airoha_eth: PPE flow entries and EGRESS TRTCM shaping

[Wayen Yan <win847@gmail.com>]

Hi Lorenzo,

While reviewing the airoha HTB offload code and your recent fix series,
I noticed that EGRESS TRTCM rate-limit buckets configured by
airoha_qdma_set_tx_rate_limit() do not appear to be referenced by
either the CPU xmit path or PPE-accelerated flows.

Specifically:

1) CPU xmit path always disables the meter:

   In airoha_dev_xmit(), TXMSG.METER is hardcoded to 0x7f:

     msg1 = FIELD_PREP(QDMA_ETH_TXMSG_METER_MASK, 0x7f);

   The register comment notes "0x7f no meters", so even though
   airoha_tc_htb_alloc_leaf_queue() configures TRTCM bucket[channel]
   with the requested rate, CPU-path packets never hit those buckets.

2) PPE flow entries never bind to a TRTCM bucket:

   In airoha_ppe_foe_entry_prepare(), the FOE 'data' field is
   initialized with:

     qdata = FIELD_PREP(AIROHA_FOE_SHAPER_ID, 0x7f);

   And neither CHANNEL[15:11] nor QID[10:8] are ever set — they
   remain zero in all PPE entry creation paths (tc offload,
   HW autolearn, L2 subflow commit). This means PPE-accelerated
   flows bypass TRTCM shaping entirely.

   The only QoS-related bit set is IB2.PSE_QOS for wired LAN ports,
   which is a flag rather than an index.

3) airoha_ppe_foe_flow_stats_update() does read CHANNEL|QID from
   the data field and moves them to ACTDP, but since they were
   never populated, this is effectively a no-op in practice.

I have a few questions:

- Does AIROHA_FOE_SHAPER_ID map to the same QDMA TRTCM meter index
  space used by airoha_qdma_set_tx_rate_limit()? If so, setting
  SHAPER_ID = channel for flows belonging to an HTB class would
  enable per-flow egress shaping via PPE.

- Is the current 0x7f / disabled behavior intentional — i.e., PPE
  flow shaping is simply not yet implemented, or is there a hardware
  constraint I'm missing?

- For the CPU xmit path, is there a reason TXMSG.METER cannot be
  set to the channel derived from skb_get_queue_mapping(), so that
  CPU-path packets also respect the TRTCM rate limits?

Thanks,
Wayen