linux-kernel - Re: [PATCH v3 1/6] MIPS: Loongson64: Do not write the read only field LPA of CP0

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Date:   Wed, 4 Nov 2020 11:50:53 +0800
From:   Tiezhu Yang <yangtiezhu@...ngson.cn>
To:     Huacai Chen <chenhc@...ote.com>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Xuefeng Li <lixuefeng@...ngson.cn>
Subject: Re: [PATCH v3 1/6] MIPS: Loongson64: Do not write the read only field
 LPA of CP0_CONFIG3

On 11/04/2020 10:00 AM, Huacai Chen wrote:
> Hi, Tiezhu,
>
> On Tue, Nov 3, 2020 at 3:13 PM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
>> The field LPA of CP0_CONFIG3 register is read only for Loongson64, so the
>> write operations are meaningless, remove them.
>>
>> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
>> ---
>>
>> v2: No changes
>> v3: No changes
>>
>>   arch/mips/include/asm/mach-loongson64/kernel-entry-init.h | 8 --------
>>   arch/mips/loongson64/numa.c                               | 3 ---
>>   2 files changed, 11 deletions(-)
>>
>> diff --git a/arch/mips/include/asm/mach-loongson64/kernel-entry-init.h b/arch/mips/include/asm/mach-loongson64/kernel-entry-init.h
>> index 87a5bfb..e4d77f4 100644
>> --- a/arch/mips/include/asm/mach-loongson64/kernel-entry-init.h
>> +++ b/arch/mips/include/asm/mach-loongson64/kernel-entry-init.h
>> @@ -19,10 +19,6 @@
>>          .macro  kernel_entry_setup
>>          .set    push
>>          .set    mips64
>> -       /* Set LPA on LOONGSON3 config3 */
>> -       mfc0    t0, CP0_CONFIG3
>> -       or      t0, (0x1 << 7)
>> -       mtc0    t0, CP0_CONFIG3
> Sorry for the late response, I have the same worry as Jiaxun. As you
> know, Loongson's user manuals are not always correct, but the original
> code comes from Loongson are usually better. So, my opinion is "Don't
> change it if it doesn't break anything".

Hi Huacai,

Thanks for your reply, I have confirmed by Loongson user manuals and
hardware designers, CP0_CONFIG3 register is read only.

Without this patch, the related kernel code is meaningless, with
this patch, it can reflect the reality.

Thanks,
Tiezhu

>
> Huacai
>
>>          /* Set ELPA on LOONGSON3 pagegrain */
>>          mfc0    t0, CP0_PAGEGRAIN
>>          or      t0, (0x1 << 29)
>> @@ -54,10 +50,6 @@
>>          .macro  smp_slave_setup
>>          .set    push
>>          .set    mips64
>> -       /* Set LPA on LOONGSON3 config3 */
>> -       mfc0    t0, CP0_CONFIG3
>> -       or      t0, (0x1 << 7)
>> -       mtc0    t0, CP0_CONFIG3
>>          /* Set ELPA on LOONGSON3 pagegrain */
>>          mfc0    t0, CP0_PAGEGRAIN
>>          or      t0, (0x1 << 29)
>> diff --git a/arch/mips/loongson64/numa.c b/arch/mips/loongson64/numa.c
>> index cf9459f..c7e3cced 100644
>> --- a/arch/mips/loongson64/numa.c
>> +++ b/arch/mips/loongson64/numa.c
>> @@ -40,9 +40,6 @@ static void enable_lpa(void)
>>          unsigned long value;
>>
>>          value = __read_32bit_c0_register($16, 3);
>> -       value |= 0x00000080;
>> -       __write_32bit_c0_register($16, 3, value);
>> -       value = __read_32bit_c0_register($16, 3);
>>          pr_info("CP0_Config3: CP0 16.3 (0x%lx)\n", value);
>>
>>          value = __read_32bit_c0_register($5, 1);
>> --
>> 2.1.0
>>