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From: Maarten Brock <>
To: Daniel Mack <>
Subject: Re: [PATCH 4/4] sc16is7xx: Use threaded IRQ
Date: Tue, 19 May 2020 18:32:32 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 2020-05-18 18:57, Daniel Mack wrote:
> Hi Maarten,
> On 5/18/20 1:14 PM, Maarten Brock wrote:
>> On 2020-05-17 22:44, Daniel Mack wrote:
>> Summerizing:
>> - After switching to a threaded IRQ, the trigger could be switched to
>> IRQF_TRIGGER_LOW and with that interrupt sharing can be enabled for
>> this device with IRQF_SHARED.
> Yes, but we don't need that. As discussed, the UART driver can cope 
> with
> edge IRQs just fine.
>> - Some (your) interrupt controllers do not support IRQF_TRIGGER_LOW.
>> For those only IRQF_TRIGGER_FALLING can be used for this device and
>> thus IRQF_SHARED cannot be used.
> True. Interrupts cannot be shared for this device then. That's a fair
> limitation, and it has always been like that.

It has always been like that for this driver. But that should be no
reason why the driver might not be improved. I wonder how the 8250
handles this. PC's have always shared interrupts for COM1/2/3/4 AFAIK.

>> - The driver for your interrupt controller should be improved to 
>> support
>> level IRQs.
> 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 
> directly, that's just as good :)

That depends on the IRQ coming out of the interrupt controller. If that 
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.

> 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.

>> This makes me wonder if it would be better to let the device tree 
>> specify
>> the interrupt configuration.
> There can be flags in the 2nd cell of the node, but their meaning is
> specific to the controller. Hence the SPI/I2C layers don't pass that
> information up.
> 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.

> Thanks,
> Daniel

Kind regards,

  reply	other threads:[~2020-05-19 16:33 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-08 14:37 [PATCH 0/4] sc16is7xx: Add IrDA mode and threaded IRQ Daniel Mack
2020-05-08 14:37 ` [PATCH 1/4] dt-bindings: sc16is7xx: Add flag to activate IrDA mode Daniel Mack
2020-05-18 18:08   ` Rob Herring
2020-05-18 18:41     ` Daniel Mack
2020-05-19 12:01       ` Maarten Brock
2020-05-08 14:37 ` [PATCH 2/4] " Daniel Mack
2020-05-08 14:37 ` [PATCH 3/4] sc16is7xx: Always use falling edge IRQ Daniel Mack
2020-05-09 12:41   ` Maarten Brock
2020-05-08 14:37 ` [PATCH 4/4] sc16is7xx: Use threaded IRQ Daniel Mack
2020-05-09 12:55   ` Maarten Brock
2020-05-17 20:44     ` Daniel Mack
2020-05-18 11:14       ` Maarten Brock
2020-05-18 16:57         ` Daniel Mack
2020-05-19 16:32           ` Maarten Brock [this message]
2020-05-19 17:37             ` Daniel Mack

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