Re: [RFC] mt76: fix tx hung regression on MT7630E

From: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
To: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: linux-wireless@vger.kernel.org, Felix Fietkau <nbd@nbd.name>,
	Ryder Lee <ryder.lee@mediatek.com>, Roy Luo <royluo@google.com>
Subject: Re: [RFC] mt76: fix tx hung regression on MT7630E
Date: Mon, 29 Jul 2019 16:02:41 +0200	[thread overview]
Message-ID: <20190729140241.GC4030@localhost.localdomain> (raw)
In-Reply-To: <20190729125351.GA3086@redhat.com>

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> On Fri, Jul 26, 2019 at 02:10:56PM +0200, Stanislaw Gruszka wrote:
> > Since 41634aa8d6db ("mt76: only schedule txqs from the tx tasklet")
> > I can observe firmware hangs on MT7630E on station mode: tx stop
> > functioning after minor activity (rx keep working) and on module
> > unload device fail to stop with messages:
> > 
> > [ 5446.141413] mt76x0e 0000:06:00.0: TX DMA did not stop
> > [ 5449.176764] mt76x0e 0000:06:00.0: TX DMA did not stop
> > 
> > Loading module again results in failure to associate with AP.
> > Only machine power off / power on cycle can make device work again.
> > 
> > I have no idea why the commit caused F/W hangs. Most likely some proper
> > fix is needed of changing registers programming (or assuring proper order
> > of actions), but so far I can not came up with any better fix than
> > a partial revert of 41634aa8d6db.
> 
> The difference is that with 41634aa8d6db we can run mt76x02_poll_tx()
> and mt76x02_tx_tasklet() in parallel on 2 different CPUs. Without
> the commit, since tasklet is not scheduled from mt76_wake_tx_queue(),
> it does not happen.
> 
> I'm not quite sure why, but MT7630E does not like when we poll tx status
> on 2 cpus at once. Change like below:

Hi Stanislaw,

have you tried to look at the commit: 6fe533378795f87
("mt76: mt76x02: remove irqsave/restore in locking for tx status fifo")?
Could it be a race between a registermap update and a stats load? (we are using a
different lock now)

> 
> diff --git a/drivers/net/wireless/mediatek/mt76/mt76x02_mmio.c b/drivers/net/wireless/mediatek/mt76/mt76x02_mmio.c
> index 467b28379870..622251faa415 100644
> --- a/drivers/net/wireless/mediatek/mt76/mt76x02_mmio.c
> +++ b/drivers/net/wireless/mediatek/mt76/mt76x02_mmio.c
> @@ -170,7 +170,7 @@ static int mt76x02_poll_tx(struct napi_struct *napi, int budget)
>  					       mt76.tx_napi);
>  	int i;
>  
> -	mt76x02_mac_poll_tx_status(dev, false);
> +	mt76x02_mac_poll_tx_status(dev, true);

I am not sure if we really need mt76x02_mac_poll_tx_status() here since we run
it in mt76x02_tx_complete_skb() and in mt76x02_tx_tasklet(). Anyway the only
difference doing so is we do not run mt76x02_send_tx_status().

Regards,
Lorenzo

>  
>  	for (i = MT_TXQ_MCU; i >= 0; i--)
>  		mt76_queue_tx_cleanup(dev, i, false);
> 
> is sufficient to avoid the hangs.
> 
> > diff --git a/drivers/net/wireless/mediatek/mt76/tx.c b/drivers/net/wireless/mediatek/mt76/tx.c
> > index 5397827668b9..fefe0ee52584 100644
> > --- a/drivers/net/wireless/mediatek/mt76/tx.c
> > +++ b/drivers/net/wireless/mediatek/mt76/tx.c
> > @@ -598,7 +598,7 @@ void mt76_wake_tx_queue(struct ieee80211_hw *hw, struct ieee80211_txq *txq)
> >  	if (!test_bit(MT76_STATE_RUNNING, &dev->state))
> >  		return;
> >  
> > -	tasklet_schedule(&dev->tx_tasklet);
> > +	mt76_txq_schedule(dev, txq->ac);
> 
> However I'm not sure if change to tasklet_schedule() is indeed correct,
> since on tasklet we schedule all queues, hence queues that could
> possibly be still blocked? And on mt76_wake_tx_queue() we schedule only
> one queue.
> 
> Stanislaw

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