Re: [RFC] Serial port aliases in DT

[Wolfram Sang <wsa@the-dreams.de>]

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On Tue, Mar 11, 2014 at 12:56:50PM +0100, Laurent Pinchart wrote:
> Hi Wolfram,
> 
> On Monday 10 March 2014 13:38:37 Wolfram Sang wrote:
> > > I've recently reviewed a patch adding serial port aliases to the device
> > > tree and would like to pick your brains about a disagreement I had with
> > > the developer.
> > 
> > And here is the developer :)
> > 
> > > The SoC includes 8 serial ports. They are all disabled in the SoC .dtsi,
> > > and enabled selectively by board DT files. As not all serial ports are
> > > available on all boards, the question was whether to add aliases for all
> > > ports (in the .dtsi in this case) like
> > > 
> > >          serial0 = &scif0;
> > >          serial1 = &scif1;
> > >          serial2 = &scif2;
> > >          serial3 = &scif3;
> > >          serial4 = &scif4;
> > >          serial5 = &scif5;
> > >          serial6 = &scif6;
> > >          serial7 = &scif7;
> > > 
> > > or to just add aliases for the enabled ports (in the board DT file) like
> > > 
> > >          serial0 = &scif2;
> > >          serial1 = &scif3;
> > > 
> > > Note the numbering in the latter case: as the board doesn't use serial
> > > ports 0 and 1, hardware ports 2 and 3 become logical ports 0 and 1.
> > > 
> > > I considered that having Linux create ttySC0 and ttySC1 devices for the
> > > first two ports of the board, regardless of which hardware ports are
> > > used, is simpler from a user point of view (it allows sharing the same
> > > inittab settings for the console serial port across several boards for
> > > instance). I'd appreciate feedback on that.
> > 
> > First, I don't think this is restricted to serial ports but how to use
> > aliases in general. We may decide this or that way, yet we should do it
> > consistently. Using aliases this way for serial ports and that way for
> > I2C busses will create a mess.
> > And currently, I only know of 1:1 mappings for I2C/SPI. So, on the same
> > board, you'll need to open /dev/i2c-2, not /dev/i2c-0.
> > 
> > From my experience, things get complicated when stuff gets added and the
> > numbers go wild:
> > 
> >           serial0 = &scif2;
> >           serial1 = &scif3;
> >           serial2 = &scif6;
> >           serial3 = &scif0;
> >           serial4 = &scif7;
> > 
> > When debugging here, trying to remember which port to open for the
> > terminal, and which number to scan for in the schematics is error-prone
> > and a PITA.

No comments on those? Are the experiences from the stable ethernet
naming which would speak for/against those arguments?

> > Yeah, the drawback is that the console might be at different places
> > across boards. I suggest to update inittab at runtime anyhow, since not
> > only the number but also the naming often changes (ttyXYZ to ttyABC).
> 
> Just out of curiosity (as I could use it), do you have a sample implementation 
> of dynamic inittab updates ?

Sure. Can be optimized and improved, but does its job (for an initramfs
at least, where the default 'ttyS0' keeps constant):

=== S02inittab

#!/bin/sh
#
# Update inittab at runtime - by Wolfram Sang, WTFPLv2

case "$1" in
  start)
	echo "Adapting inittab to kernel console"
	l=$(cat /proc/cmdline)
	l=${l##*console=}
	l=${l%%,*}
	sed -i s/ttyS0/$l/g /etc/inittab
	kill -s SIGHUP 1
	;;
  *)
	echo "Usage: $0 {start}"
	exit 1
esac

exit $?

===

Regards,

   Wolfram


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