netdev - Re: [PATCH net v6 4/4] tg3: Fix tx_pending checks for tg3_tso

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Message-ID: <1412137489.16704.26.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Tue, 30 Sep 2014 21:24:49 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Prashant <prashant@...adcom.com>
Cc:	David Miller <davem@...emloft.net>, bpoirier@...e.de,
	mchan@...adcom.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v6 4/4] tg3: Fix tx_pending checks for tg3_tso_bug

On Tue, 2014-09-30 at 20:14 -0700, Prashant wrote:

> Sorry about the late reply, out of all the HW bug conditions checked in 
> tg3_tx_frag_set() the most frequently hit condition is the short 8 byte 
> dma bug, where the chip cannot handle TX descriptors whose data buffer 
> is 8 bytes or less. Most of the LSO skb's given to the driver has their 
> fragments filled upto PAGE_SIZE (expect the last fragment depending on 
> skb->len). And if such a LSO skb's last fragment meets the 8 bytes HW 
> bug condition the above routine will not help workaround this particular 
> case.

Thats pretty easy to work around.

Say rebuilt skb has N frags (N > 1 given your description)

They are numbered 0, ... N-2, N-1

Instead of filling N-2 completely, fill it to PAGE_SIZE-8, so that last
frag has at least 8 bytes in it.

Also take a look at commit 2e4e44107176d552f8bb1bb76053e850e3809841
("net: add alloc_skb_with_frags() helper")



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