linux-kernel - Re: Question about SPIs' interrupt trigger type restrictions

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Date:   Wed, 25 May 2022 20:14:16 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     richard clark <richard.xnu.clark@...il.com>, maz@...nel.org
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: Question about SPIs' interrupt trigger type restrictions

On 2022-05-25 11:01, richard clark wrote:
> Hi Marc,
> 
> For below code snippet about SPI interrupt trigger type:
> 
> static int gic_set_type(struct irq_data *d, unsigned int type)
> {
>          ...
>          /* SPIs have restrictions on the supported types */
>          if ((range == SPI_RANGE || range == ESPI_RANGE) &&
>              type != IRQ_TYPE_LEVEL_HIGH && type != IRQ_TYPE_EDGE_RISING)
>                  return -EINVAL;
>          ...
> }
> 
> We have a device at hand whose interrupt type is SPI, Falling edge
> will trigger the interrupt. But the request_irq(50, handler,
> IRQ_TYPE_EDGE_FALLING, ...) will return -EINVAL.
> 
> The question is, why must the SPI interrupt use IRQ_TYPE_EDGE_RISING
> instead of IRQ_TYPE_EDGE_FALLING?

Because that's what the GIC architecture[1] says. From section 1.2.1 
"Interrupt Types":

"An interrupt that is edge-triggered has the following property:
	• It is asserted on detection of a rising edge of an interrupt signal 
and then, regardless of the state of the signal, remains asserted until 
the interrupt is acknowledged by software."

External signals with the wrong polarity may need external logic to 
invert them (which might even be offered by the GIC implementation 
itself, e.g. [2]), but the programmer's model neither knows nor cares 
about such details, it only knows notions of "edge-triggered" and 
"level-sensitive", where from its point of view the asserted states are 
rising and high respectively.

Robin.

[1] https://developer.arm.com/documentation/ihi0069/latest
[2] 
https://developer.arm.com/documentation/100336/0106/components-and-configuration/spi-collator/spi-collator-wires?lang=en