[PATCH net-next 0/7] Allow controlling PHY loopback and isolate modes

[Florian Fainelli]

On 9/12/24 11:21, Andrew Lunn wrote:
>> The loopback control from that API is added as it fits the API
>> well, and having the ability to easily set the PHY in MII-loopback
>> mode is a helpful tool to have when bringing-up a new device and
>> troubleshooting the link setup.
> 
> We might want to take a step back and think about loopback some more.
> 
> Loopback can be done at a number of points in the device(s). Some
> Marvell PHYs can do loopback in the PHY PCS layer. Some devices also
> support loopback in the PHY SERDES layer. I've not seen it for Marvell
> devices, but maybe some PHYs allow loopback much closer to the line?
> And i expect some MAC PCS allow loopback.
> 
> So when talking about loopback, we might also want to include the
> concept of where the loopback occurs, and maybe it needs to be a NIC
> wide concept, not a PHY concept?

Agreed, you can loop pretty much anywhere in the data path, assuming the 
hardware allows it. For the hardware I maintain, we can loop back within 
the MAC as close as possible from the interface to DRAM, or as "far" as 
possible, within the MII signals, but without actually involving the PHY.

Similarly, the PHY can loop as close as possible from the electrical 
data lines, or as far as possible by looping the *MII pins, before 
hitting the MAC.

So if nothing else, we have at least 4 kinds of loopbacks that could be 
supported, it is not clear whether we want to define all of those as 
standardized "modes" within Linux, and let drivers implement the ones 
they can, or if we just let drivers implement the mode they have, and 
advertise those. Meaning your user experience could vary.
-- 
Florian