Subject: Re: [PATCH net-next v3 10/12] net: airoha: Do not stop GDM port if it is shared

From: Lorenzo Bianconi <[email-masked]>
Date: Fri, 10 Apr 2026 12:35:03 +0200

> This is an AI-generated review of your patch. The human sending this
> email says "For your consideration."
> ---
> net: airoha: Do not stop GDM port if it is shared
> 
> Theoretically, in the current codebase, two independent net_devices can
> be connected to the same GDM port so we need to check the GDM port is not
> used by any other running net_device before setting the forward
> configuration to FE_PSE_PORT_DROP.
> 
> Tested-by: Xuegang Lu <[email-masked]>
> 
> > diff --git a/drivers/net/ethernet/airoha/airoha_eth.c b/drivers/net/ethernet/airoha/airoha_eth.c
> > index fd27ba13029bd..5b0cd37b155e4 100644
> > --- a/drivers/net/ethernet/airoha/airoha_eth.c
> > +++ b/drivers/net/ethernet/airoha/airoha_eth.c
> 
> [ ... ]
> 
> > @@ -1688,18 +1693,18 @@ static int airoha_dev_stop(struct net_device *netdev)
> >  	struct airoha_gdm_dev *dev = netdev_priv(netdev);
> >  	struct airoha_gdm_port *port = dev->port;
> >  	struct airoha_qdma *qdma = dev->qdma;
> > -	int i, err;
> > +	int i;
> >  
> >  	netif_tx_disable(netdev);
> > -	err = airoha_set_vip_for_gdm_port(dev, false);
> > -	if (err)
> > -		return err;
> > -
> >  	for (i = 0; i < ARRAY_SIZE(qdma->q_tx); 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);
> > +	}
> >  
> >  	if (atomic_dec_and_test(&qdma->users)) {
> >  		airoha_qdma_clear(qdma, REG_QDMA_GLOBAL_CFG,
> > 				  GLOBAL_CFG_TX_DMA_EN_MASK |
> > 				  GLOBAL_CFG_RX_DMA_EN_MASK);
> > 
> > 		for (i = 0; i < ARRAY_SIZE(qdma->q_tx); i++) {
> > 			if (!qdma->q_tx[i].ndesc)
> > 				continue;
> > 
> > 			airoha_qdma_cleanup_tx_queue(&qdma->q_tx[i]);
> > 		}
> > 	}
> 
> This problem wasn't introduced by this patch, but does the QDMA teardown
> path have a DMA corruption vulnerability here?
> 
> When GLOBAL_CFG_TX_DMA_EN_MASK is cleared, it fails to poll
> GLOBAL_CFG_TX_DMA_BUSY_MASK, proceeding to unmap memory while the hardware
> DMA engine may still be processing in-flight descriptors.
> 
> Furthermore, airoha_qdma_cleanup_tx_queue() unmaps the DMA addresses and
> frees the SKBs, but it does not appear to clear the physical hardware
> descriptors or reset the hardware ring pointers (REG_TX_CPU_IDX /
> REG_TX_DMA_IDX).
> 
> When the device is later reopened in airoha_dev_open(),
> GLOBAL_CFG_TX_DMA_EN_MASK is set again without re-initializing the rings.
> Will the hardware DMA engine resume fetching from its previous index,
> processing stale descriptors whose backing memory was already unmapped
> and freed?

ack, I will work on it with a dedicated patch.

> 
> [ ... ]
> 
> > @@ -1851,10 +1856,22 @@ 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 = FIELD_GET(GDM_LONG_LEN_MASK,
> > +			    airoha_fe_rr(eth, REG_GDM_LEN_CFG(port->id)));
> > +	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;
> > +	}
> > +
> 
> Does this MTU decrease validation create a state trap on shared ports?
> 
> If both eth0 and eth1 (sharing a port) are up, and a user increases eth0 MTU
> to 9000, the hardware MTU updates to 9000. If the user then tries to lower
> eth0 MTU back to 1500, len < cur_len evaluates to true. Because eth1 is also
> up (port_refcnt > 1), this returns -EBUSY.
> 
> This would prevent either interface from decreasing its MTU without fully
> bringing down the sibling interface.
> 
> Should the software netdev->mtu be decoupled from the hardware validation,
> allowing valid MTU changes in software while dynamically programming the
> hardware MTU to the maximum of all currently up interfaces on the shared
> port?

It seems more correct to me avoid receiving in the stack packet bigger than the
configured 'SW MTU".

Regards,
Lorenzo