[Linux-stm32] STM32MP1 level triggered interrupts

[Uwe Kleine-König]

On Thu, Jan 23, 2020 at 09:22:48AM +0000, Marc Zyngier wrote:
> On 2020-01-23 08:27, Alexandre Torgue wrote:
> > On 1/22/20 8:29 PM, Marek Vasut wrote:
> > > On 1/22/20 6:19 PM, Alexandre Torgue wrote:
> > > 
> > > Hi,
> > > 
> > > [...]
> > > 
> > > > > > Concerning, your question:
> > > > > > 
> > > > > > Setting your gpioC interruption as "falling edge" should
> > > > > > be enough. On
> > > > > > gpioCx falling edge, a high-level signal is generated by
> > > > > > exti and sent
> > > > > > to GIC (which triggers GIC interrupt). This signal
> > > > > > remains high until
> > > > > > stm32_irq_ack is called.
> > > > > > 
> > > > > > So you only need: (ex for gpioc 1).
> > > > > > 
> > > > > > interrupt-parent = <&gpioc>;
> > > > > > interrupts = <1 IRQ_TYPE_EDGE_FALLING>;
> > > > > 
> > > > > How does this deal with the case where the device holds the
> > > > > interrupt
> > > > > line low (since it's level-sensitive, active low) after the driver
> > > > > interrupt handler finishes ? Does such condition generate another
> > > > > interrupt and call the driver interrupt handler again ? I
> > > > > would expect
> > > > > the answer is no, because the interrupt is edge-triggered
> > > > > and there is
> > > > > no edge.
> > > > 
> > > > Your assumption is good. If your device continue to hold the
> > > > line to low
> > > > at the end of your interrupt handler, no more interrupt will be
> > > > generated.
> > > 
> > > But does that basically mean that such a device cannot be used with
> > > STM32MP1 or am I fundamentally mistaken and don't understand how a
> > > level-triggered interrupt works ? :)
> > 
> > You need to release the line in your device interrupt handler. If not,
> > yes, you will miss interrupts :$
> 
> So to sum it up, this SoC doesn't support external level interrupts
> on its own, full stop. You'd need some additional external sampling
> HW to retrigger an edge on EOI.

Or you need software support that marks the irq pending again if on
unmask the irq line is still active.

Best regards
Uwe

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Pengutronix e.K.                           | Uwe Kleine-König            |
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