[RFC PATCH v3 1/2] arch: riscv: cpu: Add callback to init each core

[Sean Anderson]

On 3/26/21 10:18 PM, Green Wan wrote:
> Add a callback riscv_hart_early_init() to ./arch/riscv/cpu/start.S to
> allow different riscv hart perform setup code for each hart as early
> as possible. Since all the harts enter the callback, they must be able
> to run the same setup.
> 
> Signed-off-by: Green Wan <green.wan at sifive.com>
> ---
>   arch/riscv/cpu/cpu.c   | 15 +++++++++++++++
>   arch/riscv/cpu/start.S | 14 ++++++++++++++
>   2 files changed, 29 insertions(+)
> 
> diff --git a/arch/riscv/cpu/cpu.c b/arch/riscv/cpu/cpu.c
> index 85592f5bee..c779a477c7 100644
> --- a/arch/riscv/cpu/cpu.c
> +++ b/arch/riscv/cpu/cpu.c
> @@ -140,3 +140,18 @@ int arch_early_init_r(void)
>   {
>   	return riscv_cpu_probe();
>   }
> +
> +/**
> + * riscv_hart_early_init() - A dummy function called by
> + * ./arch/riscv/cpu/start.S to allow to disable/enable features of each core.
> + * For example, to turn on or clear chicken bits.

What's a chicken bit?

> + *
> + * This function is executed by each core after stack is initialized and not
> + * expect to access gd since gd is not initialized. All operations in this
> + * function should affect core itself only. In multi-core system, any access
> + * to common resource or registers outside core should be avoided or need a
> + * protection for multicore.

Perhaps just

In a multi-core system, this function must not access shared resources.

Any access to such resources would probably be better done with
available_harts_lock held. However, I doubt that any such access will be
necessary.

--Sean

> + */
> +__weak void riscv_hart_early_init(void)
> +{
> +}
> diff --git a/arch/riscv/cpu/start.S b/arch/riscv/cpu/start.S
> index 8589509e01..ab73008f23 100644
> --- a/arch/riscv/cpu/start.S
> +++ b/arch/riscv/cpu/start.S
> @@ -117,6 +117,20 @@ call_board_init_f_0:
>   	mv	sp, a0
>   #endif
>   
> +#if CONFIG_IS_ENABLED(RISCV_MMODE)
> +	/*
> +	 * Jump to riscv_hart_early_init() to perform init for each core. Not
> +	 * expect to access gd since gd is not initialized. All operations in the
> +	 * function should affect core itself only. In multi-core system, any access
> +	 * to common resource or registers outside core should be avoided or need a
> +	 * protection for multicore.
> +	 *
> +	 * A dummy implementation is provided in ./arch/riscv/cpu/cpu.c.
> +	 */
> +call_riscv_hart_early_init:
> +	jal	riscv_hart_early_init
> +#endif
> +
>   #ifndef CONFIG_XIP
>   	/*
>   	 * Pick hart to initialize global data and run U-Boot. The other harts
>