Re: earlyprintk=xdbc seems broken

[Peter Zijlstra]

On Tue, Jan 25, 2022 at 02:09:18PM +0100, Sven Schnelle wrote:
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
> 
> > Now the documentation states we need this super speed A<->A cable, but
> > could you also update the documentation for usb-c ? There's a fair
> > number of usb-c only devices out there now.
> 
> Stupid beginners question: Would every USB3 A-A cable work, or are the
> debug cables special? I've read the RX/TX pairs have to be swapped, but
> to me it looks like that's always the case?

I'm using a random USB3 A-A cable (it came with an HDMI frame grabber of
of AliExpress). The official one has a wire missing I think, to avoid
ground loops or something like that.

The easiest thing to do is try the runtime DBC option:

  echo enable > /sys/bus/pci/devices/0000:00:14.0/dbc

If that works you can see what it takes to make the earlyprintk one
working. This thread seems to have much of that covered.