USB-C Power Delivery Events #63
Description
The USC subsystem is used for USB-C power delivery management on the Pro X via a SurfaceUsbCMuxAcpiFilterDriver.sys. It's possible that other (newer) devices use that as well.
Quoting myself from the issue where this was first discovered (linux-surface/surface-pro-x#33):
There is indeed some SAM stuff going on in the SurfaceUsbCMuxAcpiFilterDriver.sys using the USC subsystem. There seem to be two methods, specifically. A FilterGetPortNumbers and a FilterSendUSBCAck. In addition to that, there's a PdEventInfoUpdateFromSamToSoc function, which might be some part of a PD event handler by its name.
In fact all three functions are called in the same ProcessUsbCResponse function, first getting the port number(s?), then updating the PD event info, then sending the ACK back. I think what's happening is that SAM might send a PD event when a USB-C source is plugged in. This is then handled by that process-response function.
And from the related issue for the Pro X (the device on which this was first discovered; linux-surface/surface-pro-x#35):
Turns out we can enable and receive events on Linux (using the standard SAM event registry) when I plug in my phone for charging, but I should probably get some USB-C power meter and a powerful enough USB-C charger to see if there are any changes. Also I don't know yet what to send back to SAM via the ACK command.
Interestingly, when I enable SAM events, the port seems to do something (at very least cut power for a brief moment) as my phone screen turns on. Phone says "fast charging" regardless of events being enabled/disabled though...
Events can be enabled with
./scripts/ssam/events.py enable 0x01 0x01 0x0b 0x0c 0x1b 0x00 0x01
and listened to with
./scripts/ssam/events.py listen 0x1b
It would be interesting to know whether that's also a thing on other devices.