netdev - Re: [PATCH 1/3] net: add a new NETDEV

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Date:	Tue, 20 Aug 2013 18:10:12 +0200
From:	Jiri Pirko <jiri@...nulli.us>
To:	Florian Fainelli <f.fainelli@...il.com>
Cc:	Johannes Berg <johannes@...solutions.net>,
	netdev <netdev@...r.kernel.org>, amwang <amwang@...hat.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	kaber <kaber@...sh.net>, David Miller <davem@...emloft.net>,
	vyasevic <vyasevic@...hat.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH 1/3] net: add a new NETDEV_CHANGEROOM event type

Tue, Aug 20, 2013 at 05:30:48PM CEST, f.fainelli@...il.com wrote:
>2013/8/20 Johannes Berg <johannes@...solutions.net>:
>> On Tue, 2013-08-20 at 13:45 +0100, Florian Fainelli wrote:
>>
>>>  /**
>>> + *   dev_set_headroom - Change device needed headroom
>>> + *   @dev: device
>>> + *   @new_headroom: new headroom size
>>> + *
>>> + *   Change the network device headroom space.
>>> + */
>>> +int dev_set_headroom(struct net_device *dev, unsigned short new_headroom)
>>
>> It seems that you need to invoke these under RTNL, might be worth
>> documenting that.
>
>Good point, yes.
>
>>
>> Also, maybe it would be worth doing it in one call? If you need to
>> change both, then you'd end up calling the notifier twice, which is less
>> efficient?
>
>I have mixed feelings about this. I do not expect changing the
>headroom/tailroom to be in a hot-path, and we would need to have a
>name such as dev_set_head_and_tailroom() or something that clearly
>states that it operates on both quantities. Looking at the subsystems
>and drivers, there are quite a lot of users which only set one or the
>other, occasionaly both before registration.
>
>> I suppose you could make them 'int' arguments and reserve -1

Ugh, -1, I don't like this. I think that they should be set separate. Not
real need to do it in one function.

>> for no changes, or just require both new values to be given (if doing
>> this at all.)
>
>What I like about keeping them separate is that we can use the
>"native" storage type that is used in struct net_device, and have
>compile-time checking of this.
>-- 
>Florian
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