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

[Daniel Golle <daniel@makrotopia.org>]

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...)