Re: Help: NVIC, level-triggered interrupts and interrupt pending

[Peter Maydell]

On Mon, 30 May 2022 at 12:34, Igor Kotrasiński <i.kotrasinsk@samsung.com> wrote:
>
> On 30.05.2022 12:49, Peter Maydell wrote:
> > On Mon, 30 May 2022 at 10:33, Igor Kotrasiński <i.kotrasinsk@samsung.com> 
> > wrote:
> >>
> >> Hi,
> >>
> >> I've been hacking on QEMU recently, adding support for a custom Cortex-M
> >> board. One of the devices that I'm emulating is using level-triggered
> >> interrupts, for which I assume qemu_irq_raise/lower is the right tool.
> >> However, I'm having trouble receiving interrupts for my device which I
> >> *think* are caused by interaction with setting/clearing
> >> interrupt-pending status.
> >>
> >> The way I'm handling my interrupt currently is as follows:
> >>
> >> 1. The device raises an interrupt with qemu_irq_raise.
> >> 2. In interrupt handler, I set the clear-enable nvic register and queue
> >> a handler.
> >> 3. In handler, I read data from the device and set a register that
> >> lowers its interrupt with qemu_irq_lower.
> >> 4. Then I set the clear-pending nvic register.
> >> 5. Finally I set the set-enable nvic register.
> >
> > You don't say in this sequence where "return from the interrupt
> > exception handler" happens, which is an important part of the
> > sequence. (That is, I'm not sure whether your design has only
> > step 2 in the CPU exception handler with the "queued" handler
> > being run after exception-return, or whether all of 2-5 are
> > in the CPU exception handler for the interrupt.)
> >
>
> Sorry, I lied a bit here. I'm running someone else's code and I misread
> it a bit. The exact sequence here is:
>
> 1. The device raises the interrupt.
> 2. Interrupt handler sets the set-pending register, then the
> clear-enable register, then returns.
> 3. A handler running outside exception context reads data from the
> device and lowers the irq.
> 3.5. The device raises the irq again.
> 4. The handler sets the clear-pending register, then the set-enable
> register.

Does this code work on real hardware?

Do you have a test case image you could provide?

thanks
-- PMM