[U-Boot] [PATCH v5 1/4] riscv: Add kconfig option to run U-Boot in S-mode

[Anup Patel]

On Tue, Nov 27, 2018 at 12:09 PM Rick Chen <rickchen36 at gmail.com> wrote:
>
> > > Subject: [PATCH v5 1/4] riscv: Add kconfig option to run U-Boot in S-mode
> > >
> > > 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.
> > >
> > > 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>
> > > Reviewed-by: Lukas Auer <lukas.auer at aisec.fraunhofer.de>
> > > ---
> > >  arch/riscv/Kconfig                |  5 +++++
> > >  arch/riscv/cpu/start.S            | 33 ++++++++++++++++++++++++++++
> > >  arch/riscv/include/asm/encoding.h |  2 ++
> > >  arch/riscv/lib/interrupts.c       | 36 +++++++++++++++++++++++--------
> > >  4 files changed, 67 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig index 3e0af55e71..732a357a99
> > > 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 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
> > > 15e1b8199a..704190f946 100644
> > > --- a/arch/riscv/cpu/start.S
> > > +++ b/arch/riscv/cpu/start.S
> > > @@ -41,10 +41,18 @@ _start:
> > >       li      t0, CONFIG_SYS_SDRAM_BASE
> > >       SREG    a2, 0(t0)
> > >       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
> > > @@ -166,7 +174,11 @@ 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 */
> > > @@ -238,17 +250,34 @@ 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
>
> It only the difference between s and m in csr.
> The code seem to be duplicate too much.
> Can you implement it in macro ?
> or consider to separate it in different .S file
>
> It may too complex to maintain in the future.
>

Here are few reasons why not to do this:
1. We don't have same set of CSRs for M-mode and S-mode. For
certain, M-mode CSRs we don't have any corresponding S-mode
CSRs.
2. Number of CSRs for each mode are really few so there won't be
much #ifdef in code. Having separate .S will be total overkill and
too much code duplication.
3. Using macro would mean we use incomplete CSR names
examples: "status" for "mstatus"/"sstatus", "epc" for "mepc"/"sepc",
etc. This means we cannot grep code for use of a CSR. I think this
is less readable compared to using #ifdef

Despite above reasons, above can always be attempted as
separate patch.

Regards,
Anup