Subject: Re: DSA: phy polling

From: Florian Fainelli <[email-masked]>
Date: Mon, 14 Sep 2015 10:28:55 -0700

On 14/09/15 03:42, Russell King - ARM Linux wrote:
> Andrew,
> 
> I think you're the current maintainer of the Marvell DSA code, as being
> the most recent author of changes to it. :)
> 
> I've noticed in my testing that the Marvell DSA code seems to poll the
> internal phy link status in mv88e6xxx_poll_link(), and set the network
> device carrier status according to the results.
> 
> However, the internal phys are created using phylib, which also polls
> the phys for their link status, and controls the associated netdev
> carrier status.
> 
> The side effect of this is that I see duplicated link status messages in
> the kernel log when connecting or disconnecting cables from the switch,
> caused by the code in mv88e6xxx_poll_link() racing with the phylib code.
> From what I can see, the code in mv88e6xxx_poll_link() is entirely
> redundant as the phylib layer will take care of any phy attached to the
> switch.
> 
> To prove this, I have the following code in my tree, which disables the
> polling on a port where we have a phy attached (either an internal or
> external phy).  The result is that the per-port network devices are still
> updated with the link status even though this code is disabled - thanks
> to the phylib polling.
> 
> I'm left wondering whether the DSA specific phy polling does anything
> useful, or whether the entire polling code both in mv88e6xxx.c and
> net/dsa can be removed (mv88e6xxx.c seems to be its only user.)

Just my 2 cents here, I suspect the original intention behind this code
was to help utilize the switch's built-in PHY polling unit when
available, and use the HW to collect the state of all PHYs in fewer
register to read, instead of having to do individual (and quite possibly
expensive) MDIO reads towards each individual per-port PHYs (at least
two reads per PHY to latch MII_BMSR).

Now, I do agree there is a duplication of functionality here, and a
potential fix would be to avoid starting the PHY state machine if/when
the switch supports such a feature (not call phy_start*), that should
still get you consistent consistent link partner advertised/status
values, question is, does that really benefit anybody though?

> 
> diff --git a/drivers/net/dsa/mv88e6xxx.c b/drivers/net/dsa/mv88e6xxx.c
> index 26ec2fbfaa89..4c324eafeef2 100644
> --- a/drivers/net/dsa/mv88e6xxx.c
> +++ b/drivers/net/dsa/mv88e6xxx.c
> @@ -400,6 +400,13 @@ void mv88e6xxx_poll_link(struct dsa_switch *ds)
>  		if (dev == NULL)
>  			continue;
>  
> +		/*
> +		 * Ignore ports which have a phy; phylib will take care
> +		 * of polling the link status for these.
> +		 */
> +		if (dsa_slave_has_phy(dev))
> +			continue;
> +
>  		link = 0;
>  		if (dev->flags & IFF_UP) {
>  			port_status = mv88e6xxx_reg_read(ds, REG_PORT(i),
> diff --git a/include/net/dsa.h b/include/net/dsa.h
> index fbca63ba8f73..b31e9da43ea7 100644
> --- a/include/net/dsa.h
> +++ b/include/net/dsa.h
> @@ -176,6 +176,8 @@ static inline bool dsa_is_port_initialized(struct dsa_switch *ds, int p)
>  	return ds->phys_port_mask & (1 << p) && ds->ports[p];
>  }
>  
> +extern bool dsa_slave_has_phy(struct net_device *);
> +
>  static inline u8 dsa_upstream_port(struct dsa_switch *ds)
>  {
>  	struct dsa_switch_tree *dst = ds->dst;
> diff --git a/net/dsa/slave.c b/net/dsa/slave.c
> index 35c47ddd04f0..a107242816ff 100644
> --- a/net/dsa/slave.c
> +++ b/net/dsa/slave.c
> @@ -10,6 +10,7 @@
>  
>  #include <linux/list.h>
>  #include <linux/etherdevice.h>
> +#include <linux/export.h>
>  #include <linux/netdevice.h>
>  #include <linux/phy.h>
>  #include <linux/phy_fixed.h>
> @@ -873,6 +874,14 @@ int dsa_slave_resume(struct net_device *slave_dev)
>  	return 0;
>  }
>  
> +bool dsa_slave_has_phy(struct net_device *slave_dev)
> +{
> +	struct dsa_slave_priv *p = netdev_priv(slave_dev);
> +
> +	return p->phy != NULL;
> +}
> +EXPORT_SYMBOL_GPL(dsa_slave_has_phy);
> +
>  int dsa_slave_create(struct dsa_switch *ds, struct device *parent,
>  		     int port, char *name)
>  {
> 
> 

-- 
Florian