linux-kernel - Re: earlyprintk=xdbc seems broken

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Message-ID: <3f657ffc-e137-de8f-c89e-90676bc42448@infradead.org>
Date:   Wed, 16 Feb 2022 19:44:35 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>,
        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 08:24, Dave Hansen wrote:
> 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. ;)


This one works OK:
https://www.datapro.net/products/usb-3-0-super-speed-a-a-debugging-cable.html

> 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.

-- 
~Randy