Re: [PATCH] riscv: Setup exception vector for K210 properly

[Qiu Wenbo <qiuwenbo@phytium.com.cn>]

That is a more reasonable approach.

On 8/12/20 2:45 AM, Atish Patra wrote:
> On Tue, Aug 11, 2020 at 1:41 AM Anup Patel <anup@brainfault.org> wrote:
>> On Tue, Aug 11, 2020 at 12:07 PM Qiu Wenbo <qiuwenbo@phytium.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@phytium.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
> -
>          /* 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)
>
>
>> Regards,
>> Anup
>>
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>
>