Re: [PATCH 1/3 RFC] Driver core: Add offline/online device operations

[Toshi Kani <toshi.kani@hp.com>]

On Thu, 2013-05-02 at 02:58 +0200, Rafael J. Wysocki wrote:
> On Tuesday, April 30, 2013 05:38:38 PM Toshi Kani wrote:
> > On Mon, 2013-04-29 at 14:26 +0200, Rafael J. Wysocki wrote:
> > > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
 :
> > > + */
> > > +int device_offline(struct device *dev)
> > > +{
> > > +	int ret;
> > > +
> > > +	if (dev->offline_disabled)
> > > +		return -EPERM;
> > > +
> > > +	ret = device_for_each_child(dev, NULL, device_check_offline);
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	device_lock(dev);
> > > +	if (device_supports_offline(dev)) {
> > > +		if (dev->offline) {
> > > +			ret = 1;
> > > +		} else {
> > > +			ret = dev->bus->offline(dev);
> > > +			if (!ret) {
> > > +				kobject_uevent(&dev->kobj, KOBJ_OFFLINE);
> > > +				dev->offline = true;
> > 
> > Shouldn't this offline flag be set before sending KOBJ_OFFLINE?
> > 
> > > +			}
> > > +		}
> > > +	}
> > > +	device_unlock(dev);
> > > +
> > > +	return ret;
> > > +}
> > > +
> > > +/**
> > > + * device_online - Put the device back online after successful device_offline().
> > > + * @dev: Device to be put back online.
> > > + *
> > > + * If device_offline() has been successfully executed for @dev, but the device
> > > + * has not been removed subsequently, execute its bus type's .online() callback
> > > + * to indicate that the device can be used again.
> > 
> > There is another use-case for online().  When a device like CPU is
> > hot-added, it is added in offline.  I am not sure why, but it has been
> > this way.  So, we need to call online() to make a new device available
> > for use after a hot-add.
> 
> Actually, in the CPU case that is left to user space as far as I can say.
> That is, the device appears initially offline and user space is supposed to
> bring it online via sysfs.
> 
> > > + *
> > > + * Call under device_offline_lock.
> > > + */
> > > +int device_online(struct device *dev)
> > > +{
> > > +	int ret = 0;
> > > +
> > > +	device_lock(dev);
> > > +	if (device_supports_offline(dev)) {
> > > +		if (dev->offline) {
> > > +			ret = dev->bus->online(dev);
> > > +			if (!ret) {
> > > +				kobject_uevent(&dev->kobj, KOBJ_ONLINE);
> > > +				dev->offline = false;
> > 
> > Same comment as KOBJ_OFFLINE.
> 
> I wonder why the ordering may be important?

I do not think it causes any race condition (so this isn't a big deal),
but it seems to make more sense to emit an ONLINE/OFFLINE event after
its object is marked online/offline.

Thanks,
-Toshi