Re: lantiq_xrx200: Ethernet MAC with multiple TX queues

On Wed, Mar 24, 2021 at 04:07:47PM -0700, Florian Fainelli wrote:

quoted

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

For systemport and bcm_sf2 this was explained in this commit:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d156576362c07e954dc36e07b0d7b0733a010f7d

in a nutshell, the switch hardware can return the queue status back to
the systemport's transmit DMA such that it can automatically pace the TX
completion interrupts. To do that we need to establish a mapping between
the DSA slave and master that is comprised of the switch port number and
TX queue number, and tell the HW to inspect the congestion status of
that particular port and queue.

What this is meant to address is a "lossless" (within the SoC at least)
behavior when you have user ports that are connected at a speed lower
than that of your internal connection to the switch typically Gigabit or
more. If you send 1Gbits/sec worth of traffic down to a port that is
connected at 100Mbits/sec there will be roughly 90% packet loss unless
you have a way to pace the Ethernet controller's transmit DMA, which
then ultimately limits the TX completion of the socket buffers so things
work nicely. I believe that per queue flow control was evaluated before
and an out of band mechanism was preferred but I do not remember the
details of that decision to use ACB.

Interesting system design.

Just to clarify, this port to queue mapping is completely optional, right?
You can send packets to a certain switch port through any TX queue of
the systemport?