Re: earlyprintk=xdbc seems broken

[Sven Schnelle]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:

> 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.

Thanks, the sysfs test works for me. I have two A-A cables, one is from
a cheap PCI-e extender, the other one from a KVM switch. Both work,
which is great. So i assume that most A-A cable would work. I'm building
a new kernel now with the patch on top, lets see.

Thanks!