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Message-ID: <1416917249.32327.15.camel@debian.org>
Subject: Re: [PATCH] of: support passing console options with stdout-path
From: Ian Campbell <ijc@debian.org>
To: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
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Date: Tue, 25 Nov 2014 12:07:29 +0000
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On Tue, 2014-11-25 at 11:17 +0000, Leif Lindholm wrote:
> On Tue, Nov 25, 2014 at 10:35:04AM +0000, Mark Rutland wrote:
> > On Mon, Nov 24, 2014 at 10:23:58PM +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";
> > 
> > I would very much like to be able to use this -- it will allow
> > distributions to boot on a board without having to know _anything_ about
> > the console UART until userspace is up, which would make it possible to
> > have a completely generic installer image, without requiring the
> > platform to provide bootargs.
> > 
> > My only concern is that this conflates the set of kernel command line
> > options for the UART wit the DT binding for it. I don't know how good
> > people are at keeping those options stable, and I know they are
> > currently not documented -- we would need to add a stdout-path options
> > section to relevant UART bindings.
> 
> I don't disagree.
> 
> Current options are fairly well defined and stable, at least for any
> driver that uses uart_parse_options() (documented in
> Documentation/serial/driver).

My concern is that this is Linux specific, other OSes may have different
ideas about how stdout options should be formatted within this property.
(At least I don't know of any standardisation of the 115200n8 thing --
I'd love to be corrected!)

If I were a firmware author I'd be wary of specifying a stdout-path with
a Linux specific suffix.

Search ePAPR for baud it seems that the generic serial binding includes
a current-speed property in 6.2.1.2. It then goes on a bit ambiguously
to talk about the NS16550 in 6.2.2 but I think 6.2.1.2 was intended to
be generic. No mention of stop-bits/parity etc though, they are assumed
to be set already I think

One thought I had was to define a dt-stdout "pseudo-console" so that
console=dt-stdout,115200n8 or something could be used.

Anyway I applied your patch to v3.18-rc5 and ran it on a Mustang and it
didn't work for some reason. I'm using:

        fdt set /chosen stdout-path "/soc/serial@1c020000:115200"
        setenv bootargs "earlycon=uart8250,mmio32,0x1c020000 root=/dev/sda3 rw debug"

So I get earlycon but then no proper console. Removing earlycon just
makes that stop working.

With:

diff --git a/drivers/of/base.c b/drivers/of/base.c
index 89c6b33..5dc1718 100644
--- a/drivers/of/base.c
+++ b/drivers/of/base.c
@@ -1840,6 +1840,8 @@ void of_alias_scan(void * (*dt_alloc)(u64 size, u64 align))
 			of_stdout_options = strchr(name, ':');
 			if (of_stdout_options != NULL)
 				of_stdout_options++;
+			printk(KERN_CRIT "%s: name=%s of_stdout=%p options=%s\n",
+			       __func__, name, of_stdout, of_stdout_options);
 		}
 	}
 

I can see in dmesg:
[    0.000000] of_alias_scan: name=/soc/serial@1c020000:115200 of_stdout=          (null) options=115200

So it seems like of_find_node_by_path() is confused by the ":".

I've not tried it but I'd have expected something more like:
		if (name) {
			of_stdout_options = strchr(name, ':');
			if (of_stdout_options != NULL) {
				*of_stdout_options = '0';
				of_stdout_options++;
			}
			of_stdout = of_find_node_by_path(name);
		}

i.e. strip the options then do the patch lookup. name is const so this
won't actually work, but something like it...

Ian.

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