netdev - Re: [PATCH net-next 5/9] net: dsa: Add support for deferred xmit

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Date:   Sat, 4 May 2019 05:10:52 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     vivien.didelot@...il.com, Andrew Lunn <andrew@...n.ch>,
        "David S. Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 5/9] net: dsa: Add support for deferred xmit

On Sat, 4 May 2019 at 05:07, Florian Fainelli <f.fainelli@...il.com> wrote:
>
>
>
> On 5/3/2019 6:18 PM, Vladimir Oltean wrote:
> > Some hardware needs to take some convincing work in order to receive
> > frames on the CPU port (such as the sja1105 which takes temporary L2
> > forwarding rules over SPI that last for a single frame). Such work needs
> > a sleepable context, and because the regular .ndo_start_xmit is atomic,
> > this cannot be done in the tagger. So introduce a generic DSA mechanism
> > that sets up a transmit skb queue and a workqueue for deferred
> > transmission.
> >
> > The new driver callback (.port_deferred_xmit) is in dsa_switch and not
> > in the tagger because the operations that require sleeping typically
> > also involve interacting with the hardware, and not simply skb
> > manipulations. Therefore having it there simplifies the structure a bit
> > and makes it unnecessary to export functions from the driver to the
> > tagger.
> >
> > The driver is responsible of calling dsa_enqueue_skb which transfers it
> > to the master netdevice. This is so that it has a chance of performing
> > some more work afterwards, such as cleanup or TX timestamping.
> >
> > To tell DSA that skb xmit deferral is required, I have thought about
> > changing the return type of the tagger .xmit from struct sk_buff * into
> > a enum dsa_tx_t that could potentially encode a DSA_XMIT_DEFER value.
> >
> > But the trailer tagger is reallocating every skb on xmit and therefore
> > making a valid use of the pointer return value. So instead of reworking
> > the API in complicated ways, right now a boolean property in the newly
> > introduced DSA_SKB_CB is set.
> >
> > Signed-off-by: Vladimir Oltean <olteanv@...il.com>
> > ---
>
> [snip]
>
> >  static inline struct dsa_port *dsa_slave_to_port(const struct net_device *dev)
> >  {
> >       struct dsa_slave_priv *p = netdev_priv(dev);
> > diff --git a/net/dsa/slave.c b/net/dsa/slave.c
> > index 8ad9bf957da1..cfb8cba6458c 100644
> > --- a/net/dsa/slave.c
> > +++ b/net/dsa/slave.c
> > @@ -120,6 +120,9 @@ static int dsa_slave_close(struct net_device *dev)
> >       struct net_device *master = dsa_slave_to_master(dev);
> >       struct dsa_port *dp = dsa_slave_to_port(dev);
> >
> > +     cancel_work_sync(&dp->xmit_work);
> > +     skb_queue_purge(&dp->xmit_queue);
> > +
> >       phylink_stop(dp->pl);
>
> Don't you also need to do that for dsa_slave_suspend() in case the
> xmit() raced with netif_device_detach() somehow?
>
>
> --
> Florian

Hi Florian,
Thanks for pointing that out. Not having power management ops I didn't
even think about that.
-Vladimir