netdev - RE: [PATCH] Check for SKBTX_HW

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Date:   Tue, 12 Mar 2019 21:39:26 +0000
From:   "Keller, Jacob E" <jacob.e.keller@...el.com>
To:     "Keller, Jacob E" <jacob.e.keller@...el.com>,
        Paul Thomas <pthomas8589@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH] Check for SKBTX_HW_TSTAMP in macb driver

> -----Original Message-----
> From: netdev-owner@...r.kernel.org [mailto:netdev-owner@...r.kernel.org] On
> Behalf Of Keller, Jacob E
> Sent: Tuesday, March 12, 2019 2:35 PM
> To: Paul Thomas <pthomas8589@...il.com>; netdev@...r.kernel.org
> Subject: RE: [PATCH] Check for SKBTX_HW_TSTAMP in macb driver
> 
> > -----Original Message-----
> > From: netdev-owner@...r.kernel.org [mailto:netdev-owner@...r.kernel.org] On
> > Behalf Of Paul Thomas
> > Sent: Tuesday, March 12, 2019 12:51 PM
> > To: netdev@...r.kernel.org
> > Cc: Paul Thomas <pthomas8589@...il.com>
> > Subject: [PATCH] Check for SKBTX_HW_TSTAMP in macb driver
> >
> > Make sure SKBTX_HW_TSTAMP (i.e. SOF_TIMESTAMPING_TX_HARDWARE) has
> been
> > enabled for this skb
> > This is a concept for discussion, more testing is needed.
> > It does fix the issue where normal socks that aren't expecting a timestamp will not
> > wake
> > up on select.
> > ---
> >  drivers/net/ethernet/cadence/macb_main.c | 12 +++++++-----
> >  1 file changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/net/ethernet/cadence/macb_main.c
> > b/drivers/net/ethernet/cadence/macb_main.c
> > index ad099fd01b45..b2f184fc1370 100644
> > --- a/drivers/net/ethernet/cadence/macb_main.c
> > +++ b/drivers/net/ethernet/cadence/macb_main.c
> > @@ -898,11 +898,13 @@ static void macb_tx_interrupt(struct macb_queue
> *queue)
> >
> >  			/* First, update TX stats if needed */
> >  			if (skb) {
> > -				if (gem_ptp_do_txstamp(queue, skb, desc) == 0) {
> > -					/* skb now belongs to timestamp buffer
> > -					 * and will be removed later
> > -					 */
> > -					tx_skb->skb = NULL;
> > +				if(unlikely(skb_shinfo(skb)->tx_flags &
> > SKBTX_HW_TSTAMP)) {
> 
> This should be && for logical AND. Technically a bitwise & will likely work, but it's not
> what you intended.
> 
> Thanks,
> Jake
> 

Sorry, I misread the code.

You have two conditionals inside, and I misread where you were doing the checking of the SKBTX_HW_TSTAMP flag.

I would do the following :

if (unlikely(skb_shinfo(skb)->tx_flags & SKBTX_HWTSTAMP) &&
    gem_ptp_do_txstamp(queue, skb, desc)) {
  ....
}

Rather than have two nested if statements, relying on the short circuiting to exit early.

The patch looks technically correct to me now that I'm reading it properly.

Jake

> > +					if (gem_ptp_do_txstamp(queue, skb, desc)
> > == 0) {
> > +						/* skb now belongs to timestamp
> > buffer
> > +						 * and will be removed later
> > +						 */
> > +						tx_skb->skb = NULL;
> > +					}
> >  				}
> >  				netdev_vdbg(bp->dev, "skb %u (data %p) TX
> > complete\n",
> >  					    macb_tx_ring_wrap(bp, tail),
> > --
> > 2.17.1