It seems that Mac notebooks and Chromebooks support DisplayPort alt mode over USB Type C.
Beyond that, things get very muddled.
There are five possible video alt modes over USB Type C:
- DisplayPort
- HDMI
- MHL
- Thunderbolt
- VirtualLink
It seems to me that whether a particular device supports one of these requires first that the processor supports USB 3.x and further that it supports the specific video mode.
This information seems very hard to find out, even for a single manufacturer.
The Qualcomm Snapdragon 730G, 845 and 855 appear to support DisplayPort. Any others?
Usually switching to video alt modes requires PD communication over a CC line.
There is also the fallback of the Billboard USB class over the USB 2.0 data lines.
How universally is the Billboard USB class supported? Is it guaranteed if a video alt mode is supported?
Also, there are no class-specific requests. Why didn't they throw us a sop and have current video alt mode reported?
I will get some active adapters to try out some stuff, but in the interim I wanted to throw together a Billboard device.
I wouldn't do anything with the video data coming down on the SS pairs. I merely want to see if I can get to see some reaction in the dmesg.
Has anybody home-brewed PD control messages?
Any thoughts?
