netdev - Re: [PATCH net-next v2 4/4] net: hdlc_fr: Add support for any Ethertype

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Message-ID: <CA+FuTSdy38GOTKRr9Ze3jvfTQqxR7s_c67J0z75AFZ-yihAVuw@mail.gmail.com>
Date:   Thu, 29 Oct 2020 20:53:17 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Xie He <xie.he.0141@...il.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Krzysztof Halasa <khc@...waw.pl>
Subject: Re: [PATCH net-next v2 4/4] net: hdlc_fr: Add support for any Ethertype

On Thu, Oct 29, 2020 at 8:49 PM Xie He <xie.he.0141@...il.com> wrote:
>
> On Thu, Oct 29, 2020 at 4:53 PM Xie He <xie.he.0141@...il.com> wrote:
> >
> > > Does it make sense to define a struct snap_hdr instead of manually
> > > casting all these bytes?
> >
> > > And macros or constant integers to self document these kinds of fields.
> >
> > Yes, we can define a struct snap_hdr, like this:
> >
> > struct snap_hdr {
> >         u8 oui[3];
> >         __be16 pid;
> > } __packed;
> >
> > And then the fr_snap_parse function could be like this:
> >
> > static int fr_snap_parse(struct sk_buff *skb, struct pvc_device *pvc)
> > {
> >        struct snap_hdr *hdr = (struct snap_hdr *)skb->data;
> >
> >        if (hdr->oui[0] == OUI_ETHERTYPE_1 &&
> >            hdr->oui[1] == OUI_ETHERTYPE_2 &&
> >            hdr->oui[2] == OUI_ETHERTYPE_3) {
> >                if (!pvc->main)
> >                        return -1;
> >                skb->dev = pvc->main;
> >                skb->protocol = hdr->pid; /* Ethertype */
> >                skb_pull(skb, 5);
> >                skb_reset_mac_header(skb);
> >                return 0;
> >
> >        } else if (hdr->oui[0] == OUI_802_1_1 &&
> >                   hdr->oui[1] == OUI_802_1_2 &&
> >                   hdr->oui[2] == OUI_802_1_3) {
> >                if (hdr->pid == htons(PID_ETHER_WO_FCS)) {
> >
> > Would this look cleaner?
>
> Actually I don't think this is significantly cleaner than the previous
> version of code. A reader of this code may still wonder what are the
> values of all these macros, and he/she may still want to look up the
> values of them. The comment in the previous version of code has made
> the meaning of these values very clear, and the reader of the code
> would not need to go to another place of this file to find the values.
>
> The struct snap_hdr eliminates a cast, but only one cast. So it might
> not be very necessary, either. Introducing this struct also makes the
> reader need to go to another place of this file to look up the
> definition of this struct. So it does not significantly improve the
> readability (IMHO).

Thanks for coding up an example. Yes, seeing the alternative, I agree.