netdev - Re: [PATCH 1/1] AX88179_178A: Add FLAG_HW_IPALIGN to determine whether reserving NET_IP

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Date:	Mon, 16 Dec 2013 14:54:32 +0800
From:	Freddy Xin <freddy@...x.com.tw>
To:	David Laight <David.Laight@...LAB.COM>, netdev@...r.kernel.org,
	davem@...emloft.net, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org, louis@...x.com.tw, allan@...x.com.tw
Subject: Re: [PATCH 1/1] AX88179_178A: Add FLAG_HW_IPALIGN to determine whether
 reserving NET_IP_ALIGN bytes for an SKB.


On 2013年12月13日 18:36, David Laight wrote:
>> From: freddy@...x.com.tw
> ...
>> -	skb = __netdev_alloc_skb_ip_align(dev->net, size, flags);
>> +	if (dev->driver_info->flags & FLAG_HW_IPALIGN)
>> +		skb = __netdev_alloc_skb(dev->net, size, flags);
>> +	else
>> +		skb = __netdev_alloc_skb_ip_align(dev->net, size, flags);
> Given the definition:
> static inline struct sk_buff *__netdev_alloc_skb_ip_align(struct net_device *dev,
>                  unsigned int length, gfp_t gfp)
> {
>          struct sk_buff *skb = __netdev_alloc_skb(dev, length + NET_IP_ALIGN, gfp);
>
>          if (NET_IP_ALIGN && skb)
>                  skb_reserve(skb, NET_IP_ALIGN);
>          return skb;
> }
>
> It really ought to be possible to code that without an extra conditional.
>
> 	David
>
>
>
>
The AX88179_178A driver do need to know the value of NET_IP_ALIGN
to determine whether enabling the feature that makes IP header align
on a dword-aligned address, but according to the comments from David
Miller, I need to consider all situations, not just for the case that
NET_IP_ALIGN is zero, so the condition added in rx_submit is just used to
determine whether reserving NET_IP_ALIGN bytes for each SKB.

Freddy
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