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

[Lorenzo Bianconi <lorenzo.bianconi@redhat.com>]

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> On Mon, Jul 29, 2019 at 04:02:41PM +0200, Lorenzo Bianconi wrote:
> > > 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,
> 
> Hi
> 
> > 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)
> 
> This commit seems to be fine, reverting it did not solve the issue.

ack

> 
> > > 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().
> 
> I thought this is the problem, but it was my mistake during testing.
> I tested the above change together with mt76_txq_schedule(dev, txq->ac)
> change and get wrong impression it fixes the issue. But above change
> alone does not help.
> 
> I tried to add some locking to avoid parallel execution of mt76x02_poll_tx()
> and mt76x02_tx_tasklet(), but it didn't help either. So far only patch
> originally posted here make the problem gone.

so, in order to be on the same page, if you comment out mt76x02_mac_poll_tx_status()
in mt76x02_poll_tx() the issue will still occur. The only to 'fix' it is to run
mt76_txq_schedule_all() in mt76x02_poll_tx(), right?

Lorenzo

> 
> Stanislaw
> 

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