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Date:   Thu, 15 Jun 2023 00:51:22 +0300
From:   Vladimir Oltean <olteanv@gmail.com>
To:     arinc9.unal@gmail.com
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Subject: Re: [PATCH net v4 6/7] net: dsa: introduce
 preferred_default_local_cpu_port and use on MT7530
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On Mon, Jun 12, 2023 at 10:59:44AM +0300, arinc9.unal@gmail.com wrote:
> From: Vladimir Oltean <olteanv@gmail.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

I keep mentally objecting to this patch and then I need to remind myself
why this decision was taken. I believe that a key element influencing
that decision is not sufficiently highlighted.

You can add, right here, after "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@gmail.com>
> Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
> ---