Re: [PATCH 4/4] sc16is7xx: Use threaded IRQ

[Daniel Mack <daniel@zonque.org>]

On 5/19/20 6:32 PM, Maarten Brock wrote:
> On 2020-05-18 18:57, Daniel Mack wrote:

>> It's a controller that sits behind another hardware bus itself, so
>> polling is expensive. If the controller would need to check for level
>> IRQs it would need to poll, and then we could as well just poll the UART
>> directly, that's just as good :)
> 
> That depends on the IRQ coming out of the interrupt controller. If that is
> a level interrupt itself, then it is easy to see if all interrupts are
> handled. Further polling zooms in on the devices that require attention.

Yeah, I'm familiar with the concept, but it's not like that here,
unfortunately.

>> But again - the UART driver works perfectly fine with edge IRQs as long
>> as the interrupt is not shared.
> 
> If you would require multiple sc16is7xx devices on I2C would you like to
> connect multiple interrupt lines? Or just SCL,SDA and *one* IRQ?
> 
> OTOH for SPI you would require multiple CS already.

Right. Nevertheless, we can allow sharing the IRQ line for level-trigger
capable IRQ controllers, you're right.

>> What many drivers do is try with one setting, and if that fails because
>> the interrupt controller returns an error, they fall back to something
>> else. We could do the same here of course, but it'd be another patch on
>> top, as it's unrelated to the concrete change the patch we're commenting
>> on is bringing in.
>>
>> So what I can add is logic that first tries with IRQF_LOW|IRQF_SHARED,
>> and if that fails, we fall back to IRQF_FALLING and retry. WDYT?
> 
> That sounds like a decent plan.

Okay, I'll add a patch to the series then and resend.


Thanks for your feedback!
Daniel