linux-kernel - RE: [PATCH 1/1] AX88179_178A: Add FLAG_HW_IPALIGN to determine whether reserving NET_IP

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6026B74AF@saturn3.aculab.com>
Date:	Thu, 19 Dec 2013 10:01:54 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Freddy Xin" <freddy@...x.com.tw>, <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.

> From: Freddy Xin
> On 2013年12月16日 18:09, David Laight wrote:
> >> I was thinking of something like: skb = netdev_alloc_skb(dev, length
> >> + dev->skb_align, gfp); if (NET_IP_ALIGN && skb && !(ev->driver_flags
> >> & FLAG_HW_IPALIGN)) skb_reserve(skb, NET_IP_ALIGN); It might even be
> >> reasonable to remove the length adjustment - provided that all the
> >> later code uses the skb length. David
> 
> Thanks for your advice.
> In the way you advised, does the dev->skb_align equal
> to NET_IP_ALIGN in the case that HW doesn't supoort
> IP alignment?

Yes

> In other words, dev->skb_align should be initialized to
> NET_IP_ALIGN in USBNET, and I can change its value to
> 0 in AX88179_178A driver, right?

I'd probably set a flag to request that usbnet set the offset
to the correct value. That makes the initialisation a bit clearer.

	David