linux-kernel - RE: Strange output on the console

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Date:   Fri, 25 Feb 2022 11:11:49 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Willy Tarreau' <w@....eu>
CC:     'Steven Rostedt' <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Petr Mladek <pmladek@...e.com>,
        "Sergey Senozhatsky" <senozhatsky@...omium.org>,
        John Ogness <john.ogness@...utronix.de>
Subject: RE: Strange output on the console

From: Willy Tarreau
> Sent: 25 February 2022 10:33
> 
> On Fri, Feb 25, 2022 at 10:11:43AM +0000, David Laight wrote:
> > From: Willy Tarreau
> > > Sent: 25 February 2022 06:37
> > >
> > > On Fri, Feb 25, 2022 at 06:12:35AM +0000, David Laight wrote:
> > > > From: Steven Rostedt
> > > > > Sent: 25 February 2022 04:01
> > > > >
> > > > > I've been noticing that my tests have been spitting out strange output on
> > > > > the console. It would happen at boot up and then clear up. It looks like
> > > > > something screwed up with the serial timing.
> > > > >
> > > > > Attached is a dmesg of one of my test runs as an example.
> > > > >
> > > > > I've noticed this on both 32 bit and 64 bit x86.
> > > > >
> > > > > I haven't had time to look deeper into this, but I figured I let you know
> > > > > about it.
> > > > >
> > > > > And it always seems to happen right after:
> > > > >
> > > > >   Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
> > > > >
> > > > > Maybe this is a serial issue and not a printk one? :-/
> > > >
> > > > Looks very much like the serial baud rate is being reset.
> > >
> > > I don't think it's the baud rate, characters are still readable, it
> > > looks more like a fifo being too short and causing lots of chars to
> > > be dropped.
> >
> > Just before it recovers there is this output:
> >     ATaitoscic nitahi tuPiet mfba Ae:   aD nCt AH0 nP0
> > That is probably 'fifo not enabled'.
> >
> > But the earlier output doesn't have many different characters in it.
> > Which is typical of the baud rate being wrong.
> 
> I don't think so, here's the beginning of the capture:
> 
>   Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
>   0SI 15Nto
>   LgtAsri[dnei00ieC nm
>   i:0eom:Soce
>   [ad000Ade s:ii SSLtbueludAis002h00 1)ASrPn
>   Ars004h00 7ARrPn
> 
> All of these chars are within the printable set, so there's a probability
> of ~37% per char, so even for a short string of 9 chars like the first
> line, that's a 0.01% chance of it respecting the set. That's why for me
> it definitely doesn't correspond to a baud rate issue. With wrong baud
> rates you get lots of garbage in the full 8-bit range.
> 
> In addition there are even upper case at the beginning of the lines which
> probably correspond to the ones that are printed on these lines.

Ok, that bit is probably output after the fifo is disabled.

But a few lines later you get:

TntiTet_TetkT t:TetyTvt:TntaTttTet TeteTvtnTntnTvtnT teT teTettTettKTeteTttnTnteTttaTttaTetKTttOTet:TvtnTvtltTntw Tnt:T tOTvt_Tnt:TvtKTntnT tsTvtsTttsTttsTttosTvtotTtt:TntrTntOTet T ttTvtKT tKT teTnttTettTntaTntdTttaT taTeteT tTtt TttrTntlTvt:TetkT tTtttTnteTttOTetkTvtsTeteT tlTttoTntoTetpTvtpTetsTvtvTvteTntOTet
TttTetsTetsTntsTetKT t TttiTttT tnT tOTnt_TttrTtttTntKTetOTtteTnt:T tKTttpTvdoT dxTvdnTtd TedtTedoTvdiTedaTedT d Ttd:TedtTtd TeddTvdKTediTndgTednTtdgTtd TedKT snTesTtsOTesKTnstTvsOTvsTnsKTvs:Tts:TesTvs:Tns Tes T sdTts TtsoTesOTtslTesnTesKTesOTns
TnsTesOT s T sKTesOT sKTvsOTtsTtsKTtsT sKTesTesKT sKTesOT siT stTnsaTvsaTesKTnsOT s
TesKTnsT s TvsKTeslT sOTesT sKTtseTnseTvs_Tts_T sOTes TnsOT s T spTespTesdTesdTeseTtseTeseTt_rTe_rTv_oTes:T s:Tes:TnfwOTvfrKT fwT frTefpuTefpKTefpTvf:TnsOTvsOTnsoTvs Tvs TvsTtssTnsT sKTesTnsTvsTtsoTesmTnsgTesiTeslTtsTvsKTnsTvsOTtsT s_Tnsu

which really doesn't have enough different characters in it to be a fifo problem.
That looks like a UART struggling to find valid start and stop bits
on a continuous data stream that doesn't match the baud rate.

It may also be that whatever 'terminal' is being used is masking off the 0x80 bit.

Another possibility is that the count of characters is about right.
But the receiver is misaligned on the 10 bit async characters.
Because these is no line idle, it never synchronises properly.
ISTR a real async terminal behaving that way.
But I think that gives a much wider range of characters - just the wrong ones.

There is also a third error pattern:
^[[A^[[A^[[A^[[A^[[A^[[A^[[A^[[A^[[A^[[A^[[A^[[A^[[A^[[A^[[A^[[
which might be an 'even more wrong' baud rate.

So maybe we are both right - for different bits of the error.

	David

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