linux-kernel - Re: [PATCH RFC net-next 20/23] net: dsa: lantiq_gswip: add registers specific for MaxLinear GSW1xx

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Message-ID: <aKdnTk7bjWEyty5H@pidgin.makrotopia.org>
Date: Thu, 21 Aug 2025 19:37:02 +0100
From: Daniel Golle <daniel@...rotopia.org>
To: "Sverdlin, Alexander" <alexander.sverdlin@...mens.com>
Cc: "hauke@...ke-m.de" <hauke@...ke-m.de>,
	"olteanv@...il.com" <olteanv@...il.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"andrew@...n.ch" <andrew@...n.ch>,
	"linux@...linux.org.uk" <linux@...linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"arkadis@...lanox.com" <arkadis@...lanox.com>,
	"kuba@...nel.org" <kuba@...nel.org>,
	"pabeni@...hat.com" <pabeni@...hat.com>,
	"edumazet@...gle.com" <edumazet@...gle.com>,
	"f.fainelli@...il.com" <f.fainelli@...il.com>,
	"horms@...nel.org" <horms@...nel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"john@...ozen.org" <john@...ozen.org>,
	"Stockmann, Lukas" <lukas.stockmann@...mens.com>,
	"yweng@...linear.com" <yweng@...linear.com>,
	"fchan@...linear.com" <fchan@...linear.com>,
	"lxu@...linear.com" <lxu@...linear.com>,
	"jpovazanec@...linear.com" <jpovazanec@...linear.com>,
	"Schirm, Andreas" <andreas.schirm@...mens.com>,
	"Christen, Peter" <peter.christen@...mens.com>,
	"ajayaraman@...linear.com" <ajayaraman@...linear.com>,
	"bxu@...linear.com" <bxu@...linear.com>,
	"lrosu@...linear.com" <lrosu@...linear.com>
Subject: Re: [PATCH RFC net-next 20/23] net: dsa: lantiq_gswip: add registers
 specific for MaxLinear GSW1xx

Hi Alenxander,

thank you for the thorough review of the RFC series!

On Thu, Aug 21, 2025 at 06:11:36PM +0000, Sverdlin, Alexander wrote:
> On Sat, 2025-08-16 at 20:56 +0100, Daniel Golle wrote:
> > [...]
> > +#define GSW1XX_PORTS				6
>  
> a tricky questions for you ;-)
> 
> There are scarce references to Port 6 across GSW145 datasheet, the 7th port.
> One could also enable/disable it (bit P6) in GSWIP_CFG (0xF400).
> 
> Yes, it's sold as 6-port switch, but the question is, shall the port 6 be
> disabled to save power and avoid unnecessary EMI?

I understood that port 6 can be used for packet injection or monitoring
over the management interface (MDIO or SPI). For a moment I thought it'd
be cool to implement a (very slow) Ethernet netdev allowing to use that
feature in Linux -- but it would be very very slow and also useless on
systems which anyway got either port 4 or 5 connected to the host as CPU
port. I guess it is intended for basic managed switches which come with
a simple microcontroller connected to the GSW1xx chip: ~1 MBit/s
"Ethernet-over-MDIO" or (slightly faster due to duplex)
"Ethernet-over-SPI" can be fast enough to serve a simple Web UI, Telnet,
SSH, or respond to SNMP or LLDP, for example.
I wonder if that port also supports the DSA special tag (insertion and
egress)...

Anyway, fantasies aside, at least for now it does make sense to disable
it, though I wouldn't expect significant power saving or reduced EMI
from that (but you never know...)