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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
References: <Yao35lElOkwtBYEb@kroah.com>
        <c2b5c9bb-1b75-bf56-3754-b5b18812d65e@linux.intel.com>
        <YbyWuxoBSicFBGuv@hirez.programming.kicks-ass.net>
        <YcGhIm7yqYPk4Nuu@hirez.programming.kicks-ass.net>
        <YeE4rtq6t73OxOi+@hirez.programming.kicks-ass.net>
        <cd534ff9-e500-c7ea-426a-347ac2b0830b@linux.intel.com>
        <YeLxE3zQ7Vexk3gv@hirez.programming.kicks-ass.net>
        <dfb311e3-1a83-31a2-3c82-fd982c0757f6@linux.intel.com>
        <Ye/w/lOf4f8+8fDt@hirez.programming.kicks-ass.net>
        <yt9dbl00rmgx.fsf@linux.ibm.com>
        <YfAA+age1zo3lxIB@hirez.programming.kicks-ass.net>
Date:   Tue, 25 Jan 2022 15:01:36 +0100
In-Reply-To: <YfAA+age1zo3lxIB@hirez.programming.kicks-ass.net> (Peter
        Zijlstra's message of "Tue, 25 Jan 2022 14:54:01 +0100")
Message-ID: <yt9dh79rnccf.fsf@linux.ibm.com>
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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!