linux-kernel - Re: Device to write to all (serial) consoles

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Date:   Sat, 3 Aug 2019 08:46:39 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Paul Menzel <pmenzel@...gen.mpg.de>
Cc:     linux-serial@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Donald Buczek <buczek@...gen.mpg.de>
Subject: Re: Device to write to all (serial) consoles

On Fri, Aug 02, 2019 at 09:59:06PM +0200, Paul Menzel wrote:
> Dear Greg,
> 
> 
> 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.
> > > 
> > > In user space, we also would like to write to both consoles to not worry
> > > about the numbering. Writing to `/dev/console` only write to the active
> > > console.
> > 
> > So the same data to multiple console devices with just one userspace
> > call?  Why?
> 
> 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.
> 
> > >      $ more /proc/consoles # I think
> > >      tty0                 -WU (EC p  )    4:1
> > >      ttyS0                -W- (E  p a)    4:65
> > >      ttyS1                -W- (E  p a)    4:65
> > > 
> > > Does a device exist, or can a device be configured so you can write to
> > > all (serial) consoles from user space?
> > 
> > With one syscall, not that I know of, sorry.
> 
> Can such a device be created with udev or so?

No, a single device node can not reference multiple device nodes at the
same time, sorry.  That's just not how they work at all.

thanks,

greg k-h