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Message-ID: <7ae6c8cc-d347-2eda-364c-dcefd8a2ce24@gmail.com>
Date: Mon, 19 Jun 2023 17:05:39 +0100
From: Florian Fainelli <f.fainelli@...il.com>
To: arinc9.unal@...il.com,
Arınç ÜNAL <arinc.unal@...nc9.com>,
Daniel Golle <daniel@...rotopia.org>,
Landen Chao <Landen.Chao@...iatek.com>,
DENG Qingfang <dqfext@...il.com>,
Sean Wang <sean.wang@...iatek.com>,
Andrew Lunn <andrew@...n.ch>,
Vladimir Oltean <olteanv@...il.com>,
"David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>,
Matthias Brugger <matthias.bgg@...il.com>,
AngeloGioacchino Del Regno
<angelogioacchino.delregno@...labora.com>,
Russell King <linux@...linux.org.uk>
Cc: Frank Wunderlich <frank-w@...lic-files.de>,
Bartel Eerdekens <bartel.eerdekens@...stell8.be>,
mithat.guner@...ont.com, erkin.bozoglu@...ont.com,
linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org,
linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH net v6 5/6] net: dsa: introduce
preferred_default_local_cpu_port and use on MT7530
On 6/17/2023 7:26 AM, arinc9.unal@...il.com wrote:
> From: Vladimir Oltean <olteanv@...il.com>
>
> Since the introduction of the OF bindings, DSA has always had a policy that
> in case multiple CPU ports are present in the device tree, the numerically
> smallest one is always chosen.
>
> The MT7530 switch family, except the switch on the MT7988 SoC, has 2 CPU
> ports, 5 and 6, where port 6 is preferable on the MT7531BE switch because
> it has higher bandwidth.
>
> The MT7530 driver developers had 3 options:
> - to modify DSA when the MT7531 switch support was introduced, such as to
> prefer the better port
> - to declare both CPU ports in device trees as CPU ports, and live with the
> sub-optimal performance resulting from not preferring the better port
> - to declare just port 6 in the device tree as a CPU port
>
> Of course they chose the path of least resistance (3rd option), kicking the
> can down the road. The hardware description in the device tree is supposed
> to be stable - developers are not supposed to adopt the strategy of
> piecemeal hardware description, where the device tree is updated in
> lockstep with the features that the kernel currently supports.
>
> Now, as a result of the fact that they did that, any attempts to modify the
> device tree and describe both CPU ports as CPU ports would make DSA change
> its default selection from port 6 to 5, effectively resulting in a
> performance degradation visible to users with the MT7531BE switch as can be
> seen below.
>
> Without preferring port 6:
>
> [ ID][Role] Interval Transfer Bitrate Retr
> [ 5][TX-C] 0.00-20.00 sec 374 MBytes 157 Mbits/sec 734 sender
> [ 5][TX-C] 0.00-20.00 sec 373 MBytes 156 Mbits/sec receiver
> [ 7][RX-C] 0.00-20.00 sec 1.81 GBytes 778 Mbits/sec 0 sender
> [ 7][RX-C] 0.00-20.00 sec 1.81 GBytes 777 Mbits/sec receiver
>
> With preferring port 6:
>
> [ ID][Role] Interval Transfer Bitrate Retr
> [ 5][TX-C] 0.00-20.00 sec 1.99 GBytes 856 Mbits/sec 273 sender
> [ 5][TX-C] 0.00-20.00 sec 1.99 GBytes 855 Mbits/sec receiver
> [ 7][RX-C] 0.00-20.00 sec 1.72 GBytes 737 Mbits/sec 15 sender
> [ 7][RX-C] 0.00-20.00 sec 1.71 GBytes 736 Mbits/sec receiver
>
> Using one port for WAN and the other ports for LAN is a very popular use
> case which is what this test emulates.
>
> As such, this change proposes that we retroactively modify stable kernels
> (which don't support the modification of the CPU port assignments, so as to
> let user space fix the problem and restore the throughput) to keep the
> mt7530 driver preferring port 6 even with device trees where the hardware
> is more fully described.
>
> Fixes: c288575f7810 ("net: dsa: mt7530: Add the support of MT7531 switch")
> Signed-off-by: Vladimir Oltean <olteanv@...il.com>
> Signed-off-by: Arınç ÜNAL <arinc.unal@...nc9.com>
Reviewed-by: Florian Fainelli <florian.fainelli@...adcom.com>
--
Florian
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