[U-Boot] [PATCH v3 1/3] riscv: Add kconfig option to run u-boot in S-mode

[Palmer Dabbelt]

On Tue, 20 Nov 2018 19:41:10 PST (-0800), anup at brainfault.org wrote:
> This patch adds kconfig option RISCV_SMODE to run u-boot in
> S-mode. When this opition is enabled we use s<xyz> CSRs instead
> of m<xyz> CSRs.
>
> It is important to note that there is no equivalent S-mode CSR
> for misa and mhartid CSRs so we expect M-mode runtime firmware
> (BBL or equivalent) to emulate misa and mhartid CSR read.
>
> In-future, we will have more patches to avoid accessing misa and
> mhartid CSRs from S-mode.

Ya, I don't like this.  Our current boot protocol puts mhartid in a1 upon 
entering the supervisor, and allows the detection of misa via a device tree 
pointer provided in a0.  

As long as everyone agrees this isn't what we're actually looking for then I'm 
fine with the patch, I just don't want to end up requiring this "M mode 
emulates some CSRs for S mode" interface.  Since we don't have an actual 
platform spec we've got to be careful to avoid a bunch of defacto interfaces 
that we'll need to support later.

>
> Signed-off-by: Anup Patel <anup at brainfault.org>
> Reviewed-by: Bin Meng <bmeng.cn at gmail.com>
> Tested-by: Bin Meng <bmeng.cn at gmail.com>
> ---
>  arch/riscv/Kconfig     |  5 +++++
>  arch/riscv/cpu/start.S | 33 +++++++++++++++++++++++++++++++++
>  2 files changed, 38 insertions(+)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 3e0af55e71..8f2139ff60 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -55,6 +55,11 @@ config RISCV_ISA_C
>  config RISCV_ISA_A
>  	def_bool y
>
> +config RISCV_SMODE
> +	bool "Run in S-Mode"
> +	help
> +	  Enable this option to build an U-Boot for RISC-V S-Mode
> +
>  config 32BIT
>  	bool
>
> diff --git a/arch/riscv/cpu/start.S b/arch/riscv/cpu/start.S
> index 5af189b338..e4276e8e19 100644
> --- a/arch/riscv/cpu/start.S
> +++ b/arch/riscv/cpu/start.S
> @@ -39,10 +39,18 @@ _start:
>  	mv	s1, a1
>
>  	la	t0, trap_entry
> +#ifdef CONFIG_RISCV_SMODE
> +	csrw	stvec, t0
> +#else
>  	csrw	mtvec, t0
> +#endif
>
>  	/* mask all interrupts */
> +#ifdef CONFIG_RISCV_SMODE
> +	csrw	sie, zero
> +#else
>  	csrw	mie, zero
> +#endif
>
>  	/* Enable cache */
>  	jal	icache_enable
> @@ -164,7 +172,11 @@ fix_rela_dyn:
>  */
>  	la	t0, trap_entry
>  	add	t0, t0, t6
> +#ifdef CONFIG_RISCV_SMODE
> +	csrw	stvec, t0
> +#else
>  	csrw	mtvec, t0
> +#endif
>
>  clear_bss:
>  	la	t0, __bss_start		/* t0 <- rel __bss_start in FLASH */
> @@ -236,17 +248,34 @@ trap_entry:
>  	SREG	x29, 29*REGBYTES(sp)
>  	SREG	x30, 30*REGBYTES(sp)
>  	SREG	x31, 31*REGBYTES(sp)
> +#ifdef CONFIG_RISCV_SMODE
> +	csrr	a0, scause
> +	csrr	a1, sepc
> +#else
>  	csrr	a0, mcause
>  	csrr	a1, mepc
> +#endif
>  	mv	a2, sp
>  	jal	handle_trap
> +#ifdef CONFIG_RISCV_SMODE
> +	csrw	sepc, a0
> +#else
>  	csrw	mepc, a0
> +#endif
>
> +#ifdef CONFIG_RISCV_SMODE
> +/*
> + * Remain in S-mode after sret
> + */
> +	li	t0, SSTATUS_SPP
> +	csrs	sstatus, t0
> +#else
>  /*
>   * Remain in M-mode after mret
>   */
>  	li	t0, MSTATUS_MPP
>  	csrs	mstatus, t0
> +#endif
>  	LREG	x1, 1*REGBYTES(sp)
>  	LREG	x2, 2*REGBYTES(sp)
>  	LREG	x3, 3*REGBYTES(sp)
> @@ -279,4 +308,8 @@ trap_entry:
>  	LREG	x30, 30*REGBYTES(sp)
>  	LREG	x31, 31*REGBYTES(sp)
>  	addi	sp, sp, 32*REGBYTES
> +#ifdef CONFIG_RISCV_SMODE
> +	sret
> +#else
>  	mret
> +#endif