linux-kernel - Re: Calling device_init_wakeup() on driver removal

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Message-ID: <9bf96934-0d22-e10b-f5cb-e3caf9db9c06@roeck-us.net>
Date:   Mon, 16 Jan 2017 20:37:36 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Calling device_init_wakeup() on driver removal

On 01/16/2017 01:50 PM, Rafael J. Wysocki wrote:
> On Sun, Jan 15, 2017 at 4:23 PM, Guenter Roeck <linux@...ck-us.net> wrote:
>> On 01/15/2017 06:49 AM, Rafael J. Wysocki wrote:
>>>
>>> On Saturday, January 14, 2017 08:46:05 PM Guenter Roeck wrote:
>>>>
>>>> Hi folks,
>>>
>>>
>>> Hi,
>>>
>>>> while looking through driver initialization and removal functions, I
>>>> noticed that many drivers
>>>> call device_init_wakeup(dev, false) in the removal function. Given that
>>>> the driver is about
>>>> to be removed, that doesn't make much sense to me, especially since
>>>> device_wakeup_disable()
>>>> is called from device_pm_remove() anyway.
>>>>
>>>> Is it safe to assume that all those calls can be removed, or is there a
>>>> possible reason for
>>>> keeping them around ?
>>>
>>>
>>> Removing them automatically might break things, because
>>> device_init_wakeup(dev, false)
>>> also clears the power.can_wakeup flag and removes the "wakeup" attribute
>>> from sysfs.
>>>
>>
>> I had the same concern, but I concluded that the wakeup attribute should be
>> removed
>> automatically, since it is added with sysfs_merge_group(), and the matching
>> unmerge call
>> is also made in dpm_sysfs_remove(). power.can_wakeup is part of the device
>> structure,
>> which is in the process of being removed, so I am not sure I understand how
>> that can be
>> problematic.
>
> That's when the device goes away, but ->remove() is invoked when the
> driver goes away too and that's the more common case (by far IMO).
>
Confused ... ah, no, I am using sloppy terminology again. Sorry for
that. When I talked about "driver remove function, I meant the remove
function defined in 'struct platform_driver' or 'struct i2c_driver',
which is the device remove function, not the driver remove function.
I am not talking about functions like platform_driver_unregister()
or i2c_del_driver().

>>> I guess they could be removed safely in the majority of cases, though.
>>
>>
>> How would one decide if it is needed ? I see some drivers call it on remove,
>> but others
>> don't. I don't see a clear pattern; unless I am missing something, it seems
>> to be
>> more or less random.
>
> My guess would be that drivers calling it don't want the wakeup
> capability to be exposed after they have gone away.  Most likely these
> are the ones that expose the wakeup attribute on probe.
>
> IMO the rule should be that whoever exposes the wakeup attribute one
> init, should also hide it on cleanup.
>
The probe function is a device probe function. Are you saying that calling
device_init_wakeup(dev, false) in the device remove function is necessary
if the probe function called device_init_wakeup(dev, true) ?

That is exactly what I am wondering. Because, if so, several drivers violate
that. Also, I don't really see the point because the device structure is in
the process of being removed (and I don't see anything in the code that isn't
already cleaned up).

Thanks,
Guenter