PS mode STA trigger driver bug

[Jouni Malinen]

On Thu, Apr 08, 2004 at 03:01:15PM -0400, Joe Parks wrote:

> When a STA with Power Saving mode enabled connects to one of my hostap boxes (firmware revisions 1.3.6 through 1.7.4 tested with driver revisions 0.1.1 through current CVS) it will *eventually* trigger the following:
> 
> wlan1: driver bug - prism2_transmit() called when previous TX was pending

It looks like the FIX comment in pspoll_send_buffered() (hostap_ap.c) of
development branch is quite valid.. The driver is trying skip the
possible TX queue when sending a buffered frame after a PS poll from the
station. This bypasses some locking in the kernel and prism2_tx_80211()
ends up being called concurrently by the next "normal" data frame.

I need to take a bit closer look at this at some point, but if you are
interested in experimental changed, you could try to modify that
pspoll_send_buffered() function. One trivial change would be to change
the call to use kernel queue:

        if (skb->dev->hard_start_xmit(skb, skb->dev)) {
		PDEBUG(DEBUG_AP, "%s: TX failed for buffered frame (PS Poll)"
		       "\n", skb->dev->name);
		dev_kfree_skb(skb);
	}

into

	dev_queue_xmit(skb);

another alternative would be to change this into

        spin_lock_bh(&skb->dev->xmit_lock);
        if (skb->dev->hard_start_xmit(skb, skb->dev)) {
		spin_unlock_bh(&skb->dev->xmit_lock);
		PDEBUG(DEBUG_AP, "%s: TX failed for buffered frame (PS Poll)"
		       "\n", skb->dev->name);
		dev_kfree_skb(skb);
        }
        spin_unlock_bh(&skb->dev->xmit_lock);

this would at least try to send the frame faster (which would be useful
for this PS Poll case)


I haven't tried these, so if you happen to test them, I'm certainly
interested in hearing whether they helped with this issue.

-- 
Jouni Malinen                                            PGP id EFC895FA