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

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Message-Id: <20171207.145024.1434883857028947517.davem@davemloft.net>
Date:   Thu, 07 Dec 2017 14:50:24 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     ivan.khoronzhuk@...aro.org
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

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.