netdev - lantiq_xrx200: Ethernet MAC with multiple TX queues

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Date:   Wed, 24 Mar 2021 21:04:16 +0100
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     netdev@...r.kernel.org
Cc:     Hauke Mehrtens <hauke@...ke-m.de>, andrew@...n.ch,
        vivien.didelot@...il.com, f.fainelli@...il.com, olteanv@...il.com,
        davem@...emloft.net, kuba@...nel.org
Subject: lantiq_xrx200: Ethernet MAC with multiple TX queues

Hello,

the PMAC (Ethernet MAC) IP built into the Lantiq xRX200 SoCs has
support for multiple (TX) queues.
This MAC is connected to the SoC's built-in switch IP (called GSWIP).

Right now the lantiq_xrx200 driver only uses one TX and one RX queue.
The vendor driver (which mixes DSA/switch and MAC functionality in one
driver) uses the following approach:
- eth0 ("lan") uses the first TX queue
- eth1 ("wan") uses the second TX queue

With the current (mainline) lantiq_xrx200 driver some users are able
to fill up the first (and only) queue.
This is why I am thinking about adding support for the second queue to
the lantiq_xrx200 driver.

My main question is: how do I do it properly?
Initializing the second TX queue seems simple (calling
netif_tx_napi_add for a second time).
But how do I choose the "right" TX queue in xrx200_start_xmit then?

If my description is too vague then please let me know about any
specific questions you have.
Also if there's an existing driver that "does things right" I am happy
to look at that one.

Thank you and best regards,
Martin