linux-kernel - Re: [PATCH v2] powerpc/mm: Add mask of always present MMU features

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Date:   Wed, 21 Oct 2020 12:32:39 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] powerpc/mm: Add mask of always present MMU features



Le 12/10/2020 à 17:39, Christophe Leroy a écrit :
> On the same principle as commit 773edeadf672 ("powerpc/mm: Add mask
> of possible MMU features"), add mask for MMU features that are
> always there in order to optimise out dead branches.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> ---
> v2: Features must be anded with MMU_FTRS_POSSIBLE instead of ~0, otherwise
>      MMU_FTRS_ALWAYS is ~0 when no #ifdef matches.

This is still not enough. For BOOK3S/32, MMU_FTRS_POSSIBLE is still too much.
We need a #ifdef CONFIG_PPC_BOOK3S_32 with 0.

Christophe

> ---
>   arch/powerpc/include/asm/mmu.h | 25 +++++++++++++++++++++++++
>   1 file changed, 25 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/mmu.h b/arch/powerpc/include/asm/mmu.h
> index 255a1837e9f7..64e7e7f7cda9 100644
> --- a/arch/powerpc/include/asm/mmu.h
> +++ b/arch/powerpc/include/asm/mmu.h
> @@ -201,8 +201,30 @@ enum {
>   		0,
>   };
>   
> +enum {
> +	MMU_FTRS_ALWAYS =
> +#ifdef CONFIG_PPC_8xx
> +		MMU_FTR_TYPE_8xx &
> +#endif
> +#ifdef CONFIG_40x
> +		MMU_FTR_TYPE_40x &
> +#endif
> +#ifdef CONFIG_PPC_47x
> +		MMU_FTR_TYPE_47x &
> +#elif defined(CONFIG_44x)
> +		MMU_FTR_TYPE_44x &
> +#endif
> +#if defined(CONFIG_E200) || defined(CONFIG_E500)
> +		MMU_FTR_TYPE_FSL_E &
> +#endif
> +		MMU_FTRS_POSSIBLE,
> +};
> +
>   static inline bool early_mmu_has_feature(unsigned long feature)
>   {
> +	if (MMU_FTRS_ALWAYS & feature)
> +		return true;
> +
>   	return !!(MMU_FTRS_POSSIBLE & cur_cpu_spec->mmu_features & feature);
>   }
>   
> @@ -231,6 +253,9 @@ static __always_inline bool mmu_has_feature(unsigned long feature)
>   	}
>   #endif
>   
> +	if (MMU_FTRS_ALWAYS & feature)
> +		return true;
> +
>   	if (!(MMU_FTRS_POSSIBLE & feature))
>   		return false;
>   
>