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

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Date:   Thu, 25 Mar 2021 00:21:14 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Florian Fainelli <f.fainelli@...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

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:
> > 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 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?