idpf: adjust TxQ ring count minimum

idpf: adjust TxQ ring count minimum

Set the TxQ ring count minimum to 128 descriptors. Any lower than this,
and the queue will stall and trigger Tx timeouts in flow based
scheduling mode. This is because next_to_clean might never be updated.

In flow based scheduling mode, next_to_clean is only updated after a
descriptor completion is processed, i.e. after the RE bit is set in the
last descriptor of a Tx packet. This will never happen with a ring size
of 64 and an IDPF_TX_SPLITQ_RE_MIN_GAP of 64. No matter what the value
of last_re is initialized/set to, the calculated gap will be at most 63
and never trigger the RE bit.

Even a ring size of 96 does not solve this. Because of how infrequent
next_to_clean is updated and how small the ring is, IDPF_DESC_UNUSED
will be much smaller on average. This increases the chance the queue
will be stopped because a multi-descriptor packet, e.g. a large LSO
packet, does not see enough resources on the ring. In this case, the
queue will trigger the stop logic. The queue permanently stalls because
there is no chance for a descriptor completion to update next_to_clean
since it is dependent on a packet being sent.

Fixes: 5f417d551324 ("idpf: replace flow scheduling buffer ring with buffer pool")
Signed-off-by: Joshua Hay <joshua.a.hay@intel.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Tested-by: Samuel Salin <Samuel.salin@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>