On Tue, Sep 8, 2020 at 2:17 AM Sean Anderson <sean...@gmail.com> wrote: > > This adds comments regarding the ordering and purpose of certain > instructions as I understand them. > > Signed-off-by: Sean Anderson <sean...@gmail.com> > --- > > arch/riscv/cpu/start.S | 19 +++++++++++++++++-- > 1 file changed, 17 insertions(+), 2 deletions(-) > > diff --git a/arch/riscv/cpu/start.S b/arch/riscv/cpu/start.S > index 59d3d7bbf4..c659c6df53 100644 > --- a/arch/riscv/cpu/start.S > +++ b/arch/riscv/cpu/start.S > @@ -43,7 +43,10 @@ _start: > csrr a0, CSR_MHARTID > #endif > > - /* save hart id and dtb pointer */ > + /* > + * Save hart id and dtb pointer. The thread pointer register is not
U-Boot does not use tp register in C code. > + * modified by C code, and may be used in trap handlers. > + */ > mv tp, a0 > mv s1, a1 > > @@ -54,10 +57,18 @@ _start: > */ > mv gp, zero > > + /* > + * Set the trap handler. This must happen after initializing tp and gp > + * because the handler may use these registers. > + */ > la t0, trap_entry > csrw MODE_PREFIX(tvec), t0 > > - /* mask all interrupts */ > + /* > + * Mask all interrupts. Interrupts are disabled globally (in > m/sstatus) > + * for U-Boot, but we will need to read m/sip to determine if we get > an > + * IPI > + */ > csrw MODE_PREFIX(ie), zero > > #if CONFIG_IS_ENABLED(SMP) > @@ -407,6 +418,10 @@ secondary_hart_relocate: > mv gp, a2 > #endif > > +/* > + * Interrupts are disabled globally, but they can still be read from m/sip. > The > + * wfi function will wake us up if we get an IPI, even if we do not trap. > + */ > secondary_hart_loop: > wfi Regards, Bin
