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

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Date:   Wed, 24 Mar 2021 14:09:02 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc:     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 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.
-- 
Florian