| 1 Jun 2025 |
 K900 | For a specific module | 12:48:25 |
 emily | right I'm not talking about the modules themselves | 12:48:37 |
| emily | I've sorta accepted I just have to keep gambling on those | 12:48:43 |
| emily | I'm just trying to establish what the actual chip can do | 12:48:48 |
| emily | "The BCM84891L can communicate with Ethernet MACs that support 2500BASE-X interface through XFI at 3.125 Gbaud (Gb/s) through the RDN[x]/RDP[x] and TDN[x]/TDP[x] pins." | 12:49:20 |
| emily | that seems promising | 12:49:22 |
| emily | that would mean I could use the on-SoC MAC in the R4 and the BCM84891L would just handle the actual 2.5GBASE-T physical layer conversion? | 12:50:16 |
 magic_rb | cant you buy a 5g copper module and then plug 2.5g copper into it? wouldnt it figure out the autoneg with the other end? | 12:50:46 |
| emily | there are 2.5GBASE-T modules that will talk 10G to the host with pause frames or whatever | 12:51:23 |
| emily | but I think that implies the kernel not being able to directly talk to the MAC | 12:51:52 |
| emily | because it means stuff has to happen on the transceiver end | 12:52:04 |
| emily | also, this is the only chip you can find in twisted pair transceivers that actually runs relatively cool, so I'm sorta locked in to buying a transceiver that has it specifically | 12:52:34 |
| emily | from skimming the data sheet, I guess the BCM84891L has an integrated MAC that it can use, but it can also talk directly to a MAC on the host end if you use it in one of these modes? | 12:53:24 |
| K900 | Yes that's usually how those work | 12:53:35 |
| emily | something that worries me a little is that, even if the SFP+ vendor did absolutely everything I wanted, how is the Broadcom chip going to know it's meant to talk 2500BASE-X when the R4 can only talk 2500BASE-X if it skips any in-band negotiation? | 12:54:47 |
| K900 | It would read the EEPROM | 12:55:49 |
| K900 | Or the fake-EEPROM | 12:55:52 |
| K900 | But hopefully the real EEPROM | 12:56:00 |
| magic_rb | In reply to @emilazy:matrix.org
also, this is the only chip you can find in twisted pair transceivers that actually runs relatively cool, so I'm sorta locked in to buying a transceiver that has it specifically honestly, bpir4 <-fiber-> switch <-copper-> whatever | 12:56:10 |
| K900 | The autonegotiation stuff doesn't mean nothing is negotiated | 12:56:16 |
| K900 | Or rather | 12:56:18 |
| K900 | It means there is no negotiation happening | 12:56:24 |
| K900 | But there is capability information passed around | 12:56:33 |
| emily | this is about WAN | 12:56:46 |
| magic_rb | oh | 12:56:51 |
| emily | my uplink to the WAN is (going to be) 2.5GBASE-T | 12:56:52 |
| K900 | SFP modules always have an EEPROM | 12:56:55 |
| magic_rb | you get wan over copper? | 12:56:56 |
| magic_rb | damn | 12:56:57 |
| emily | it's fibre | 12:57:02 |
