spi->irq == 0 on module reload of driver using IRQF_TRIGGER_LOW

Marc Zyngier
2017-11-13T09:35:57Z

On Sun, Nov 12 2017 at 5:49:39 pm GMT, <kernel@martin.sperl.org> wrote:
>> On 12.11.2017, at 16:41, Marc Zyngier <marc.zyngier@arm.com> wrote:
>>> +
>>> + can0: mcp2517fd at 0 {
>>> + reg = <0>;
>>> + compatible = "microchip,mcp2517fd";
>>> + pinctrl-names = "default";
>>> + pinctrl-0 = <&can0_pins>;
>>> + spi-max-frequency = <12500000>;
>>> + interrupt-parent = <&gpio>;
>>> + interrupts = <16 0x2>;
>>
>> This indicates a falling edge. No wonder the kernel is confused (I
>> don't know why this isn't enforced the first time though, probably an
>> issue in the GPIO irqchip driver...). Replacing this 2 with a 8 should
>> allow you to make some progress.
>
> Thanks for the clarification - with that change it works!
>
> For a better understanding:
> Isn?t the interrupt type to use more of a driver decision than a
> HW implementation detail that needs to get defined in the device tree?

Absolutely *not*. The signalling of an interrupt is completely HW
dependent, and the driver *must* use abide by the HW rule. That's
because edge and level interrupts signal entirely different things:

- An edge interrupt signals a an event. Something has happened, and many
of these events can be signalled independently without the CPU doing
anything.

- A level interrupt indicates a change of state, and this state persist
until the CPU has services the interrupt.

That's the difference between receiving a SMS each time you pay
something your bank card, and receiving a phone call from your bank
because your account is overdrawn.

> In my case I probably could write some more code that would allow edge
> interrupts to work (without race-conditions on spi transfers - probably
> by using spi_async to reenable interrupts on the HW device), but it
> would not be as straight-forward and a bit more complex.

And more or less wrong, given that the spec sheet calls out "active low"
interrupts.

> Summary: Essentially the driver has to match the interrupt type -
> otherwise it will fail (on second initialization).

And even that is not quite right. The driver should fail straight
away. on the first request. I don't have the HW to track it down, but
I'd appreciate it if you could have a look and enlighten me.

Thanks,

M.
--
Jazz is not dead, it just smell funny.