pussuw commented on a change in pull request #5782:
URL: https://github.com/apache/incubator-nuttx/pull/5782#discussion_r837265005
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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:
As you already mentioned, this acking of the timer is not used in the
RISC-V SBI spec. but my analysis of the spec is that the timer is unusable as
is anyway. The format of sbi_set_timer(absolute_time) is not safe to use, as
S-mode does not know the timer's value, so it cannot calculate absolute_time >
mtimecmp anyway. Setting the timer will very likely result in either two things:
- Immediate interrupt, because absolute_time > mtimecmp. As I said, the IRQ
is a level detect, so whenever mtime > mtimecmp, an interrupt will fire. Other
compare match implementations use level detect, so when time == cmpvalue, an
interrupt is triggered exactly once.
- Interrupt will fire much later than expected, or perhaps even never, as
mtimecmp > mtime. The counter is 64-bit so mtimecmp can become so outrageously
large compared to mtime, tha the interrupt will never fire (the sun will
explode before).
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