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

[Siddharth Vadapalli <s-vadapalli@ti.com>]

On 05/02/26 5:05 PM, Chintan Vankar wrote:
> 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.

Consider two MAC Ports P1 and P2. P1 belongs to multicast group M1 and 
M2, while P2 belongs only to multicast group M2.

If M1 is deleted from the interface for P1, it will trigger the 
execution of 'am65_cpsw_nuss_ndo_slave_set_rx_mode()' function.
In the function, we have:

	port_mask = BIT(port->port_id) | ALE_PORT_HOST;
	cpsw_ale_flush_multicast(common->ale, port_mask, -1);

	=> At this point, M2 would have also been removed in addition
	   to M1 for P1. As a result, Multicast traffic M2 from P2 to
	   P1 is dropped.

	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);
		}
	}

	=> We are adding back M2 to P1 above, but since it is not
	   instantaneous, it will take time for the Multicast traffic
	   M2 to be forwarded from P2 to P1.

Please let me know if I have overlooked something.

Regards,
Siddharth.