RE: [net-next, v2, 3/7] net: dsa: free skb->cb usage in core driver

[Y.b. Lu]

> -----Original Message-----
> From: Tobias Waldekranz <tobias@xxxxxxxxxxxxxx>
> Sent: 2021年4月29日 4:30
> To: Y.b. Lu <yangbo.lu@xxxxxxx>; netdev@xxxxxxxxxxxxxxx
> Cc: Y.b. Lu <yangbo.lu@xxxxxxx>; Richard Cochran
> <richardcochran@xxxxxxxxx>; Vladimir Oltean <vladimir.oltean@xxxxxxx>;
> David S . Miller <davem@xxxxxxxxxxxxx>; Jakub Kicinski <kuba@xxxxxxxxxx>;
> Jonathan Corbet <corbet@xxxxxxx>; Kurt Kanzenbach <kurt@xxxxxxxxxxxxx>;
> Andrew Lunn <andrew@xxxxxxx>; Vivien Didelot <vivien.didelot@xxxxxxxxx>;
> Florian Fainelli <f.fainelli@xxxxxxxxx>; Claudiu Manoil
> <claudiu.manoil@xxxxxxx>; Alexandre Belloni
> <alexandre.belloni@xxxxxxxxxxx>; UNGLinuxDriver@xxxxxxxxxxxxx;
> linux-doc@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [net-next, v2, 3/7] net: dsa: free skb->cb usage in core driver
> 
> On Mon, Apr 26, 2021 at 17:37, Yangbo Lu <yangbo.lu@xxxxxxx> wrote:
> > Free skb->cb usage in core driver and let device drivers decide to use
> > or not. The reason having a DSA_SKB_CB(skb)->clone was because
> > dsa_skb_tx_timestamp() which may set the clone pointer was called
> > before p->xmit() which would use the clone if any, and the device
> > driver has no way to initialize the clone pointer.
> >
> > Although for now putting memset(skb->cb, 0, 48) at beginning of
> > dsa_slave_xmit() by this patch is not very good, there is still way to
> > improve this. Otherwise, some other new features, like one-step
> 
> Could you please expand on this improvement?
> 
> This memset makes it impossible to carry control buffer information from
> driver callbacks that run before .ndo_start_xmit, for
> example .ndo_select_queue, to a tagger's .xmit.
> 
> It seems to me that if the drivers are to handle the CB internally from now on,
> that should go for both setting and clearing of the required fields.
> 

For the timestamping case, dsa_skb_tx_timestamp may not touch .port_txtstamp callback, so we had to put skb->cb initialization at beginning of dsa_slave_xmit.
To avoid breaking future skb->cb usage you mentioned, do you think we can back to Vladimir's idea initializing only field required, or even just add a callback for cb initialization for timestamping?

Thanks.


> > timestamp which needs a flag of skb marked in dsa_skb_tx_timestamp(),
> > and handles as one-step timestamp in p->xmit() will face same
> > situation.
> >
> > Signed-off-by: Yangbo Lu <yangbo.lu@xxxxxxx>