Re: lantiq_xrx200: Ethernet MAC with multiple TX queues

Hi Florian,

On Wed, Mar 24, 2021 at 02:09:02PM -0700, Florian Fainelli wrote:

On 3/24/2021 1:13 PM, Vladimir Oltean wrote:

quoted

Hi Martin,

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

quoted

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 you use DSA you will have a DSA slave network device which will be
calling into dev_queue_xmit() into the DSA master which will be the
xrx200 driver, so it's fairly simple for you to implement a queue
selection within the xrx200 tagger for instance.

You can take a look at how net/dsa/tag_brcm.c and
drivers/net/ethernet/broadcom/bcmsysport.c work as far as mapping queues
from the DSA slave network device queue/port number into a queue number
for the DSA master.

What are the benefits of mapping packets to TX queues of the DSA master
from the DSA layer?