netdev - Re: lantiq_xrx200: Ethernet MAC with multiple TX queues

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Date:   Wed, 24 Mar 2021 20:08:40 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        netdev@...r.kernel.org, Hauke Mehrtens <hauke@...ke-m.de>,
        andrew@...n.ch, vivien.didelot@...il.com, davem@...emloft.net,
        kuba@...nel.org
Subject: Re: lantiq_xrx200: Ethernet MAC with multiple TX queues



On 3/24/2021 6:18 PM, Vladimir Oltean wrote:
> On Wed, Mar 24, 2021 at 04:07:47PM -0700, Florian Fainelli wrote:
>>> 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?

Absolutely, the Broadcom tags allow you to steer traffic towards any TX
queue of any switch port. If ring inspection is disabled then there is
no flow control applied whatsoever.
-- 
Florian