netdev - Re: [PATCH 4/5] r8169: more alignment for the 0x8168

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Message-ID: <20070228214839.GA23772@electric-eye.fr.zoreil.com>
Date:	Wed, 28 Feb 2007 22:48:39 +0100
From:	Francois Romieu <romieu@...zoreil.com>
To:	Philip Craig <philipc@...pgear.com>
Cc:	Mike Isely <isely@...ox.com>, Jeff Garzik <jeff@...zik.org>,
	netdev@...r.kernel.org
Subject: Re: [PATCH 4/5] r8169: more alignment for the 0x8168

Sorry for the delay, I took some time to check the history of the
r8169 alignment issues.

Philip Craig <philipc@...pgear.com> :
[...]
> This only partially helps.  Many of the packets are greater than 200
> bytes so copybreak doesn't apply to them.

Yes.

> Can we assume anything about the alignment of skb->data?  I think it
> should be 4 byte aligned, otherwise the whole NET_IP_ALIGN thing
> won't work.  All the drivers I looked at just reserve NET_IP_ALIGN
> without checking the alignment first.
> 
> So can you do something like set align to 0 for RTL_CFG_0 and change
> rtl8169_alloc_rx_skb() to:
> 	skb_reserve(skb, align ? (align - 1) & (u32)skb->data : NET_IP_ALIGN);

The "So" part assumes that the 0x8169 can DMA at any address.

/me ponders...

It's easy to debug if it misbehaves now or in 6 months on some obscure
system. It's consistent with the preprevious code. Ok, good idea, I like it.

[...]
> BTW, should the alignment expression be:
> 	(((u32)skb->data + (align - 1)) & ~(align - 1)) - (u32)skb->data

I'll see if something can be hacked with a zero or power of two alignment.

-- 
Ueimor
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