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Date:   Sat, 3 Aug 2019 15:55:37 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Adam Borowski <kilobyte@angband.pl>
Cc:     Paul Menzel <pmenzel@molgen.mpg.de>, linux-serial@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Donald Buczek <buczek@molgen.mpg.de>
Subject: Re: Device to write to all (serial) consoles
Message-ID: <20190803135537.GA1743@kroah.com>
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 <cab1fe06-0dc7-e7c1-50ac-cc01773c5ef5@molgen.mpg.de>
 <20190803132323.GB6703@angband.pl>
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On Sat, Aug 03, 2019 at 03:23:23PM +0200, Adam Borowski wrote:
> On Fri, Aug 02, 2019 at 09:59:06PM +0200, Paul Menzel wrote:
> > On 02.08.19 18:02, Greg Kroah-Hartman wrote:
> > > On Fri, Aug 02, 2019 at 03:23:08PM +0200, Paul Menzel wrote:
> > > > On a lot of devices, like servers, you have more than one serial console,
> > > > and you do not always know, how they are numbered. Therefore, we start a
> > > > console on ttyS0 and ttyS1.
> > 
> > Because the cable is always connected to the port on the back side, and
> > sometimes the port in the front has ID 0, and the one in the back 1, and
> > other times vice versa. We do not want to track that, and it would be
> > convenient to just write to both ports.
> 
> Sounds like an XY problem then: what you want is not writing to all ports,
> but to have the port assignments stable (see also: disk device reordering).

You can get that information from the symlinks in /dev/serial/ which
udev creates.