Re: [RFC] Serial port aliases in DT

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

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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.
> 
> 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 ?

-- 
Regards,

Laurent Pinchart

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