Re: earlyprintk=xdbc seems broken

From: Sven Schnelle <svens@linux.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
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
Date: Tue, 25 Jan 2022 15:01:36 +0100	[thread overview]
Message-ID: <yt9dh79rnccf.fsf@linux.ibm.com> (raw)
In-Reply-To: <YfAA+age1zo3lxIB@hirez.programming.kicks-ass.net> (Peter Zijlstra's message of "Tue, 25 Jan 2022 14:54:01 +0100")

Peter Zijlstra <peterz@infradead.org> writes:

> 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.

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!