linux-kernel - Re: earlyprintk=xdbc seems broken

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Date:   Tue, 25 Jan 2022 15:01:36 +0100
From:   Sven Schnelle <svens@...ux.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mathias Nyman <mathias.nyman@...ux.intel.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
        Chunfeng Yun <chunfeng.yun@...iatek.com>
Subject: Re: earlyprintk=xdbc seems broken

Peter Zijlstra <peterz@...radead.org> writes:

> On Tue, Jan 25, 2022 at 02:09:18PM +0100, Sven Schnelle wrote:
>> Peter Zijlstra <peterz@...radead.org> 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!