Re: earlyprintk=xdbc seems broken

From: Dave Hansen <dave.hansen@intel.com>
To: Sven Schnelle <svens@linux.ibm.com>,
	Peter Zijlstra <peterz@infradead.org>
Cc: Mathias Nyman <mathias.nyman@linux.intel.com>,
	Greg KH <gregkh@linuxfoundation.org>,
	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
Date: Tue, 25 Jan 2022 08:24:53 -0800	[thread overview]
Message-ID: <a34be5ca-47e7-95a6-602f-da054e409e85@intel.com> (raw)
In-Reply-To: <yt9dbl00rmgx.fsf@linux.ibm.com>

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.