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

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Date:   Wed, 24 Mar 2021 23:50:23 +0100
From:   Hauke Mehrtens <hauke@...ke-m.de>
To:     Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc:     netdev@...r.kernel.org, andrew@...n.ch, vivien.didelot@...il.com,
        davem@...emloft.net, kuba@...nel.org
Subject: Re: lantiq_xrx200: Ethernet MAC with multiple TX queues

On 3/24/21 10:09 PM, 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.
> 

Hi,

The PMAC in the xrx200 has 4 TX queues and 8 RX queues. We can not map 
one queue to each port as there are more ports than queues. I am also 
unsure if the DSL part which is using an out of tree driver uses some of 
these DMA resources.

Is it possible to configure a mapping between a DSA bridge and a queue 
on the mater device with tc from user space? We could expose these 4 TX 
queues on the mac driver to Linux and then Linux configure somehow a 
mapping between ports or bridges and queues.

Hauke