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

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Date:   Mon, 31 Jul 2023 10:49:44 +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 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.)

Best Regards
Michał Mirosław