linux-kernel - Re: PPC_FPU, ALTIVEC: enable_kernel_fp, put_vr, get

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Message-ID: <1f337b4c-940e-110c-d0a2-2ad95cfb2dc8@csgroup.eu>
Date:   Tue, 20 Apr 2021 06:55:32 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Randy Dunlap <rdunlap@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Segher Boessenkool <segher@...nel.crashing.org>
Cc:     PowerPC <linuxppc-dev@...ts.ozlabs.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: PPC_FPU, ALTIVEC: enable_kernel_fp, put_vr, get_vr



Le 19/04/2021 à 23:39, Randy Dunlap a écrit :
> On 4/19/21 6:16 AM, Michael Ellerman wrote:
>> Randy Dunlap <rdunlap@...radead.org> writes:
> 
>>> Sure.  I'll post them later today.
>>> They keep FPU and ALTIVEC as independent (build) features.
>>
>> Those patches look OK.
>>
>> But I don't think it makes sense to support that configuration, FPU=n
>> ALTVEC=y. No one is ever going to make a CPU like that. We have enough
>> testing surface due to configuration options, without adding artificial
>> combinations that no one is ever going to use.
>>
>> IMHO :)
>>
>> So I'd rather we just make ALTIVEC depend on FPU.
> 
> That's rather simple. See below.
> I'm doing a bunch of randconfig builds with it now.
> 
> ---
> From: Randy Dunlap <rdunlap@...radead.org>
> Subject: [PATCH] powerpc: make ALTIVEC depend PPC_FPU
> 
> On a kernel config with ALTIVEC=y and PPC_FPU not set/enabled,
> there are build errors:
> 
> drivers/cpufreq/pmac32-cpufreq.c:262:2: error: implicit declaration of function 'enable_kernel_fp' [-Werror,-Wimplicit-function-declaration]
>             enable_kernel_fp();
> ../arch/powerpc/lib/sstep.c: In function 'do_vec_load':
> ../arch/powerpc/lib/sstep.c:637:3: error: implicit declaration of function 'put_vr' [-Werror=implicit-function-declaration]
>    637 |   put_vr(rn, &u.v);
>        |   ^~~~~~
> ../arch/powerpc/lib/sstep.c: In function 'do_vec_store':
> ../arch/powerpc/lib/sstep.c:660:3: error: implicit declaration of function 'get_vr'; did you mean 'get_oc'? [-Werror=implicit-function-declaration]
>    660 |   get_vr(rn, &u.v);
>        |   ^~~~~~
> 
> In theory ALTIVEC is independent of PPC_FPU but in practice nobody
> is going to build such a machine, so make ALTIVEC require PPC_FPU
> by depending on PPC_FPU.
> 
> Signed-off-by: Randy Dunlap <rdunlap@...radead.org>
> Reported-by: kernel test robot <lkp@...el.com>
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Cc: linuxppc-dev@...ts.ozlabs.org
> Cc: Christophe Leroy <christophe.leroy@...roup.eu>
> Cc: Segher Boessenkool <segher@...nel.crashing.org>
> Cc: lkp@...el.com
> ---
>   arch/powerpc/platforms/86xx/Kconfig    |    1 +
>   arch/powerpc/platforms/Kconfig.cputype |    2 ++
>   2 files changed, 3 insertions(+)
> 
> --- linux-next-20210416.orig/arch/powerpc/platforms/86xx/Kconfig
> +++ linux-next-20210416/arch/powerpc/platforms/86xx/Kconfig
> @@ -4,6 +4,7 @@ menuconfig PPC_86xx
>   	bool "86xx-based boards"
>   	depends on PPC_BOOK3S_32
>   	select FSL_SOC
> +	select PPC_FPU
>   	select ALTIVEC
>   	help
>   	  The Freescale E600 SoCs have 74xx cores.
> --- linux-next-20210416.orig/arch/powerpc/platforms/Kconfig.cputype
> +++ linux-next-20210416/arch/powerpc/platforms/Kconfig.cputype
> @@ -186,6 +186,7 @@ config E300C3_CPU
>   config G4_CPU
>   	bool "G4 (74xx)"
>   	depends on PPC_BOOK3S_32
> +	select PPC_FPU
>   	select ALTIVEC
>   
>   endchoice
> @@ -309,6 +310,7 @@ config PHYS_64BIT
>   
>   config ALTIVEC
>   	bool "AltiVec Support"
> +	depends on PPC_FPU

Shouldn't we do it the other way round ? In extenso make ALTIVEC select PPC_FPU and avoid the two 
selects that are above ?

>   	depends on PPC_BOOK3S_32 || PPC_BOOK3S_64 || (PPC_E500MC && PPC64)
>   	help
>   	  This option enables kernel support for the Altivec extensions to the
>