netdev - Re: [patch net-next v2 1/9] Introduce devlink infrastructure

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Date:	Wed, 24 Feb 2016 11:12:07 -0500
From:	Jarod Wilson <jarod@...hat.com>
To:	Jiri Pirko <jiri@...nulli.us>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, idosch@...lanox.com,
	eladr@...lanox.com, yotamg@...lanox.com, ogerlitz@...lanox.com,
	yishaih@...lanox.com, dledford@...hat.com, sean.hefty@...el.com,
	hal.rosenstock@...il.com, eugenia@...lanox.com,
	roopa@...ulusnetworks.com, nikolay@...ulusnetworks.com,
	hadarh@...lanox.com, jhs@...atatu.com, john.fastabend@...il.com,
	jeffrey.t.kirsher@...el.com, brouer@...hat.com, ivecera@...hat.com,
	rami.rosen@...el.com, hannes@...essinduktion.org,
	gospo@...ulusnetworks.com
Subject: Re: [patch net-next v2 1/9] Introduce devlink infrastructure

On Wed, Feb 24, 2016 at 08:19:58AM +0100, Jiri Pirko wrote:
> Tue, Feb 23, 2016 at 10:19:48PM CET, jarod@...hat.com wrote:
> >On Tue, Feb 23, 2016 at 04:51:26PM +0100, Jiri Pirko wrote:
> >> From: Jiri Pirko <jiri@...lanox.com>
> >> 
> >> Introduce devlink infrastructure for drivers to register and expose to
> >> userspace via generic Netlink interface.
> >> 
> >> There are two basic objects defined:
> >> devlink - one instance for every "parent device", for example switch ASIC
> >> devlink port - one instance for every physical port of the device.
> >> 
> >> This initial portion implements basic get/dump of objects to userspace.
> >> Also, port splitter and port type setting is implemented.
> >...
> >> +/**
> >> + *	devlink_alloc - Allocate new devlink instance resources
> >> + *
> >> + *	@ops: ops
> >> + *	@priv_size: size of user private data
> >> + *
> >> + *	Allocate new devlink instance resources, including devlink index
> >> + *	and name.
> >> + */
> >> +struct devlink *devlink_alloc(const struct devlink_ops *ops, size_t priv_size)
> >> +{
> >> +	static atomic_t dev_counter = ATOMIC_INIT(0);
> >> +	struct devlink *devlink;
> >> +
> >> +	devlink = kzalloc(sizeof(*devlink) + priv_size, GFP_KERNEL);
> >> +	if (!devlink)
> >> +		return NULL;
> >> +	devlink->ops = ops;
> >> +	devlink_net_set(devlink, &init_net);
> >> +
> >> +different_name:
> >> +	devlink->index = atomic_inc_return(&dev_counter);
> >> +	if (devlink->index < 0) {
> >> +		/* wrapped */
> >> +		atomic_dec(&dev_counter);
> >> +		kfree(devlink);
> >> +		return NULL;
> >> +	}
> >> +	/* atomic_inc_return makes it start at 1, make it start at 0 */
> >> +	devlink->index--;
> >> +
> >> +	dev_set_name(&devlink->dev, DEVLINK_GENL_NAME "%d", devlink->index);
> >> +	if (devlink_name_exists(devlink_net(devlink), devlink_name(devlink)))
> >> +		goto different_name;
> >> +
> >> +	INIT_LIST_HEAD(&devlink->port_list);
> >> +	device_initialize(&devlink->dev);
> >> +	devlink->dev.class = &devlink_class;
> >> +	devlink->dev.platform_data = devlink;
> >> +	return devlink;
> >> +}
> >> +EXPORT_SYMBOL_GPL(devlink_alloc);
> >
> >The increment then decrement bit feels... Clunky and a little confusing.
> >Why not just atomic_read(&dev_counter) before different_name:, then an
> >atomic_inc_return(&dev_counter) under the if clause before the goto, and
> >eliminate the devlink->index-- bit?
> 
> If I split it it would not be atomic op anymore. But the
> devlink->index-- is just to adjust the start, nothing else.
> This is copied from nl80211 code.

Argh, I'm a moron. You need to increment dev_counter, can't just read it's
current value. Still fugly, but okay, does actually serve a purpose this
way.

-- 
Jarod Wilson
jarod@...hat.com