netdev - Re: [Kgdb-bugreport] [PATCH 2.6.20-rc7] 8139too KGDBoE fix

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Date:	Fri, 23 Feb 2007 11:22:46 -0800
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Sergei Shtylyov <sshtylyov@...mvista.com>
Cc:	Mark Huth <mhuth@...sta.com>,
	"Amit S. Kale" <amitkale@...syssoft.com>, netdev@...r.kernel.org,
	Mithlesh Thukral <mithlesh@...syssoft.com>,
	Vitaly Wool <vwool@...mvista.com>
Subject: Re: [Kgdb-bugreport] [PATCH 2.6.20-rc7] 8139too KGDBoE fix

On Fri, 23 Feb 2007 22:16:59 +0300
Sergei Shtylyov <sshtylyov@...mvista.com> wrote:

> Hello.
> 
> Stephen Hemminger wrote:
> 
> >>>>>This thread came up on kgdb-bugreport mailing list. Could you please suggest 
> >>>>>us what's the correct way of fixing this problem?
> 
> >>>>>1. When running a kgdb on RTL8139 ethernet interface: 8139too driver prints 
> >>>>>too many "Out-of-sync dirty pointer" messages on console and gdb can't 
> >>>>>connect to kgdb stub. These messages can be suppressed, though it still 
> >>>>>results in connection failures frequently. 
> 
> >>>>We think this comes from calling the driver while the queue is stopped.  
> >>>>Drivers should not do horrible things when hard start is called with the 
> >>>>queue stopped, but unfortunately, at this time, at least some drivers 
> >>>>do  explode or complain under that condition.
> 
> >>>The kernel is built on a set of assumptions about calling context. Your
> >>>out of tree code is violating one of them. Why not check for stopped queue
> >>>and do some action to try and clear it, that is what netconsole does.
> 
> >>    The queue can't be stopped when the netpoll traffic trapping is enabled 
> >>(cause this effectively bypasses queue control), So, the stopped queue 
> >>indoication doesn't work also -- *that* is the problem. It's not at all 
> >>specific to KGBoE -- only to traffic trapping.
> 
> > You can't ask a device to send a packet when it has no resources.
> 
>     When traffic trapping is enabled, and driver stops the queue, the 
> __LINK_STATE_XOFF flag does *not* get set, so netif_queue_stopped() resturns 
> *zero*.  What may be done in this situation?

Read netpoll_send_skb()

	int status = NETDEV_TX_BUSY;
...
		if (netif_tx_trylock(dev)) {
			/* try until next clock tick */
			for (tries = jiffies_to_usecs(1)/USEC_PER_POLL;
					tries > 0; --tries) {
				if (!netif_queue_stopped(dev))
					status = dev->hard_start_xmit(skb, dev);

				if (status == NETDEV_TX_OK)
					break;

				/* tickle device maybe there is some cleanup */
				netpoll_poll(np);

				udelay(USEC_PER_POLL);
			}
			netif_tx_unlock(dev);

netpoll_poll() allows device to try and cleanup transmit resources.

-- 
Stephen Hemminger <shemminger@...ux-foundation.org>
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