console vs earlycon ?
arnd at arndb.de
Wed Oct 21 03:36:45 PDT 2015
On Wednesday 21 October 2015 18:30:05 Masahiro Yamada wrote:
> >> I am trying to implement OF_EARLYCON_DECLARE() for that.
> >> I was just wondering if console_initcall() should work as well.
> >> As I said, I noticed the console_initcall() in 8250_core.c
> >> only works on very limited platforms.
> > It works with all those that use of_serial.c, right?
> I doubt it.
> I have a board with a pure 8250-compat device working.
> ( compatible = "ns16550a")
> It uses of_serial.c, of course.
> As far as I tested, it would never enable the console at console_init().
Ok, that sounds like a bug. Peter and others have changed that code
a lot in the last year. I wonder if this has never worked then or
if it has regressed.
> The console is enabled when the UART driver is probed.
> (or earlycon if "earlycon" is specified in the kernel-parameter.)
Got it. Let me know if you think this interpretation is correct:
* univ8250_console_match() looks for the earlycon argument format like
"console=uart,io|mmio|mmio32,<addr>[,<options>]", and that
would work, but we don't want to have to pass all that data when
the information should already be there.
* univ8250_console_setup() looks for ports that have been added by
one of several other methods (including earlycon) already, but
we don't do that on ARM without earlycon being enabled.
* arch/powerpc/kernel/legacy_serial.c does everything we need, but
does not live in architecture independent code and does a few
things that we probably don't need or want there. It relies
on scanning the device tree for known UART device nodes before
the platform devices are added.
* for console_initcall() to do the right thing, we want both the
ttyS devices to get added early for console=ttyS1 to work, as well
as having the preferred console work based on the stdout-property.
* we parse the /chosen/stdout-path property in drivers/of/base and
store the device node pointer in the global 'of_stdout' variable,
but do not use it until the uart is added by the tty driver
and calls of_console_check() to add the default console device.
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