Re: lantiq_xrx200: Ethernet MAC with multiple TX queues

Hi Martin,

On Wed, Mar 24, 2021 at 09:04:16PM +0100, Martin Blumenstingl wrote:

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.

Is this question specific in any way to DSA?
Many Ethernet drivers are multi-queue. Some map one queue per CPU, some
use mqprio to map one or more queues per priority.