linux-kernel - Re: [PATCH v2 3/5] counter: new TI eQEP driver

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Date:   Sun, 11 Aug 2019 14:15:14 -0500
From:   David Lechner <david@...hnology.com>
To:     Jonathan Cameron <jic23@...nel.org>, linux-omap@...r.kernel.org
Cc:     linux-iio@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Benoît Cousson <bcousson@...libre.com>,
        Tony Lindgren <tony@...mide.com>,
        William Breathitt Gray <vilhelm.gray@...il.com>,
        Thierry Reding <thierry.reding@...il.com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pwm@...r.kernel.org
Subject: Re: [PATCH v2 3/5] counter: new TI eQEP driver

On 8/11/19 4:23 AM, Jonathan Cameron wrote:
> On Wed,  7 Aug 2019 14:40:21 -0500
> David Lechner <david@...hnology.com> wrote:
>> +
>> +	pm_runtime_enable(dev);
>> +	pm_runtime_get(dev);
> I'm a little confused on the flow here.
> 
> pm_runtime_enable turns on runtime pm in general.
> 
> pm_runtime_get basically calls runtime_resume to ensrue the
> device has power.
> 
>> +
>> +	return devm_counter_register(dev, &priv->counter);
> 
> This registers the userspace interfaces and exposes the device.
> 
>> +}
>> +
>> +static int ti_eqep_remove(struct platform_device *pdev)
>> +{
>> +	struct device *dev = &pdev->dev;
>> +
>> +	pm_runtime_put(dev),
>> +	pm_runtime_disable(dev);
> 
> pm_runtime_put notifies the system that the device is idle
> (and hence potentially turns it off).
> 
> Not good if the counter is still registered.
> 
> I'm assuming the presence of runtime pm at all is to interact
> with a parent driver and hence stop that turning off if this
> driver is probed?  That's probably fine, but add a few comments
> to make it clear that this driver itself doesn't really do
> runtime pm at all.
> 

To be honest, despite reading the runtime PM docs more than once,
I still don't feel like I have a good grasp on how it is supposed
to work. I just know that we get a crash if it is omitted because
the IP block is not powered on.

I will fix the ordering in _remove() and add a comment in v3.