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

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Date:   Thu, 5 Nov 2020 13:57:53 +0800
From:   Huacai Chen <chenhc@...ote.com>
To:     Tiezhu Yang <yangtiezhu@...ngson.cn>
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

Hi, Tiezhu,

On Wed, Nov 4, 2020 at 11:51 AM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
>
> 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
Then you should at least test your code on Loongson-3A R1 two way machine.

Huacai
>
> >
> > 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
> >>
>