linux-kernel - Re: [PATCH 1/7] RISC-V: Top-Level Makefile for riscv{32,64}

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Date:   Mon, 05 Jun 2017 21:56:33 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     Arnd Bergmann <arnd@...db.de>
CC:     albert@...ive.com
Subject:     Re: [PATCH 1/7] RISC-V: Top-Level Makefile for riscv{32,64}

On Mon, 29 May 2017 03:50:47 PDT (-0700), Arnd Bergmann wrote:
> On Sat, May 27, 2017 at 2:57 AM, Palmer Dabbelt <palmer@...belt.com> wrote:
>> On Tue, 23 May 2017 04:30:50 PDT (-0700), Arnd Bergmann wrote:
>>> On Tue, May 23, 2017 at 2:41 AM, Palmer Dabbelt <palmer@...belt.com> wrote:
>>>> RISC-V has both 32-bit and 64-bit base ISAs, but they are very similar.
>>>> Like some other platforms, we'd like to share one arch directory between
>>>> the two of them.
>>>
>>> I think we mainly do the others for backwards-compatibility with ancient
>>> build scripts, and we don't need that here. Instead, you could add one more
>>> line to the 'SUBARCH:=' statement that interprets the uname output.
>>
>> I don't think that does the same thing.  The desired effect of this diff is:
>>
>>  * "uname -m" when running on a RISC-V machine returns either riscv32 or
>>    riscv64, as that's what tools like autoconf expect when trying to find
>>    tuples.
>>
>>  * I can cross compile for riscv32 and riscv64.  That's currently controlled by
>>    a Kconfig setting, but ARCH=riscv32 vs ARCH=riscv64 controlls what defconfig
>>    sets.
>>
>>  * I can natively compile for riscv32 and riscv64.  That uses the same Kconfig
>>    setting, and the same ARCH=riscv32 vs ARCH=riscv64 switch for defconfig.
>
> Right, but my point is that a new architecture should not rely on 'ARCH='
> to pick the defconfig, we only do that on a couple of architectures for
> backwards compatibility with old scripts.
>
>> Neither of the two Kconfig issues is a big deal, but we de need "uname -m" to
>> return "riscv64" or "riscv32" not "riscv".  I think the only way to do that is
>> to set SRCARCH, but I'd be happy to change it if there's a better way.  I think
>> if I just do this
>>
>> diff --git a/Makefile b/Makefile
>> index 0606f28..4adc609 100644
>> --- a/Makefile
>> +++ b/Makefile
>> @@ -232,7 +232,8 @@ SUBARCH := $(shell uname -m | sed -e s/i.86/x86/ -e s/x86_64/x86/ \
>>                                   -e s/arm.*/arm/ -e s/sa110/arm/ \
>>                                   -e s/s390x/s390/ -e s/parisc64/parisc/ \
>>                                   -e s/ppc.*/powerpc/ -e s/mips.*/mips/ \
>> -                                 -e s/sh[234].*/sh/ -e s/aarch64.*/arm64/ )
>> +                                 -e s/sh[234].*/sh/ -e s/aarch64.*/arm64/ \
>> +                                 -e s/riscv.*/riscv/ )
>>
>>  # Cross compiling and selecting different set of gcc/bin-utils
>>  # ---------------------------------------------------------------------------
>> @@ -269,14 +270,6 @@ ifeq ($(ARCH),x86_64)
>>          SRCARCH := x86
>>  endif
>>
>> -# Additional ARCH settings for RISC-V
>> -ifeq ($(ARCH),riscv32)
>> -       SRCARCH := riscv
>> -endif
>> -ifeq ($(ARCH),riscv64)
>> -       SRCARCH := riscv
>> -endif
>> -
>>  # Additional ARCH settings for sparc
>>  ifeq ($(ARCH),sparc32)
>>         SRCARCH := sparc
>>
>> then I'll end up with "uname -m" as "riscv" -- I haven't tried it, but that's
>> why we ended up with this diff in the first place.
>
> Do you mean the "uname -m" output comes from "${SRCARCH}" at
> the time of the kernel build? That would be easy enough to change
> by simply hardcoding it depending on CONFIG_64BIT.

OK, I didn't know about COMPAT_UTS_MACHINE.  That's a much better solution,
I'll use that.