netdev - Re: about tx_queue_len for phonet driver

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Message-ID: <2151675.UOkxvXJjp1@hector>
Date:	Thu, 12 Jan 2012 13:56:28 +0200
From:	Rémi Denis-Courmont 
	<remi.denis-courmont@...ia.com>
To:	ext Hemant-vilas RAMDASI <hemant.ramdasi@...ricsson.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: about tx_queue_len for phonet driver

	Hello,

Le Jeudi 12 Janvier 2012 11:45:48 ext Hemant-vilas RAMDASI a écrit :
> Can anyone guide me about when should a network driver set tx_queue_len to
> 0? We are facing a network congestion issue with phonet packets where one
> of the control message packet is dropped because of congestion. In our
> phonet network driver the tx_queue_len is set to 3 (taken from usb-phonet
> mostly). If phonet cannot handle packet-drop (for pipe messages), then
> should network driver always set tx_queue_len as 0?

In theory, I guess the network device queuing discipline should be avoided 
altogether, unless there is another way to prevent packet loss, that I am not 
aware of. Unfortunately dropping qdisc, and thus net_device also, would have 
major redesign implications on the Linux Phonet infrastructure to say the 
least.

In any case, Phonet devices typically fail to provide sane, reliable and race-
free congestion control. I have seen some Phonet modem firmwares that just 
crash if they receive too many and/or too large packets. Alas, this cannot be 
fixed on the Linux / application processor side.

So you have to find a value large enough to avoid packet loss, but also small 
enough to avoid overflow and who-knows-what bugs on the modem side. For the 
record, the SSI driver queue length found on Nokia N9/N950 defaults to 100.

-- 
Rémi Denis-Courmont
http://www.remlab.net/

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