On Sun, Jun 21, 2020, 11:46 AM Gregory Nutt <spudan...@gmail.com> wrote:
>
> > I'm bringing up support for a new RISC-V core but I'm running into some
> > issues around the sys tick.
> >
> > My tick timer interrupt is disabled at activating a task, clearly this is
> > not correct. This is code used across all architectures so I'm sure I
> just
> > do not understand something
> >
> > void nxtask_activate(FAR struct tcb_s *tcb)
> > {
> > irqstate_t flags = enter_critical_section();
> > up_unblock_task(tcb);
> > leave_critical_section(flags);
> > }
> >
> >
> > Won't we perform a context switch here and not leave the critical
> section?
>
> No, the critical section is left automatically when the context switch
> occurs. The critical section is not a global attribute; it is a per
> task attribute. If Task A enters the critical section then suspends (as
> above) the state of critical section is saved and the new state of the
> critical section for the newly started Task B is instantiated. For a
> new task like this, the initial state of the critical section will be
> "not-in-a-critical section".
>
> This is described in a Wiki pages somewhere, but I don't recall which.
> In know that is mentioned in the Wiki page on the critical section
> monitor but I don't think that is the authoritative reference.
>
> So, don't worry. This has all been carefully thought through and has
> worked well for 13.3 years.
>
I totally agree that the os is working as expected and that I am porting
some functionally wrong, but I am trying to understand where the interrupts
get re-enabled in this flow. The irq save/restore look like this, but when
I instrument them I see a save called on the entry to the init task, but
never a restore. At that point the timer isr is never triggered to indicate
an OS tick. Is there another place I should be hooking to renenable the
interrupt on the context switch?
irqstate_t up_irq_save(void)
{
uint64_t oldstat;
/* Read mstatus & clear machine interrupt enable (MIE) in mstatus */
asm volatile ("csrrc %0, mstatus, %1": "=r" (oldstat) : "r"(MSTATUS_MIE));
return oldstat;
}
void up_irq_restore(irqstate_t flags)
{
/* Write flags to mstatus */
asm volatile("csrw mstatus, %0" : /* no output */ : "r" (flags));
}
>
