netdev - Re: [PATCH v3 2/3] net: ethernet: actions: Add Actions Semi Owl Ethernet MAC driver

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Date:   Tue, 23 Mar 2021 10:34:09 +0200
From:   Cristian Ciocaltea <cristian.ciocaltea@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Andreas Färber <afaerber@...e.de>,
        Manivannan Sadhasivam <mani@...nel.org>,
        Philipp Zabel <p.zabel@...gutronix.de>, netdev@...r.kernel.org,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-actions@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/3] net: ethernet: actions: Add Actions Semi Owl
 Ethernet MAC driver

On Tue, Mar 23, 2021 at 12:38:10AM +0100, Andrew Lunn wrote:
> > +static void owl_emac_set_multicast(struct net_device *netdev, int count)
> > +{
> > +	struct owl_emac_priv *priv = netdev_priv(netdev);
> > +	struct netdev_hw_addr *ha;
> > +	int index = 0;
> > +
> > +	if (count <= 0) {
> > +		priv->mcaddr_list.count = 0;
> > +		return;
> > +	}
> > +
> > +	netdev_for_each_mc_addr(ha, netdev) {
> > +		if (!is_multicast_ether_addr(ha->addr))
> > +			continue;
> 
> Is this possible?

I remember I've seen this in one of the drivers I have studied, but
I'm not really sure it is actually necessary. I added it to be on the
safe side..

> > +
> > +		WARN_ON(index >= OWL_EMAC_MAX_MULTICAST_ADDRS);
> > +		ether_addr_copy(priv->mcaddr_list.addrs[index++], ha->addr);
> > +	}
> > +
> > +	priv->mcaddr_list.count = index;
> > +
> > +	owl_emac_setup_frame_xmit(priv);
> > +}
> > +
> > +static void owl_emac_ndo_set_rx_mode(struct net_device *netdev)
> > +{
> > +	struct owl_emac_priv *priv = netdev_priv(netdev);
> > +	u32 status, val = 0;
> > +	int mcast_count = 0;
> > +
> > +	if (netdev->flags & IFF_PROMISC) {
> > +		val = OWL_EMAC_BIT_MAC_CSR6_PR;
> > +	} else if (netdev->flags & IFF_ALLMULTI) {
> > +		val = OWL_EMAC_BIT_MAC_CSR6_PM;
> > +	} else if (netdev->flags & IFF_MULTICAST) {
> > +		mcast_count = netdev_mc_count(netdev);
> > +
> > +		if (mcast_count > OWL_EMAC_MAX_MULTICAST_ADDRS) {
> > +			val = OWL_EMAC_BIT_MAC_CSR6_PM;
> > +			mcast_count = 0;
> > +		}
> > +	}
> > +
> > +	spin_lock_bh(&priv->lock);
> > +
> > +	/* Temporarily stop DMA TX & RX. */
> > +	status = owl_emac_dma_cmd_stop(priv);
> > +
> > +	/* Update operation modes. */
> > +	owl_emac_reg_update(priv, OWL_EMAC_REG_MAC_CSR6,
> > +			    OWL_EMAC_BIT_MAC_CSR6_PR | OWL_EMAC_BIT_MAC_CSR6_PM,
> > +			    val);
> > +
> > +	/* Restore DMA TX & RX status. */
> > +	owl_emac_dma_cmd_set(priv, status);
> > +
> > +	spin_unlock_bh(&priv->lock);
> > +
> > +	/* Set/reset multicast addr list. */
> > +	owl_emac_set_multicast(netdev, mcast_count);
> > +}
> 
> I think this can be simplified. At least, you had me going around in
> circles a while trying to see if WARN_ON() could be triggered from
> user space.
> 
> If you have more than OWL_EMAC_MAX_MULTICAST_ADDRS MC addresses, you
> go into promisc mode. Can you then skip calling
> owl_emac_set_multicast(), which appears not to do too much useful when
> passed 0?

The main purpose of always calling owl_emac_set_multicast() is to ensure
the size of the mcaddr_list is correctly updated (either set or reset).
This prevents owl_emac_setup_frame_xmit() using obsolete data, when
invoked from different contexts (i.e. MAC address changed).

A conditional call involves splitting the mcaddr_list management logic
(i.e. moving the 'reset' operation from the callee to the caller), which
IMO would make the usage of a separate function less justified.

Thanks,
Cristi

> 
>        Andrew