When packets enter the device they are classified to a priority group
(PG) buffer based on their PCP value. After their egress port and
traffic class are determined they are moved to the switch's shared
buffer and await transmission, if:

(Ingress{Port}.Usage < Thres && Ingress{Port,PG}.Usage < Thres &&
 Egress{Port}.Usage < Thres && Egress{Port,TC}.Usage < Thres)
||
(Ingress{Port}.Usage < Min || Ingress{Port,PG} < Min ||
 Egress{Port}.Usage < Min || Egress{Port,TC}.Usage < Min)

Packets scheduled to transmission through CPU port (trapped to CPU) use
traffic class 7, which has a zero maximum and minimum quotas. However,
when such packets arrive from PG 0 they are admitted to the shared
buffer as PG 0 has a non-zero minimum quota.

Allow all packets to be trapped to the CPU - regardless of the PG they
were classified to - by assigning a 10KB minimum quota for CPU port and
TC7.

Fixes: 8e8dfe9fdf06 ("mlxsw: spectrum: Add IEEE 802.1Qaz ETS support")
Reported-by: Tamir Winetroub <tamirw@mellanox.com>
Tested-by: Tamir Winetroub <tamirw@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>