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

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Date:	Thu, 27 Nov 2014 13:16:36 +0000
From:	Leif Lindholm <leif.lindholm@...aro.org>
To:	Mark Rutland <mark.rutland@....com>
Cc:	Andrew Lunn <andrew@...n.ch>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	Ian Campbell <ijc@...ian.org>,
	"s.hauer@...gutronix.de" <s.hauer@...gutronix.de>
Subject: Re: [PATCH v2 2/2] of: support passing console options with
 stdout-path

On Thu, Nov 27, 2014 at 12:15:43PM +0000, Mark Rutland wrote:
> 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@...n.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. 

Device-specific is a bit broad though?

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

There is, however, nothing serial-specific about this functionality -
it console-specific.

Could it be bindings/console/stdout-path.txt?
 
> ---->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).

For the current situation, which _is_ serial-specific, could we then
add a serial/stdout-path.txt describing the mapping to
uart_parse_options - making that interface an implicit requirement for
the use of stdout-path?

/
    Leif
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