Re: [PATCH net 1/2] net: ethernet: ti: am65-cpsw-nuss: Update port_mask while adding multicast entry

[Chintan Vankar <c-vankar@ti.com>]

Hello Siddharth

On 05/02/26 13:45, Siddharth Vadapalli wrote:
> On Thu, 2026-02-05 at 12:39 +0530, Chintan Vankar wrote:
>> Multicast entry rules are mainly evaluated for receiving traffic and do not
>> require MAC ports to be explicitly associated with them. However, setting
>> associated MAC port's bit to the port_mask of the ALE entry ensures
>> technical correctness and helps maintaining ALE entries linked to MAC
>> ports.
>>
>> Signed-off-by: Chintan Vankar <c-vankar@ti.com>
>> ---
>>   drivers/net/ethernet/ti/am65-cpsw-nuss.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/ethernet/ti/am65-cpsw-nuss.c b/drivers/net/ethernet/ti/am65-cpsw-nuss.c
>> index 5924db6be3fe..967918050433 100644
>> --- a/drivers/net/ethernet/ti/am65-cpsw-nuss.c
>> +++ b/drivers/net/ethernet/ti/am65-cpsw-nuss.c
>> @@ -391,7 +391,7 @@ static void am65_cpsw_nuss_ndo_slave_set_rx_mode(struct net_device *ndev)
>>   	cpsw_ale_set_allmulti(common->ale,
>>   			      ndev->flags & IFF_ALLMULTI, port->port_id);
>>   
>> -	port_mask = ALE_PORT_HOST;
>> +	port_mask = BIT(port->port_id) | ALE_PORT_HOST;
> 
> Since the port_mask setup above is passed to 'cpsw_ale_flush_multicast()'
> below, wouldn't it temporarily cause the forwarded multicast traffic to be
> dropped?
> 
>>   	/* Clear all mcast from ALE */
>>   	cpsw_ale_flush_multicast(common->ale, port_mask, -1);
>>
>>
> 
> The code in the driver following the above function call is:
> 
> 	if (!netdev_mc_empty(ndev)) {
> 		struct netdev_hw_addr *ha;
> 
> 		/* program multicast address list into ALE register */
> 		netdev_for_each_mc_addr(ha, ndev) {
> 			cpsw_ale_add_mcast(common->ale, ha->addr,
> 					   port_mask, 0, 0, 0);
> 		}
> 	}
> 
> So we do add back the multicast entries for the same 'port_mask' that we
> removed earlier during cpsw_ale_flush_multicast(), but, there is a delay
> between flushing the entries in cpsw_ale_flush_multicast() and adding them
> back via cpsw_ale_add_mcast(). During that period, the multicast traffic on
> the forwarding path between MAC Ports will be momentarily dropped. Please
> clarify if that isn't the case.
> 

When "cpsw_ale_flush_multicast()" is called, it only deletes entries
where the resulting port_mask becomes "0x0" or "ALE_PORT_HOST" (as per
the function implementation "cpsw_ale_flush_multicast()" in cpsw_ale.c).

For multicast entries shared across multiple ports, the function only
clears the specific port's bit from the port_mask without deleting the
entry itself. This prevents any traffic disruption for other active
ports.

Regards,
Chintan.

> Regards,
> Siddharth.