Re: [PATCH net-next v4 08/10] net: airoha: Do not stop GDM port if it is shared

[Lorenzo Bianconi <lorenzo@kernel.org>]

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> This is an AI-generated review of your patch. The human sending this
> email has considered the AI review valid, or at least plausible.
> ---
> net: airoha: Do not stop GDM port if it is shared
> 
> This patch adds a per-port users refcount so the GDM forward configuration
> is only reset to FE_PSE_PORT_DROP when the last netdev attached to the
> shared GDM port stops, and it adjusts the MTU/length register updates to
> cope with the shared case.
> 
> > diff --git a/drivers/net/ethernet/airoha/airoha_eth.c b/drivers/net/ethernet/airoha/airoha_eth.c
> > index 0253919714e0..1c4927c1aeb0 100644
> > --- a/drivers/net/ethernet/airoha/airoha_eth.c
> > +++ b/drivers/net/ethernet/airoha/airoha_eth.c
> > @@ -1714,8 +1714,8 @@ static int airoha_dev_open(struct net_device *netdev)
> >  	int err, len = ETH_HLEN + netdev->mtu + ETH_FCS_LEN;
> >  	struct airoha_gdm_dev *dev = netdev_priv(netdev);
> >  	struct airoha_gdm_port *port = dev->port;
> > +	u32 cur_len, pse_port = FE_PSE_PORT_PPE1;
> >  	struct airoha_qdma *qdma = dev->qdma;
> > -	u32 pse_port = FE_PSE_PORT_PPE1;
> >  
> >  	netif_tx_start_all_queues(netdev);
> >  	err = airoha_set_vip_for_gdm_port(dev, true);
> > @@ -1729,10 +1729,14 @@ static int airoha_dev_open(struct net_device *netdev)
> >  		airoha_fe_clear(qdma->eth, REG_GDM_INGRESS_CFG(port->id),
> >  				GDM_STAG_EN_MASK);
> >  
> > -	airoha_fe_rmw(qdma->eth, REG_GDM_LEN_CFG(port->id),
> > -		      GDM_SHORT_LEN_MASK | GDM_LONG_LEN_MASK,
> > -		      FIELD_PREP(GDM_SHORT_LEN_MASK, 60) |
> > -		      FIELD_PREP(GDM_LONG_LEN_MASK, len));
> > +	cur_len = airoha_fe_get(qdma->eth, REG_GDM_LEN_CFG(port->id),
> > +				GDM_LONG_LEN_MASK);
> > +	if (!atomic_read(&port->users) || len > cur_len)
> > +		airoha_fe_rmw(qdma->eth, REG_GDM_LEN_CFG(port->id),
> > +			      GDM_SHORT_LEN_MASK | GDM_LONG_LEN_MASK,
> > +			      FIELD_PREP(GDM_SHORT_LEN_MASK, 60) |
> > +			      FIELD_PREP(GDM_LONG_LEN_MASK, len));
> > +	atomic_inc(&port->users);
> 
> [Medium]
> When a second netdev opens with a larger MTU, this unconditionally
> widens GDM_LONG_LEN_MASK for the already-running sibling that has a
> smaller MTU:
> 
> 	if (!atomic_read(&port->users) || len > cur_len)
> 		airoha_fe_rmw(qdma->eth, REG_GDM_LEN_CFG(port->id),
> 			      GDM_SHORT_LEN_MASK | GDM_LONG_LEN_MASK,
> 			      FIELD_PREP(GDM_SHORT_LEN_MASK, 60) |
> 			      FIELD_PREP(GDM_LONG_LEN_MASK, len));
> 
> Meanwhile airoha_dev_change_mtu() rejects shrinks (len < cur_len) with
> -EBUSY when the port is shared. Is this asymmetry intentional?

This is done intentionally since we do not want to break traffic of already
running net_device if we want to increase the MTU on a sibling net_device.

> 
> With the new policy the hardware long-length ends up as the maximum of
> the two MTUs, so the smaller-MTU sibling silently accepts frames larger
> than its advertised MTU at the MAC level. Should opens that would
> increase the shared length also be gated (symmetric policy), or should
> the commit message and a comment document the chosen ceiling/floor
> behaviour?

I will add a comment in the code in v5.

> 
> >  
> >  	airoha_qdma_set(qdma, REG_QDMA_GLOBAL_CFG,
> >  			GLOBAL_CFG_TX_DMA_EN_MASK |
> > @@ -1762,8 +1766,12 @@ static int airoha_dev_stop(struct net_device *netdev)
> >  	for (i = 0; i < netdev->num_tx_queues; i++)
> >  		netdev_tx_reset_subqueue(netdev, i);
> >  
> > -	airoha_set_gdm_port_fwd_cfg(qdma->eth, REG_GDM_FWD_CFG(port->id),
> > -				    FE_PSE_PORT_DROP);
> > +	if (atomic_dec_and_test(&port->users)) {
> > +		airoha_set_vip_for_gdm_port(dev, false);
> > +		airoha_set_gdm_port_fwd_cfg(qdma->eth,
> > +					    REG_GDM_FWD_CFG(port->id),
> > +					    FE_PSE_PORT_DROP);
> > +	}
> 
> [Low]
> Is the airoha_set_vip_for_gdm_port(dev, false) call inside the
> atomic_dec_and_test() block intentional?
> 
> Looking at airoha_dev_stop() as a whole, the same call is already made
> unconditionally a few lines above:
> 
> 	netif_tx_disable(netdev);
> 	airoha_set_vip_for_gdm_port(dev, false);
> 	for (i = 0; i < netdev->num_tx_queues; i++)
> 		netdev_tx_reset_subqueue(netdev, i);
> 
> 	if (atomic_dec_and_test(&port->users)) {
> 		airoha_set_vip_for_gdm_port(dev, false);
> 		...
> 	}
> 
> Since get_vip_port() derives the bit mask per-(port, dev->nbq) and
> airoha_fe_clear() of already-cleared bits is a no-op, the second call
> looks like dead code today. The commit log only mentions the
> FE_PSE_PORT_DROP gating, not VIP handling. Should the duplicate be
> dropped in v2?

ack, I will fix in v5.

> 
> >  
> >  	if (atomic_dec_and_test(&qdma->users)) {
> >  		airoha_qdma_clear(qdma, REG_QDMA_GLOBAL_CFG,
> > @@ -1915,10 +1923,22 @@ static void airoha_dev_get_stats64(struct net_device *netdev,
> >  static int airoha_dev_change_mtu(struct net_device *netdev, int mtu)
> >  {
> >  	struct airoha_gdm_dev *dev = netdev_priv(netdev);
> > +	u32 cur_len, len = ETH_HLEN + mtu + ETH_FCS_LEN;
> >  	struct airoha_gdm_port *port = dev->port;
> > -	u32 len = ETH_HLEN + mtu + ETH_FCS_LEN;
> >  	struct airoha_eth *eth = dev->eth;
> >  
> > +	cur_len = airoha_fe_get(eth, REG_GDM_LEN_CFG(port->id),
> > +				GDM_LONG_LEN_MASK);
> > +	if (len < cur_len) {
> > +		u8 port_refcnt = atomic_read(&port->users);
> > +
> > +		/* We can decrease the device MTU just if the GDM port is
> > +		 * not shared or if the other device is not running.
> > +		 */
> > +		if (port_refcnt > 1 || (port_refcnt && !netif_running(netdev)))
> > +			return -EBUSY;
> > +	}
> > +
> 
> [Low]
> Why is port_refcnt typed as u8 here when atomic_read() returns int?
> 
> 	u8 port_refcnt = atomic_read(&port->users);
> 
> With AIROHA_MAX_NUM_GDM_DEVS currently 2 this cannot overflow, but a
> future bump of AIROHA_MAX_NUM_GDM_DEVS, or a refcount leak, that pushed
> port->users to 256 would wrap to 0 and quietly bypass both the
> port_refcnt > 1 and port_refcnt && !netif_running(netdev) checks,
> permitting MTU shrinks that are meant to be rejected. Would using int
> here (the type atomic_read() returns) be preferable?

ack, I will fix in v5.

Regards,
Lorenzo

> 
> >  	airoha_fe_rmw(eth, REG_GDM_LEN_CFG(port->id),
> >  		      GDM_LONG_LEN_MASK,
> >  		      FIELD_PREP(GDM_LONG_LEN_MASK, len));

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