linux-kernel - Re: [PATCH 2/2 v3] MIPS: Ingenic: Fix bugs when calculate bogomips/lpj.

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Date:   Fri, 2 Aug 2019 16:13:15 +0800
From:   Zhou Yanjie <zhouyanjie@...o.com>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        ralf@...ux-mips.org, paul.burton@...s.com, jhogan@...nel.org,
        malat@...ian.org, gregkh@...uxfoundation.org, tglx@...utronix.de,
        allison@...utok.net, syq@...ian.org, chenhc@...ote.com,
        jiaxun.yang@...goat.com
Subject: Re: [PATCH 2/2 v3] MIPS: Ingenic: Fix bugs when calculate
 bogomips/lpj.



On 2019年08月02日 09:26, Paul Cercueil wrote:
> Hi Zhou,
>
>
>
> Le jeu. 1 août 2019 à 8:16, Zhou Yanjie <zhouyanjie@...o.com> a écrit :
>> Enable BTB lookups for short loops to fix bugs when calculate
>> bogomips and loops_per_jiffy.
>
> The commit description and the code comment below seem to say two
> different things. Are we enabling the BTB lookup optimization, or not?
>
By set config7 bit 4 we are disable the BTB lookup optimization and enable
BTB lookup for short loop.
> Also, maybe change the commit title to something more meaningful, e.g.
> "MIPS: ingenic: Disable broken BTB lookup optimization" or similar.
>
OK, I'll change it in v4.
>
>> Signed-off-by: Zhou Yanjie <zhouyanjie@...o.com>
>> ---
>>  arch/mips/include/asm/mipsregs.h | 4 ++++
>>  arch/mips/kernel/cpu-probe.c     | 7 +++++++
>>  2 files changed, 11 insertions(+)
>>
>> diff --git a/arch/mips/include/asm/mipsregs.h 
>> b/arch/mips/include/asm/mipsregs.h
>> index 1e6966e..bdbdc19 100644
>> --- a/arch/mips/include/asm/mipsregs.h
>> +++ b/arch/mips/include/asm/mipsregs.h
>> @@ -689,6 +689,9 @@
>>  #define MIPS_CONF7_IAR        (_ULCAST_(1) << 10)
>>  #define MIPS_CONF7_AR        (_ULCAST_(1) << 16)
>>
>> +/* Ingenic Config7 bits */
>> +#define MIPS_CONF7_BTB_LOOP_EN    (_ULCAST_(1) << 4)
>> +
>>  /* Config7 Bits specific to MIPS Technologies. */
>>
>>  /* Performance counters implemented Per TC */
>> @@ -2813,6 +2816,7 @@ __BUILD_SET_C0(status)
>>  __BUILD_SET_C0(cause)
>>  __BUILD_SET_C0(config)
>>  __BUILD_SET_C0(config5)
>> +__BUILD_SET_C0(config7)
>>  __BUILD_SET_C0(intcontrol)
>>  __BUILD_SET_C0(intctl)
>>  __BUILD_SET_C0(srsmap)
>> diff --git a/arch/mips/kernel/cpu-probe.c b/arch/mips/kernel/cpu-probe.c
>> index eb527a1..2bdd3e1 100644
>> --- a/arch/mips/kernel/cpu-probe.c
>> +++ b/arch/mips/kernel/cpu-probe.c
>> @@ -1964,6 +1964,13 @@ static inline void cpu_probe_ingenic(struct 
>> cpuinfo_mips *c, unsigned int cpu)
>>          c->cputype = CPU_XBURST;
>>          c->writecombine = _CACHE_UNCACHED_ACCELERATED;
>>          __cpu_name[cpu] = "Ingenic XBurst";
>> +        /*
>> +         * The XBurst core by default attempts to avoid branch target
>> +         * buffer lookups by detecting & special casing loops. This
>> +         * feature will cause BogoMIPS and lpj calculate in error.
>> +         * Set cp0 config7 bit 4 to disable this feature.
>> +         */
>> +        set_c0_config7(MIPS_CONF7_BTB_LOOP_EN);
>
> Shouldn't it be MIPS_CONF7_BTB_LOOP_DIS then?
> Since the feature is disabled when the bit is set.
>
According to Ingenic's explanation and Paul's old patch in

https://github.com/paulburton/linux/commit/0d72377bd615d00e99733adc0d37e6a2373fcde7

In order to further reduce power consumption, the XBurst core by default 
attempts
to avoid branch target buffer lookups by detecting & special casing 
loops, this is
what you mentioned as "BTB lookup optimization". Enabling this feature 
will avoid
branch target buffer lookups for loops. And now we are disable this 
feature, so it
should be "MIPS_CONF7_BTB_LOOP_EN".
>
>>          break;
>>      default:
>>          panic("Unknown Ingenic Processor ID!");
>> -- 
>> 2.7.4
>>
>>
>
>