> 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 <xuegang.lu@airoha.com>
> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> ---
>  drivers/net/ethernet/airoha/airoha_eth.c | 36 +++++++++++++++++++++++++-------
>  drivers/net/ethernet/airoha/airoha_eth.h |  2 ++
>  2 files changed, 30 insertions(+), 8 deletions(-)
> 
> 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);
>  
>  	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);
> +	}
>  
>  	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;
> +	}
> +
>  	airoha_fe_rmw(eth, REG_GDM_LEN_CFG(port->id),
>  		      GDM_LONG_LEN_MASK,
>  		      FIELD_PREP(GDM_LONG_LEN_MASK, len));
> diff --git a/drivers/net/ethernet/airoha/airoha_eth.h b/drivers/net/ethernet/airoha/airoha_eth.h
> index 207c75152fde..3a313ac439e7 100644
> --- a/drivers/net/ethernet/airoha/airoha_eth.h
> +++ b/drivers/net/ethernet/airoha/airoha_eth.h
> @@ -553,6 +553,8 @@ struct airoha_gdm_port {
>  	struct airoha_gdm_dev *devs[AIROHA_MAX_NUM_GDM_DEVS];
>  	int id;
>  
> +	atomic_t users;
> +
>  	struct airoha_hw_stats stats;
>  
>  	struct metadata_dst *dsa_meta[AIROHA_MAX_DSA_PORTS];
> 
> -- 
> 2.54.0
> 

Commenting on shashiko report:
https://sashiko.dev/#/patchset/20260507-airoha-eth-multi-serdes-v4-0-af613b61ae02%40kernel.org

- Is this call to airoha_set_vip_for_gdm_port() redundant?
  Looking at airoha_dev_stop(), airoha_set_vip_for_gdm_port(dev, false) is
  already called unconditionally earlier in the function. Since get_vip_port()
  returns a device-specific bitmask based on dev->nbq, does the unconditional
  call correctly clear the hardware bits for the specific device being stopped
  without affecting other devices on the same shared port?
  - I will fix it in v5.

- Does returning -EBUSY here improperly prevent valid software MTU configurations? 
  Instead of failing the ndo_change_mtu callback and preventing the software
  netdev->mtu from updating, should this code unconditionally allow the software
  update (netdev->mtu = mtu) and simply skip the hardware register
  (REG_GDM_LEN_CFG) downgrade to safely preserve the larger hardware limit
  required by active shared siblings?
  - I will fix it in v5.

- Is the use of an atomic_t necessary for tracking users here?
  The port->users reference count appears to only be modified in ndo_open and
  ndo_stop, and read in ndo_change_mtu. Since all of these callback paths are
  inherently strictly serialized by the kernel's rtnl_lock(), could a simple
  integer be used instead to avoid unnecessary atomic overhead?
  - I will fix it in v5.