netdev - Re: [PATCH 2/2] net: davinci_cpdma: reduce time holding chan->lock in cpdma_chan

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Date:	Wed, 27 Jul 2016 09:12:28 +0200
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Grygorii Strashko <grygorii.strashko@...com>
Cc:	Mugunthan V N <mugunthanvnm@...com>, linux-omap@...r.kernel.org,
	netdev@...r.kernel.org, kernel@...gutronix.de
Subject: Re: [PATCH 2/2] net: davinci_cpdma: reduce time holding chan->lock
 in cpdma_chan_submit

Hello,

On Tue, Jul 26, 2016 at 05:25:58PM +0300, Grygorii Strashko wrote:
> On 07/26/2016 03:02 PM, Uwe Kleine-König wrote:
> > Allocating and preparing a dma descriptor doesn't need to happen under
> > the channel's lock. So do this before taking the channel's lock. The only
> > down side is that the dma descriptor might be allocated even though the
> > channel is about to be stopped. This is unlikely though.
> > 
> > Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> > ---
> >  drivers/net/ethernet/ti/davinci_cpdma.c | 38 +++++++++++++++++----------------
> >  1 file changed, 20 insertions(+), 18 deletions(-)
> > 
> > diff --git a/drivers/net/ethernet/ti/davinci_cpdma.c b/drivers/net/ethernet/ti/davinci_cpdma.c
> > index 5ffa04a306c6..ba3462707ae3 100644
> > --- a/drivers/net/ethernet/ti/davinci_cpdma.c
> > +++ b/drivers/net/ethernet/ti/davinci_cpdma.c
> > @@ -542,24 +542,10 @@ int cpdma_chan_submit(struct cpdma_chan *chan, void *token, void *data,
> >  	u32				mode;
> >  	int				ret = 0;
> >  
> > -	spin_lock_irqsave(&chan->lock, flags);
> > -
> > -	if (chan->state == CPDMA_STATE_TEARDOWN) {
> > -		ret = -EINVAL;
> > -		goto unlock_ret;
> > -	}
> > -
> > -	if (chan->count >= chan->desc_num)	{
> > -		chan->stats.desc_alloc_fail++;
> > -		ret = -ENOMEM;
> > -		goto unlock_ret;
> > -	}
> 
> I'm not sure this is right thing to do. This check is expected to be strict
> and means "channel has exhausted the available descriptors, so further descs allocation does not allowed".

I developed this patch basing on a 4.4 kernel which doesn't have
742fb20fd4c7 ("net: ethernet: ti: cpdma: switch to use genalloc"). There
my patch is more obviously correct. As currently chan->count is
protected by chan->lock we must hold the lock for this check. If a
failing check means we must not call cpdma_desc_alloc in the first
place, that's bad.

But I'm not sure this is the case here. After all cpdma_desc_alloc
doesn't do anything relevant for the hardware, right?

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |