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

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Date:	Thu, 27 Nov 2014 13:45:00 +0000
From:	Grant Likely <grant.likely@...aro.org>
To:	Leif Lindholm <leif.lindholm@...aro.org>
Cc:	Mark Rutland <mark.rutland@....com>, Andrew Lunn <andrew@...n.ch>,
	"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 1:16 PM, Leif Lindholm <leif.lindholm@...aro.org> wrote:
> 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?

Not really, but the expectation should be quite strong that for UARTs,
the first argument after the ':' is the b/p/b/f string. We could also
specify that ',' is used as a separator when multiple arguments are to
be passed to a device.

>
>> 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?

Put it in Documentation/devicetree/bindings/chosen.txt

We may as well have one file for all the common stuff that goes into chosen.

g.
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