linux-kernel - Re: [PATCH 2/2] MIPS: Ingenic: Disable abandoned HPTLB function.

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Date:   Fri, 15 Nov 2019 13:37:16 -0800
From:   Paul Burton <paulburton@...nel.org>
To:     Zhou Yanjie <zhouyanjie@...o.com>
Cc:     linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        ralf@...ux-mips.org, jhogan@...nel.org, gregkh@...uxfoundation.org,
        paul.burton@...s.com, chenhc@...ote.com, paul@...pouillou.net,
        tglx@...utronix.de, jiaxun.yang@...goat.com
Subject: Re: [PATCH 2/2] MIPS: Ingenic: Disable abandoned HPTLB function.

Hi Zhou,

On Thu, Oct 24, 2019 at 05:29:01PM +0800, Zhou Yanjie wrote:
> JZ4760/JZ4770/JZ4775/X1000/X1500 has an abandoned huge page
> tlb, write 0xa9000000 to cp0 config5 sel4 to disable this
> function to prevent getting stuck.

Can you describe how we "get stuck"? What actually goes wrong on the
affected CPUs? Do they misinterpret EntryLo values? Which bits do they
misinterpret?

> Confirmed by Ingenic,
> this operation will not adversely affect processors
> without HPTLB function.
> 
> Signed-off-by: Zhou Yanjie <zhouyanjie@...o.com>
> ---
>  arch/mips/kernel/cpu-probe.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/mips/kernel/cpu-probe.c b/arch/mips/kernel/cpu-probe.c
> index 16033a4..cfebf8c 100644
> --- a/arch/mips/kernel/cpu-probe.c
> +++ b/arch/mips/kernel/cpu-probe.c
> @@ -1966,11 +1966,23 @@ static inline void cpu_probe_ingenic(struct cpuinfo_mips *c, unsigned int cpu)
>  	}
>  
>  	/*
> -	 * The config0 register in the Xburst CPUs with a processor ID of
> +	 * The config0 register in the XBurst CPUs with a processor ID of
> +	 * PRID_COMP_INGENIC_D1 has an abandoned huge page tlb, write
> +	 * 0xa9000000 to cp0 config5 sel4 to disable this function to

Saying "config5" suggests $16 sel 5 to me - Config5 is after all an
architecturally defined register & it's not this one. It'd be better to
say "cop0 register 5 sel 4".

> +	 * prevent getting stuck.
> +	 */
> +	if ((c->processor_id & PRID_COMP_MASK) == PRID_COMP_INGENIC_D1) {
> +		__asm__ (
> +			"li    $2, 0xa9000000 \n\t"
> +			"mtc0  $2, $5, 4      \n\t"
> +			"nop                  \n\t"
> +			::"r"(2));

I'd prefer that you add #defines to asm/mipsregs.h to provide a
write_c0_X() function where X is replaced with whatever the name of this
register is, and preferably also #define macros describing the fields
present in the register. Writing a magic number isn't ideal.

> +	/*
> +	 * The config0 register in the XBurst CPUs with a processor ID of
>  	 * PRID_COMP_INGENIC_D0 report themselves as MIPS32r2 compatible,
>  	 * but they don't actually support this ISA.
>  	 */
> -	if ((c->processor_id & PRID_COMP_MASK) == PRID_COMP_INGENIC_D0)
> +	} else if ((c->processor_id & PRID_COMP_MASK) == PRID_COMP_INGENIC_D0)

It might be cleaner to use a switch statement rather than writing out
the & PRID_COMP_MASK condition twice?

Thanks,
    Paul