linux-kernel - Re: earlyprintk=xdbc seems broken

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Date:   Tue, 25 Jan 2022 08:24:53 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sven Schnelle <svens@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Mathias Nyman <mathias.nyman@...ux.intel.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        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

On 1/25/22 05:09, Sven Schnelle wrote:
>> 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?

My understanding is that the 5v line is disconnected.  Here's one cable 
I have:

> https://designintools.intel.com/SVT_DCI_DbC2_3_A_to_A_Debug_Cable_1_Meter_p/itpdciamam1m.htm

I think they snip that line to prevent any chance of current flowing 
between devices.  The cable isn't expensive, but I do remember being 
astounded at how much Intel charged itself for shipping it. ;)

That said, I've also used the same cable Peter has: just a cheap A-to-A 
cable I had lying around.  It surely has VBUS connected, so I usually 
plumb it through a USB hub with a power switch.  This surely isn't the 
"right" way to do it, but it does seem to work in some situations.