netdev - Re: [net-next v4 2/3] net: dsa: tag_yt921x: add support for Motorcomm YT921x tags

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Message-ID: <CAAXyoMNjukd-=cMDLiupNDYv1NLreWkCQufhAu_1y3N0udUrQw@mail.gmail.com>
Date: Tue, 19 Aug 2025 01:32:52 +0800
From: Yangfl <mmyangfl@...il.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: netdev@...r.kernel.org, Vladimir Oltean <olteanv@...il.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Simon Horman <horms@...nel.org>, Russell King <linux@...linux.org.uk>, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [net-next v4 2/3] net: dsa: tag_yt921x: add support for Motorcomm
 YT921x tags

On Tue, Aug 19, 2025 at 1:07 AM Andrew Lunn <andrew@...n.ch> wrote:
>
> > +static struct sk_buff *
> > +yt921x_tag_xmit(struct sk_buff *skb, struct net_device *netdev)
> > +{
> > +     struct dsa_port *dp = dsa_user_to_port(netdev);
> > +     unsigned int port = dp->index;
> > +     struct dsa_port *partner;
> > +     __be16 *tag;
> > +     u16 tx;
> > +
> > +     skb_push(skb, YT921X_TAG_LEN);
> > +     dsa_alloc_etype_header(skb, YT921X_TAG_LEN);
> > +
> > +     tag = dsa_etype_header_pos_tx(skb);
> > +
> > +     /* We might use yt921x_priv::tag_eth_p, but
> > +      * 1. CPU_TAG_TPID could be configured anyway;
> > +      * 2. Are you using the right chip?
> > +      */
> > +     tag[0] = htons(ETH_P_YT921X);
> > +     /* Service VLAN tag not used */
> > +     tag[1] = 0;
> > +     tag[2] = 0;
> > +     tx = YT921X_TAG_PORT_EN | YT921X_TAG_TX_PORTn(port);
> > +     if (dp->hsr_dev)
> > +             dsa_hsr_foreach_port(partner, dp->ds, dp->hsr_dev)
> > +                     tx |= YT921X_TAG_TX_PORTn(partner->index);
>
> As far as i remember, this was not in v1. When i spotting this in v2
> that made me comment you should not add new features in revision of a
> patch.
>
> Does the current version of the DSA driver support hsr? Is this
> useful? Maybe it would be better to add hsr support as a follow up
> patch?

Sorry, this was forgotten to undo.


> > +static struct sk_buff *
> > +yt921x_tag_rcv(struct sk_buff *skb, struct net_device *netdev)
> > +{
> > +     unsigned int port;
> > +     __be16 *tag;
> > +     u16 rx;
> > +
> > +     if (unlikely(!pskb_may_pull(skb, YT921X_TAG_LEN)))
> > +             return NULL;
> > +
> > +     tag = (__be16 *)skb->data;
> > +
> > +     /* Locate which port this is coming from */
> > +     rx = ntohs(tag[1]);
> > +     if (unlikely((rx & YT921X_TAG_PORT_EN) == 0)) {
> > +             netdev_err(netdev, "Unexpected rx tag 0x%04x\n", rx);
> > +             return NULL;
> > +     }
> > +
> > +     port = FIELD_GET(YT921X_TAG_RX_PORT_M, rx);
> > +     skb->dev = dsa_conduit_find_user(netdev, 0, port);
> > +     if (unlikely(!skb->dev)) {
> > +             netdev_err(netdev, "Cannot locate rx port %u\n", port);
> > +             return NULL;
> > +     }
>
> O.K. Stop. Think.
>
> You changed the rate limiting to an unlimiting netdev_err().
>
> What is the difference? Under what conditions would you want to use
> rate limiting? When would you not use rate limiting?
>
> Please reply and explain why you made this change.
>
>         Andrew

I copied the limited version from tag_vsc73xx_8021q.

Under no conditions I expect either of them to appear: it is the case
when I did my own tests; unless something really bad happens, like
pouring a cup of coffee over your device.

I know rate limiting is a way to prevent flooding the same message
over dmesg, but if an event never happens, I would consider two
methods are exchangeable. Theoretically if an event never happens, no
warnings would ever be needed, but I placed one here in case you
destroy your device accidentally.

Thus if you think rate limiting is not appropriate here, I would fix
it with another.