xiaoxiang781216 commented on a change in pull request #5782:
URL: https://github.com/apache/incubator-nuttx/pull/5782#discussion_r837389096
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File path: arch/risc-v/src/common/supervisor/riscv_sbi.c
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@@ -41,7 +47,11 @@
void riscv_sbi_ack_timer(void)
{
+#ifdef CONFIG_NUTTSBI
+ riscv_mcall_ack_timer();
Review comment:
> I think I might be wrong on this. This detail was VERY well hidden in
the RISC-V privileged spec. But according to this:
[riscv-software-src/riscv-pk#184](https://github.com/riscv-software-src/riscv-pk/issues/184)
>
Good finding.
> mtime apparently is accessible to the supervisor, via CSR called "time"
which apparently, implicitly equals mtime, or at least can be used as a seed
for calculating values for sbi_set_timer(abs_time). This detail was not at all
apparent to me and I won't believe it works until I have tried.
>
I think we can add a general macro to mode.h, so the same macro can be used
with in S-mode and M-mode.
> The reason I'm thoroughly confused, is because the mtime/mtimecmp
registers are not really CSRs accessible with the csr instructions. They are
memory mapped registers that reside in the CLINT memory space, and are accessed
via the normal ld/sd instructions, which reference memory (not registers).
>
From my previous experience with IC designer, the source of timer CSR may
come from(connect to) the external timer.
> An example of reading / writing their values is like this:
>
> ```
> static void c906_reload_mtimecmp(void)
> {
> irqstate_t flags = spin_lock_irqsave(NULL);
>
> uint64_t current;
> uint64_t next;
>
> if (!_b_tick_started)
> {
> _b_tick_started = true;
> current = getreg64(C906_CLINT_MTIME);
> }
> else
> {
> current = getreg64(C906_CLINT_MTIMECMP);
> }
>
> uint64_t tick = TICK_COUNT;
> next = current + tick;
>
> putreg64(next, C906_CLINT_MTIMECMP);
>
> spin_unlock_irqrestore(NULL, flags);
> }
> ```
>
> However, in the risc-v privileged ISA spec there indeed is a CSR called
"time", which lives in the user ISA. If the value of time == value of mtime,
then using riscv_mcall_ack_timer() becomes unnecessary and we can do it like
the RISC-V SBI standard indicates.
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