CPUs supported by your kernel config.

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

Re: Build issue "Error: operands mismatch -- statement `movec %d2,%caar' ignored" on m68k

[Thomas Petazzoni]

Hello Geert,

On Mon, 30 Dec 2024 09:53:29 +0100
Geert Uytterhoeven <geert@linux-m68k.org> wrote:

> > /usr/libexec/gcc/m68k-unknown-linux-musl/as -v -mcpu=68020 --pcrel -o a.o a.s  
> 
> Thanks, so it fails when -mcpu=68040 is passed to the assembler.
> 
> Oh, I misread "caar" as "cacr". The latter exists on MC68040, the
> former does not. However, that doesn't explain the failures for pflusha,
> which is supported on 68040.
> 
> After disabling CONFIG_680[236]0 in the kernel config, I can build
> it fine using the buildroot toolchain.
> 
> Conclusion: make sure your toolchain supports (a superset of) the
> CPUs supported by your kernel config.

Thanks for the investigation. How should the toolchain be configured to
s upport multiple m68k CPUs? My understanding was that when you
configure gcc with --with-cpu=<something>, it means gcc will by default
emit code for <something>, but it can always be overridden when running
gcc using -mcpu=<somethingelse>.

How can one build this kernel/sys_m68k.c file if it uses instructions
that are mutually incompatible on different m68k cores?

Thomas
-- 
Thomas Petazzoni, co-owner and CEO, Bootlin
Embedded Linux and Kernel engineering and training
https://bootlin.com

Re: Build issue "Error: operands mismatch -- statement `movec %d2,%caar' ignored" on m68k

[Geert Uytterhoeven]

Hi Thomas,

On Mon, Dec 30, 2024 at 10:35 AM Thomas Petazzoni
<thomas.petazzoni@bootlin.com> wrote:
> On Mon, 30 Dec 2024 09:53:29 +0100
> Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> > > /usr/libexec/gcc/m68k-unknown-linux-musl/as -v -mcpu=68020 --pcrel -o a.o a.s
> >
> > Thanks, so it fails when -mcpu=68040 is passed to the assembler.
> >
> > Oh, I misread "caar" as "cacr". The latter exists on MC68040, the
> > former does not. However, that doesn't explain the failures for pflusha,
> > which is supported on 68040.
> >
> > After disabling CONFIG_680[236]0 in the kernel config, I can build
> > it fine using the buildroot toolchain.
> >
> > Conclusion: make sure your toolchain supports (a superset of) the
> > CPUs supported by your kernel config.
>
> Thanks for the investigation. How should the toolchain be configured to
> s upport multiple m68k CPUs? My understanding was that when you
> configure gcc with --with-cpu=<something>, it means gcc will by default
> emit code for <something>, but it can always be overridden when running
> gcc using -mcpu=<somethingelse>.

Sorry, it's been ages since I built my own cross-compiler.
IIRC, I always used the default, which gives a compiler targeting all
(non-Coldfire) variants.

The GCC documentation seems to be ambiguous
(https://gcc.gnu.org/install/configure.html):

    Example 1: to configure a compiler for SH4A only, but supporting
both endians, with little endian being the default:
       --with-cpu=sh4a --with-endian=little,big --with-multilib-list=

vs.

    --with-cpu=cpu
    --with-cpu-32=cpu
    --with-cpu-64=cpu
        Specify which cpu variant the compiler should generate code
for by default

> How can one build this kernel/sys_m68k.c file if it uses instructions
> that are mutually incompatible on different m68k cores?

Each of the CPU-specific sections depends on CPU_IS_*(), which
expand to compile-time and/or run-time checks for the CPU type,
depending on which CONFIG_680*0 kernel config options are set.

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

Re: Build issue "Error: operands mismatch -- statement `movec %d2,%caar' ignored" on m68k

[Andreas Schwab]

On Dez 30 2024, Geert Uytterhoeven wrote:

> Conclusion: make sure your toolchain supports (a superset of) the
> CPUs supported by your kernel config.

Conclusion: always use .chip directives.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
"And now for something completely different."