On Sat, Jun 05, 2021 at 01:34:55AM +0200, Pali Rohár wrote: > But as this is really confusing what each mode means for Linux, I would > suggest that documentation for these modes in ethernet-controller.yaml > file (or in any other location) could be extended. I see that it is > really hard to find exact information what these modes mean and what is > their meaning in DTS / kernel. We have been adding documentation to: Documentation/networking/phy.rst for each of the modes that have had issues. The 2500base-X entry hasn't been updated yet, as the question whether it can have in-band signaling is unclear (there is no well defined standard for this.) Some vendors state that there is no in-band signalling in 2500base-X. Others (e.g. Xilinx) make it clear that it is optional. Others don't say either way, and when testing hardware, it appears to be functional. So, coming up with a clear definition for this, when we have no real method in the DT file to say "definitely do not use in-band" is a tad difficult. It started out as described - literally, 1000base-X multiplied by 2.5x. There are setups where that is known to work - namely GPON SFPs that support 2500base-X. What that means is that we know the GPON SFP module negotiates in-band AN with 2500base-X. However, we don't know whether the module will work if we disable in-band AN. There is hardware out there as well which allows one to decide whether to use in-band AN with 2500base-X or not. Xilinx is one such vendor who explicitly documents this. Marvell on the other hand do not prohibit in-band AN with mvneta, neither to they explicitly state it is permitted. In at least one of their PHY documents, they suggest it isn't supported if the MAC side is operating in 2500base-X. Others (NXP) take the position that in-band AN is not supported at 2500base-X speeds. I think a few months ago, Vladimir persuaded me that we should disable in-band AN for 2500base-X - I had forgotten about the Xilinx documentation I had which shows that it's optional. (Practically, it's optional in hardware with 1000base-X too, but then it's not actually conforming with 802.3's definition of 1000base-X.) The result is, essentially, a total mess. 2500base-X is not a standards defined thing, so different vendors have gone off and done different things. Sometimes it's amazing that you can connect two devices together and they will actually talk to each other! -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!
