linux-kernel - Re: [PATCH v2 1/2] net: macb: Remove unnecessary alignment check for TSO

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Message-ID: <CAFcVECKXp-s-vteTzmqSDCR0ajugiDK_tnBmacea5NA+Fu02Ng@mail.gmail.com>
Date:   Tue, 4 Feb 2020 15:52:55 +0530
From:   Harini Katakam <harinik@...inx.com>
To:     David Miller <davem@...emloft.net>
Cc:     Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Claudiu Beznea <claudiu.beznea@...rochip.com>, kuba@...nel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Michal Simek <michal.simek@...inx.com>,
        Harini Katakam <harini.katakam@...inx.com>,
        Harini Katakam <harinikatakamlinux@...il.com>
Subject: Re: [PATCH v2 1/2] net: macb: Remove unnecessary alignment check for TSO

Hi David,

> > -----Original Message-----
> > From: David Miller [mailto:davem@...emloft.net]
> > Sent: Tuesday, February 4, 2020 3:07 PM
> > To: Harini Katakam <harinik@...inx.com>
> > Cc: nicolas.ferre@...rochip.com; claudiu.beznea@...rochip.com;
> > kuba@...nel.org; netdev@...r.kernel.org; linux-kernel@...r.kernel.org;
> > Michal Simek <michals@...inx.com>; harinikatakamlinux@...il.com
> > Subject: Re: [PATCH v2 1/2] net: macb: Remove unnecessary alignment check
> > for TSO
> >
> > From: Harini Katakam <harini.katakam@...inx.com>
> > Date: Mon,  3 Feb 2020 18:48:01 +0530
> >
> > > The IP TSO implementation does NOT require the length to be a multiple
> > > of 8. That is only a requirement for UFO as per IP documentation.
> > >
> > > Fixes: 1629dd4f763c ("cadence: Add LSO support.")
> > > Signed-off-by: Harini Katakam <harini.katakam@...inx.com>
> > > ---
> > > v2:
> > > Added Fixes tag
> >
> > Several problems with this.
> >
> > The subject talks about alignemnt check, but you are not changing the alignment
> > check.  Instead you are modifying the linear buffer
> > check:

Thanks for the review. Everything below that line becomes unused
when alignment check is removed. More details below.

> >
> > > @@ -1792,7 +1792,7 @@ static netdev_features_t
> > macb_features_check(struct sk_buff *skb,
> > >     /* Validate LSO compatibility */
> > >
> > >     /* there is only one buffer */
> > > -   if (!skb_is_nonlinear(skb))
> > > +   if (!skb_is_nonlinear(skb) || (ip_hdr(skb)->protocol !=
> > > +IPPROTO_UDP))
> > >             return features;
> >
> > So either your explanation is wrong or the code change is wrong.

Alignment check is not required for TSO and is ONLY required for UFO.
So, if NOT(UDP), just return.

macb_features_check()
{
If existing linear check (or) if !UDP
    no need to change features, just return

Alignment check implementation which is only necessary for UDP.
}

> >
> > Furthermore, if you add this condition then there is now dead code below this.
> > The code that checks for example:
> >
> >       /* length of header */
> >       hdrlen = skb_transport_offset(skb);
> >       if (ip_hdr(skb)->protocol == IPPROTO_TCP)
> >               hdrlen += tcp_hdrlen(skb);
> >
> > will never trigger this IPPROTO_TCP condition after your change.

Yes, this is dead code now. I'll remove it.

> >
> > A lot of things about this patch do not add up.

Please let me know if you have any further concerns.

Regards,
Harini