Re: [PATCH v2 2/2] of: support passing console options with stdout-path

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Nov 26, 2014 at 09:48:47PM +0000, Andrew Lunn wrote:
> On Wed, Nov 26, 2014 at 09:07:33PM +0000, Grant Likely wrote:
> > On Wed, Nov 26, 2014 at 6:30 PM, Andrew Lunn <andrew@lunn.ch> wrote:
> > > On Wed, Nov 26, 2014 at 05:40:40PM +0000, Leif Lindholm wrote:
> > >> Support specifying console options (like with console=ttyXN,<options>)
> > >> by appending them to the stdout-path property after a separating ':'.
> > >>
> > >> Example:
> > >>         stdout-path = "uart0:115200";
> > >
> > > Hi Leif
> > >
> > > This should be documented somewhere under
> > > Documentation/devicetree/bindings/
> > >
> > > Not sure where thought. Maybe a top level chosen.txt?
> > 
> > Actually, this one doesn't. It is already documented in ePAPR
> 
> Hi Grant
> 
> Humm, do i have an old version of ePAPR?
> 
> All i see is that in Table 3-4 It says:
> 
> stdout-path O <string> A string that specifies the full path to the
> 	      	       node representing the device to be used for
> 	      	       boot console output. If the character ":" is
> 	      	       present in the value it terminates the
> 	      	       path. The value may be an alias.
> 
> 		       If the stdin-path property is not specified,
> 		       stdout-path should be assumed to define the input device.
> 
> So what is before the : is defined. What comes afterwards,
> baudrate/parity/bits/flow control does not appear to the defined in
> ePAPR. Should we not document the extension being added here?

I believe that we should, and it should be relatively trivial to add a
document stating that the format and meaning of the parts after the ':'
are device-specific. 

So how about Documentation/devicetree/bindings/serial/stdout-path.txt,
with something like the following:

---->8----
Device trees may specify the device to be used for boot console output
with a stdout-path property under /chosen, as described in ePAPR, e.g.

/ {
	chosen {
		stdout-path = "/serial@f00:115200";
	};

	serial@f00 {
		compatible = "vendor,some-uart";
		reg = <0xf00 0x10>;
	};
};

If the character ":" is present in the value, this terminates the path.
The meaning of any cahracters following the ":" is device-specific, and
must be specified in the relevant binding documentation.
---->8----

The more difficult part is documenting those (and I'm still uneasy about
conflating the Linux driver command line options with the DT binding for
that reason).

Thanks,
Mark.