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Date:   Tue, 25 Jan 2022 14:54:01 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Sven Schnelle <svens@linux.ibm.com>
Cc:     Mathias Nyman <mathias.nyman@linux.intel.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Lu Baolu <baolu.lu@linux.intel.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, linux-usb@vger.kernel.org,
        Chunfeng Yun <chunfeng.yun@mediatek.com>
Subject: Re: earlyprintk=xdbc seems broken
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On Tue, Jan 25, 2022 at 02:09:18PM +0100, Sven Schnelle wrote:
> Peter Zijlstra <peterz@infradead.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.