3 ms·

Since I can't edit my sibling post: I wanted to address your point about the "correct" answer, because it's actually not so simple: The billboard spec allows

Since I can't edit my sibling post: I wanted to address your point about the "correct" answer, because it's actually not so simple: The billboard spec allows for a device to be both a DisplayPort 4-lane and a USB 2.0 device (This isn't theoretical--I have such a device in front of me. It's a passive DisplayPort + 2 port USB2.0 + microSD "hub"). The HDMI adapter I hypothesized in my second scenario is a passive HDMI adapter. You're correct that passive HDMI + USB2.0 is not possible, but substitute a passive DisplayPort for passive HDMI here and the idea is the same. In the third scenario I am hypothesizing an active 2-lane DisplayPort to HDMI adapter (These are also not theoretical--this is how the Apple "HDMI Multiport Adapter" works). The hub I hypothesized for scenarios 2 through 4 (which does not exist) is one that would be smart enough to do the following: 1. Recognize the video dongle's billboard 2. Send the host a "fake" billboard for "DisplayPort 2 lane + USB 3.0" or "DisplayPort 4 lane + USB 2.0" (as appropriate) 3. Re-configure the downstream USB-C as USB 3.0 or 2.0-only ports (again as appropriate) 4. Route DP to the dongle (perhaps via an internal DP -> HDMI converter, if the hub has one), Route USB 2.0 or 3.0 to the internal hub, and route the hub to the downstream USB-C ports. I don't recall if rewriting the billboard is allowed per the USB spec (I think it isn't). But you can see how it would be possible for a hub to support alt-mode routing, even if the spec doesn't formally define it.

cesarb 7y ago
You seem to be confusing the billboard device with the alternate mode negotiation. The billboard device is a normal USB device which shows up once the alternate mode negotiation times out. The alternate mode negotiation happens on the configuration pins over the USB-PD protocol. The alternate mode discovery is initiated by the host, so the only way the hub could change which alternate modes it supports would be to disconnect from the host (unplugging all the devices on it, which would be particularly bad if some of them were storage devices). (Also, I believe HDMI alternate mode with USB 2.0 is possible. The only alternate mode I know of which changes the meaning of the USB 2.0 pins is VirtualLink, which uses them for USB 3.x instead.)

cesarb 7y ago

You seem to be confusing the billboard device with the alternate mode negotiation. The billboard device is a normal USB device which shows up once the alternate mode negotiation times out. The alternate mode negotiation happens on the configuration pins over the USB-PD protocol. The alternate mode discovery is initiated by the host, so the only way the hub could change which alternate modes it supports would be to disconnect from the host (unplugging all the devices on it, which would be particularly bad if some of them were storage devices). (Also, I believe HDMI alternate mode with USB 2.0 is possible. The only alternate mode I know of which changes the meaning of the USB 2.0 pins is VirtualLink, which uses them for USB 3.x instead.)