netdev - Re: [PATCH 14/18] netdevice airo: Convert directly reference of netdev->priv to netdev->ml

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Message-ID: <48C24F66.1090803@cn.fujitsu.com>
Date:	Sat, 06 Sep 2008 17:37:42 +0800
From:	Wang Chen <wangchen@...fujitsu.com>
To:	Dan Williams <dcbw@...hat.com>
CC:	"John W. Linville" <linville@...driver.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jeff Garzik <jgarzik@...ox.com>,
	NETDEV <netdev@...r.kernel.org>, linux-wireless@...r.kernel.org
Subject: Re: [PATCH 14/18] netdevice airo: Convert directly reference of	netdev->priv
 to netdev->ml_priv

Dan Williams said the following on 2008-9-6 1:06:
> On Fri, 2008-09-05 at 08:50 -0400, John W. Linville wrote:
>> On Fri, Sep 05, 2008 at 11:27:39AM +0800, Wang Chen wrote:
>>> We have some reasons to kill netdev->priv:
>>> 1. netdev->priv is equal to netdev_priv().
>>> 2. netdev_priv() wraps the calculation of netdev->priv's offset, obviously
>>>    netdev_priv() is more flexible than netdev->priv.
>>> But we cann't kill netdev->priv, because so many drivers reference to it
>>> directly.
>>>
>>> OK, becasue Dave S. Miller said, "every direct netdev->priv usage is a bug",
>>> and I want to kill netdev->priv later, I decided to convert all the direct
>>> reference of netdev->priv first.
>>>
>>> Different to readonly reference of netdev->priv, in this driver, netdev->priv
>>> was changed. I use netdev->ml_priv to replace netdev->priv.
>>>
>>> Signed-off-by: Wang Chen <wangchen@...fujitsu.com>
>> Thank you for your patch.  However, I do not understand why you didn't
>> simply replace netdev->priv with netdev_priv()?  Can you explain?
> 
> Yeah, that would have been my first choice too...
> 

1. Why I don't use netdev_priv() to replace netdev->priv here?
Because, here
> @@ -2665,7 +2666,7 @@ static struct net_device *init_wifidev(struct airo_info *ai,
>  	struct net_device *dev = alloc_netdev(0, "wifi%d", wifi_setup);
>  	if (!dev)
>  		return NULL;
> -	dev->priv = ethdev->priv;
> +	dev->ml_priv = ethdev->ml_priv;
> @@ -2766,7 +2767,7 @@ static struct net_device *_init_airo_card( unsigned short irq, int port,
>  		return NULL;
>  	}
>  
> -	ai = dev->priv;
> +	ai = dev->ml_priv = netdev_priv(dev);
netdev->priv was changed here, but it shouldn't, the memory was allocated when alloc_netdev and
netdev->priv should always pointed to that memory.

2. Why I use netdev->ml_priv here to replace netdev->priv?
In this driver, netdev->priv are shared by multi wifidevs, that means wifidevs need
mid-layer private data, which are all same as their parent netdev.
This usage is same as Dave's commit "syncppp: Fix crashes."

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