linux-kernel - Re: [PATCH v3 2/2] i2c: Add GPIO-based hotplug gate

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Message-ID: <ZMg6m+Dru6rxCRqU@qmqm.qmqm.pl>
Date:   Tue, 1 Aug 2023 00:50:03 +0200
From:   Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     Svyatoslav Ryhel <clamor95@...il.com>,
        Andi Shyti <andi.shyti@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Wolfram Sang <wsa@...nel.org>, linux-i2c@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] i2c: Add GPIO-based hotplug gate

On Mon, Jul 31, 2023 at 02:59:41PM +0200, Krzysztof Kozlowski wrote:
> On 31/07/2023 10:49, Michał Mirosław wrote:
> > On Mon, Jul 31, 2023 at 08:58:14AM +0200, Krzysztof Kozlowski wrote:
> >> On 30/07/2023 23:55, Michał Mirosław wrote:
> >>> On Sun, Jul 30, 2023 at 10:30:56PM +0200, Krzysztof Kozlowski wrote:
> >>>> On 29/07/2023 18:08, Svyatoslav Ryhel wrote:
> >>>>> From: Michał Mirosław <mirq-linux@...e.qmqm.pl>
> >>>>>
> >>>>> Implement driver for hot-plugged I2C busses, where some devices on
> >>>>> a bus are hot-pluggable and their presence is indicated by GPIO line.
> >>> [...] 
> >>>>> +	priv->irq = platform_get_irq(pdev, 0);
> >>>>> +	if (priv->irq < 0)
> >>>>> +		return dev_err_probe(&pdev->dev, priv->irq,
> >>>>> +				     "failed to get IRQ %d\n", priv->irq);
> >>>>> +
> >>>>> +	ret = devm_request_threaded_irq(&pdev->dev, priv->irq, NULL,
> >>>>> +					i2c_hotplug_interrupt,
> >>>>> +					IRQF_ONESHOT | IRQF_SHARED,
> >>>>
> >>>> Shared IRQ with devm is a recipe for disaster. Are you sure this is a
> >>>> shared one? You have a remove() function which also points that it is
> >>>> not safe. You can:
> >>>> 1. investigate to be sure it is 100% safe (please document why do you
> >>>> think it is safe)
> >>>
> >>> Could you elaborate on what is unsafe in using devm with shared
> >>> interrupts (as compared to non-shared or not devm-managed)?
> >>>
> >>> The remove function is indeed reversing the order of cleanup. The
> >>> shutdown path can be fixed by removing `remove()` and adding
> >>> `devm_add_action_or_reset(...deactivate)` before the IRQ is registered.
> >> Shared interrupt might be triggered easily by other device between
> >> remove() and irq release function (devm_free_irq() or whatever it is
> >> called).
> > 
> > This is no different tham a non-shared interrupt that can be triggered
> > by the device being removed. Since devres will release the IRQ first,
> > before freeing the driver data, the interrupt hander will see consistent
> > driver-internal state. (The difference between remove() and devres
> > release phase is that for the latter sysfs files are already removed.)
> 
> True, therefore non-devm interrupts are recommended also in such case.
> Maybe one of my solutions is actually not recommended.
> 
> However if done right, driver with non-shared interrupts, is expected to
> disable interrupts in remove(), thus there is no risk. We have big
> discussions in the past about it, so feel free to dig through LKML to
> read more about. Anyway shared and devm is a clear no go.

Can you share pointers to some of those discussions? Quick search
about devm_request_irq() and friends found only a thread from 2013
about conversions of RTC drivers to use devres. [1] IIRC the issue was
then that the drivers requested IRQs before fully initializing the state
(as many still do). Back to the original question: what is the risk
in using devres with shared interrupts? (Let's assume the probe() is already
fixed and remove() removed.)

BTW, We have devres doc [2] in the kernel tree that, among other things,
lists IRQs as a managed resource and mentions no warnings nor restictions
for driver authors. I'd expect that if devm_request_threaded_irq() for
shared iterrupts was indeed deprecated, it should be documented in a way
easy to refer to.

[1] https://groups.google.com/g/linux.kernel/c/yi2ueo-sNJs
[2] Documentation/udriver-api/driver-model/devres.rst

Best Regards
Michał Mirosław