netdev - Re: [PATCH 4/6] sky2: reduce default Tx ring size

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Date:	Thu, 17 Nov 2011 22:07:25 +0100
From:	Sven Joachim <svenjoac@....de>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH 4/6] sky2: reduce default Tx ring size

On 2011-11-17 00:42 +0100, Stephen Hemminger wrote:

> The default Tx ring size for the sky2 driver is quite large and could
> cause excess buffer bloat for many users. The minimum ring size 
> possible and still allow handling  the worst case packet on 64bit platforms
> is 38 which gets rounded up to a power of 2. But most packets only require
> a couple of ring elements.
>
> Signed-off-by: Stephen Hemminger <shemminger@...tta.com>
>
> --- a/drivers/net/ethernet/marvell/sky2.c	2011-11-16 15:19:39.518659262 -0800
> +++ b/drivers/net/ethernet/marvell/sky2.c	2011-11-16 15:19:40.990692544 -0800
> @@ -68,7 +68,7 @@
>  #define MAX_SKB_TX_LE	(2 + (sizeof(dma_addr_t)/sizeof(u32))*(MAX_SKB_FRAGS+1))
>  #define TX_MIN_PENDING		(MAX_SKB_TX_LE+1)
>  #define TX_MAX_PENDING		1024
> -#define TX_DEF_PENDING		127
> +#define TX_DEF_PENDING		63
>  
>  #define TX_WATCHDOG		(5 * HZ)
>  #define NAPI_WEIGHT		64

It's hard to believe, but this innocuous-looking patch caused my system
to crash and burn as soon as I actually used the network.
Unfortunately, I have neither a digital camera nor a serial console at
hand, but from my recollections and having read sky2.c I conclude that

	BUG_ON(done >= sky2->tx_ring_size);

in sky2_tx_complete() seems to have been triggered.  Does this make any
sense?

Cheers,
       Sven
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