netdev - Re: Frequent spurious tx

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Message-ID: <BANLkTi=FT4v=Jvv5bQpUX15R87kPudKq=Q@mail.gmail.com>
Date:	Mon, 2 May 2011 20:59:04 +0100
From:	Daniel Drake <dsd@...top.org>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	netdev@...r.kernel.org, libertas-dev@...ts.infradead.org,
	linux-wireless <linux-wireless@...r.kernel.org>
Subject: Re: Frequent spurious tx_timeouts for libertas

On 2 May 2011 03:24, Ben Hutchings <bhutchings@...arflare.com> wrote:
>> Also, while looking at this code, I spotted a bug in dev_watchdog():
>>                               /*
>>                                * old device drivers set dev->trans_start
>>                                */
>>                               trans_start = txq->trans_start ? : dev->trans_start;
>>
>> i.e. it is trying to figure out whether to read trans_start from txq
>> or dev. In both cases, trans_start is updated based on the value of
>> jiffies, which will occasionally be 0 (as it wraps around). Therefore
>> this line of code will occasionally make the wrong decision.
>
> No, I don't think so.
>
> If only dev->trans_start is being updated then the watchdog reads that.
> If both txq->trans_start and dev->trans_start are being updated then it
> doesn't matter much which the watchdog reads.
> If only txq->trans_start is being updated then dev->trans_start is
> always set to 0, so when txq->trans_start is 0 the watchdog still gets
> 0.

dev->trans_start is unconditionally initialized by dev_activate() in
sch_generic.c:

	if (need_watchdog) {
		dev->trans_start = jiffies;
		dev_watchdog_up(dev);
	}

so it is (usually) not 0.

Thanks for your input on the tx timeout issue, now that I understand
it better I think the right plan of action is to remove it from
libertas entirely, I'll CC you on the patch.

Daniel
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