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

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Date:   Wed, 24 Mar 2021 15:58:37 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Hauke Mehrtens <hauke@...ke-m.de>,
        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 3/24/2021 3:50 PM, Hauke Mehrtens wrote:
> 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.

bcmsysport.c for the "Lite" version has the same kind of limitation, so
what it does is limit the number of DSA slave queues to continue to
support a 1:1 mapping.

> 
> 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.

There is not a way to establish a mapping from user-space between the
DSA slave queues and the DSA master queues, I am not too sure what would
be the right way to do that but it would likely require changes to the
'tc' tool and ndo_setup_tc() most likely.
-- 
Florian