linux-kernel - Re: [PATCH net-next] net: ethernet: ti: cpdma: rate is not changed

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Message-ID: <20171207201005.GC3022@khorivan>
Date:   Thu, 7 Dec 2017 22:10:06 +0200
From:   Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
To:     David Miller <davem@...emloft.net>
Cc:     grygorii.strashko@...com, netdev@...r.kernel.org,
        linux-omap@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] net: ethernet: ti: cpdma: rate is not changed -
 correct case

On Thu, Dec 07, 2017 at 02:50:24PM -0500, David Miller wrote:
> From: Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
> Date: Thu, 7 Dec 2017 21:48:56 +0200
> 
> > On Wed, Dec 06, 2017 at 04:35:45PM -0500, David Miller wrote:
> >> From: Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
> >> Date: Wed,  6 Dec 2017 16:41:18 +0200
> >> 
> >> > If rate is the same as set it's correct case.
> >> > 
> >> > Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
> >> > ---
> >> > Based on net-next/master
> >> > 
> >> >  drivers/net/ethernet/ti/davinci_cpdma.c | 2 +-
> >> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >> > 
> >> > diff --git a/drivers/net/ethernet/ti/davinci_cpdma.c b/drivers/net/ethernet/ti/davinci_cpdma.c
> >> > index e4d6edf..dbe9167 100644
> >> > --- a/drivers/net/ethernet/ti/davinci_cpdma.c
> >> > +++ b/drivers/net/ethernet/ti/davinci_cpdma.c
> >> > @@ -841,7 +841,7 @@ int cpdma_chan_set_rate(struct cpdma_chan *ch, u32 rate)
> >> >  		return -EINVAL;
> >> >  
> >> >  	if (ch->rate == rate)
> >> > -		return rate;
> >> > +		return 0;
> >> 
> >> Looking at the one and only caller of this function, cpsw_ndo_set_tx_maxrate, it
> >> makes sure this can never, ever, happen.
> > In current circumstances yes, it will never happen.
> > But I caught it while adding related code and better return 0 if upper caller
> > doesn't have such check. Suppose that cpdma module is responsible for itself
> > and if it's critical I can send this patch along with whole related series.
> 
> You have to decide one way or the other, who is responsible.
> 
> I think checking higher up is better because it's cheaper at that point to
> look at the per-netdev queue rate setting before moving down deeper into the
> driver specific data-structures.

No objection, but upper caller not always knows current rate and for doing like
this it needs read it first, and this is also some redundancy.

-- 
Regards,
Ivan Khoronzhuk