linux-kernel - Re: [PATCH v5] PM / wakeup: show wakeup sources stats in sysfs

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Date:   Wed, 31 Jul 2019 10:34:11 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Tri Vo <trong@...roid.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Hridya Valsaraju <hridya@...gle.com>,
        Sandeep Patil <sspatil@...gle.com>,
        Kalesh Singh <kaleshsingh@...gle.com>,
        Ravi Chandra Sadineni <ravisadineni@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        kbuild test robot <lkp@...el.com>
Subject: Re: [PATCH v5] PM / wakeup: show wakeup sources stats in sysfs

On Wed, Jul 31, 2019 at 1:41 AM Stephen Boyd <swboyd@...omium.org> wrote:
>
> Quoting Rafael J. Wysocki (2019-07-30 16:05:55)
> > On Wed, Jul 31, 2019 at 12:26 AM Stephen Boyd <swboyd@...omium.org> wrote:
> > >
> > > Quoting Rafael J. Wysocki (2019-07-30 15:17:55)
> > > > On Tuesday, July 30, 2019 8:48:09 PM CEST Stephen Boyd wrote:
> > > > >
> > > > > Using the same prefix for the class and the device name is quite common.
> > > > > For example, see the input, regulator, tty, tpm, remoteproc, hwmon,
> > > > > extcon classes. I'd prefer it was left as /sys/class/wakeup/wakeupN. The
> > > > > class name could be changed to wakeup_source perhaps (i.e.
> > > > > /sys/class/wakeup_source/wakeupN)?
> > > >
> > > > Alternatively /sys/class/wakeup/wsN
> > > >
> > >
> > > Or /sys/class/wakeup/eventN? It's your bikeshed to paint.
> >
> > So actually the underlying problem here is that device_wakeup_enable()
> > tries to register a wakeup source and then attach it to the device to
> > avoid calling possibly sleeping functions under a spinlock.
>
> Agreed, that is one problem.
>
> >
> > However, it should be possible to call wakeup_source_create(name)
> > first, then attach the wakeup source to the device (after checking for
> > presence), and then invoke wakeup_source_add() (after dropping the
> > lock).  If the wakeup source virtual device registration is done in
> > wakeup_source_add(), that should avoid the problem altogether without
> > having to introduce extra complexity.
>
> While reordering the code to do what you describe will fix this specific
> duplicate name problem, it won't fix the general problem with reusing
> device names from one bus on a different bus/class.

Fair enough.

> We can run into the same problem when two buses name their devices the
> same name and then we attempt to attach a wakeup source to those two
> devices. Or we can have a problem where a virtual wakeup is made with
> the same name, and again we'll try to make a duplicate named device.
> Using something like 'event' or 'wakeup' or 'ws' as the prefix avoids this
> problem and keeps things clean.

Or suffix, like "<devname-wakeup>.

But if prefixes are used by an existing convention, I would prefer
"ws-" as it is concise enough and should not be confusing.

> We should probably avoid letting the same virtual wakeup source be made
> with the same name anyway, because userspace will be confused about what
> virtual wakeup it is otherwise. I concede that using the name of the
> wakeup source catches this problem without adding extra code.
>
> Either way, I'd like to see what you outline implemented so that we
> don't need to do more work than is necessary when userspace writes to
> the file.

Since we agree here, let's make this change first.  I can cut a patch
for that in a reasonable time frame I think if no one else beats me to
that.

> I just don't want to see us need to change the name of the
> wakeup device later on and then add a 'name' attribute to the class so
> that we can avoid name collisions due to various buses controlling the
> string we use to create the name of the wakeup device.

OK