[PATCH] iio: adc: stm32-adc: fix erroneous handling of spurious IRQs

[Jonathan Cameron]

On Tue, 12 Jan 2021 16:24:42 +0100
Ahmad Fatoum <a.fatoum at pengutronix.de> wrote:

> 1c6c69525b40 ("genirq: Reject bogus threaded irq requests") makes sure
> that threaded IRQs either
>   - have IRQF_ONESHOT set
>   - don't have the default just return IRQ_WAKE_THREAD primary handler
> 
> This is necessary because level-triggered interrupts need to be masked,
> either at device or irqchip, to avoid an interrupt storm.
> 
> For spurious interrupts, the STM32 ADC driver still does this bogus
> request though:
>   - It doesn't set IRQF_ONESHOT
>   - Its primary handler just returns IRQ_WAKE_THREAD if the interrupt
>     is unexpected, i.e. !(status & enabled_mask)

This seems 'unusual'.  If this is a spurious interrupt we should be
returning IRQ_NONE and letting the spurious interrupt protection
stuff kick in.

The only reason I can see that it does this is print an error message.
I'm not sure why we need to go into the thread to do that given
it's not supposed to happen. If we need that message at all, I'd
suggest doing it in the interrupt handler then return IRQ_NONE;

>   - stm32mp151.dtsi describes the ADC interrupt as level-triggered
> 
> Fix this by setting IRQF_ONESHOT to have the irqchip mask the IRQ
> until the IRQ thread has finished.
> 
> IRQF_ONESHOT also has the effect that the primary handler is no longer
> forced into a thread. This makes the issue with spurious interrupts
> interrupts disappear when reading the ADC on a threadirqs=1 kernel.
> This used to result in following kernel error message:
> 
> 	iio iio:device1: Unexpected IRQ: IER=0x00000000, ISR=0x0000100e
> or
> 	iio iio:device1: Unexpected IRQ: IER=0x00000004, ISR=0x0000100a
> 
> But with this patch applied (or threaded IRQs disabled), this no longer
> occurs.
> 
> Cc: Lucas Stach <l.stach at pengutronix.de>
> Reported-by: Holger Assmann <has at pengutronix.de>
> Fixes: 695e2f5c289b ("iio: adc: stm32-adc: fix a regression when using dma and irq")
> Signed-off-by: Ahmad Fatoum <a.fatoum at pengutronix.de>
> ---
>  drivers/iio/adc/stm32-adc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/iio/adc/stm32-adc.c b/drivers/iio/adc/stm32-adc.c
> index c067c994dae2..7e0e21c79ac8 100644
> --- a/drivers/iio/adc/stm32-adc.c
> +++ b/drivers/iio/adc/stm32-adc.c
> @@ -1910,7 +1910,7 @@ static int stm32_adc_probe(struct platform_device *pdev)
>  
>  	ret = devm_request_threaded_irq(&pdev->dev, adc->irq, stm32_adc_isr,
>  					stm32_adc_threaded_isr,
> -					0, pdev->name, indio_dev);
> +					IRQF_ONESHOT, pdev->name, indio_dev);
>  	if (ret) {
>  		dev_err(&pdev->dev, "failed to request IRQ\n");
>  		return ret;