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Date:   Wed, 12 Aug 2020 00:41:39 -0700
From:   Atish Patra <atishp@...shpatra.org>
To:     Anup Patel <anup@...infault.org>
Cc:     Qiu Wenbo <qiuwenbo@...tium.com.cn>,
        Damien Le Moal <damien.lemoal@....com>,
        Guo Ren <guoren@...ux.alibaba.com>,
        Vincent Chen <vincent.chen@...ive.com>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        Atish Patra <atish.patra@....com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Palmer Dabbelt <palmer@...belt.com>,
        Zong Li <zong.li@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Greentime Hu <greentime.hu@...ive.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>
Subject: Re: [PATCH] riscv: Setup exception vector for K210 properly

On Tue, Aug 11, 2020 at 6:57 PM Anup Patel <anup@...infault.org> wrote:
>
> On Wed, Aug 12, 2020 at 12:16 AM Atish Patra <atishp@...shpatra.org> wrote:
> >
> > On Tue, Aug 11, 2020 at 1:41 AM Anup Patel <anup@...infault.org> wrote:
> > >
> > > On Tue, Aug 11, 2020 at 12:07 PM Qiu Wenbo <qiuwenbo@...tium.com.cn> wrote:
> > > >
> > > > Exception vector is missing on nommu platform and it is a big issue.
> > > > This patch is tested in Sipeed MAIX Bit Dev Board.
> > > >
> > > > Fixes: 79b1feba5455 ("RISC-V: Setup exception vector early")
> > > > Signed-off-by: Qiu Wenbo <qiuwenbo@...tium.com.cn>
> >
> > Thanks for testing it on the kendryte board.
> >
> > > > ---
> > > >  arch/riscv/kernel/smpboot.c |  1 +
> > > >  arch/riscv/kernel/traps.c   | 11 ++++++++++-
> > > >  2 files changed, 11 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/arch/riscv/kernel/smpboot.c b/arch/riscv/kernel/smpboot.c
> > > > index 356825a57551..23cde0ceb39d 100644
> > > > --- a/arch/riscv/kernel/smpboot.c
> > > > +++ b/arch/riscv/kernel/smpboot.c
> > > > @@ -154,6 +154,7 @@ asmlinkage __visible void smp_callin(void)
> > > >         mmgrab(mm);
> > > >         current->active_mm = mm;
> > > >
> > > > +       trap_init();
> > > >         notify_cpu_starting(curr_cpuid);
> > > >         update_siblings_masks(curr_cpuid);
> > > >         set_cpu_online(curr_cpuid, 1);
> > > > diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> > > > index ad14f4466d92..a390239818ae 100644
> > > > --- a/arch/riscv/kernel/traps.c
> > > > +++ b/arch/riscv/kernel/traps.c
> > > > @@ -174,7 +174,16 @@ int is_valid_bugaddr(unsigned long pc)
> > > >  }
> > > >  #endif /* CONFIG_GENERIC_BUG */
> > > >
> > > > -/* stvec & scratch is already set from head.S */
> > > > +/* stvec & scratch is already set from head.S when mmu is enabled */
> > > >  void trap_init(void)
> > > >  {
> > > > +#ifndef CONFIG_MMU
> > > > +       /*
> > > > +        * Set sup0 scratch register to 0, indicating to exception vector
> > > > +        * that we are presently executing in the kernel
> > > > +        */
> > > > +       csr_write(CSR_SCRATCH, 0);
> > > > +       /* Set the exception vector address */
> > > > +       csr_write(CSR_TVEC, &handle_exception);
> > > > +#endif
> > > >  }
> > > > --
> > > > 2.28.0
> > > >
> > >
> > > This issue seems to be only on the latest master branch of
> > > Linux stable tree so this fix need not be a stable fix.
> > >
> > > For MMU kernel, the CSR_TVEC is setup in relocate() function
> > > called from secondary_start_common() function of head.S
> > >
> > > For NoMMU kernel, we should set CSR_TVEC directly in
> > > secondary_start_common() function as "#else" case of the
> > > "#ifdef CONFIG_MMU".
> > >
> >
> > That would enable the trap only for secondary harts. But the exception
> > vector on boot hart
> > is still uninitialized. How about this change ?
> >
> > diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> > index d0c5c316e9bb..7822054dbd88 100644
> > --- a/arch/riscv/kernel/head.S
> > +++ b/arch/riscv/kernel/head.S
> > @@ -77,16 +77,6 @@ relocate:
> >         csrw CSR_SATP, a0
> >  .align 2
> >  1:
> > -       /* Set trap vector to exception handler */
> > -       la a0, handle_exception
> > -       csrw CSR_TVEC, a0
> > -
> > -       /*
> > -        * Set sup0 scratch register to 0, indicating to exception vector that
> > -        * we are presently executing in kernel.
> > -        */
> > -       csrw CSR_SCRATCH, zero
> > -
>
> Instead of having no trap vector setup here, we should
> at least have dummy trap vector (just like original code).
>

Ahh yes. We should include that.

> /* Set trap vector to spin forever to help debug */
> la a0, .Lsecondary_park
> csrw CSR_TVEC, a0
>
> >         /* Reload the global pointer */
> >  .option push
> >  .option norelax
> > @@ -144,9 +134,23 @@ secondary_start_common:
> >         la a0, swapper_pg_dir
> >         call relocate
> >  #endif
> > +       call setup_trap_vector
> >         tail smp_callin
> >  #endif /* CONFIG_SMP */
> >
> > +.align 2
> > +setup_trap_vector:
> > +       /* Set trap vector to exception handler */
> > +       la a0, handle_exception
> > +       csrw CSR_TVEC, a0
> > +
> > +       /*
> > +        * Set sup0 scratch register to 0, indicating to exception vector that
> > +        * we are presently executing in kernel.
> > +        */
> > +       csrw CSR_SCRATCH, zero
> > +       ret
> > +
> >  .Lsecondary_park:
> >         /* We lack SMP support or have too many harts, so park this hart */
> >         wfi
> > @@ -240,6 +244,7 @@ clear_bss_done:
> >         call relocate
> >  #endif /* CONFIG_MMU */
> >
> > +       call setup_trap_vector
> >         /* Restore C environment */
> >         la tp, init_task
> >         sw zero, TASK_TI_CPU(tp)
>
> Apart from above, this looks good.
>
> Regards,
> Anup



-- 
Regards,
Atish

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