Re: [REGRESSION] "console: don't prefer first registered if DT specifies stdout-path" breaks console on video outputs of various ARM boards

From: Paul Burton
Date: Mon Nov 07 2016 - 12:17:21 EST

Hi Hans,

On Sunday, 6 November 2016 11:54:35 GMT Hans de Goede wrote:
> > What I see in my systems, and what 05fd007e4629 ("console: don't prefer
> > first registered if DT specifies stdout-path") addressed, is that if
> > there are for example 2 UARTs uart0 & uart1 that are probed in that order
> > and stdout-path indicates that we should use uart1 we wind up essentially
> > ignoring it because>
> > the ordering of the relevant calls goes:
> > - of_console_check() for uart0
> > - add_preferred_console() for uart0
> > - register_console() for uart0
> > - of_console_check() for uart1
> > - add_preferred_console() for uart1
> > - register_console() for uart1
> >
> > Since of_check_console() doesn't get called for uart1 until after uart0
> > has
> > been probed, we don't add an entry for it to the console_cmdline array
> > until after register_console() has already decided to enable uart0
> > because preferred_console == -1.
> >
> > I'm not the only one seeing this oddity either, for example see the
> > discussion on this patch:
> >
> >
> >
> > By simply reverting my patch you restore us to a position where so far as
> > I
> > can see we simply do not honor stdout-path for the real kernel console.
> As said before, we do still honor it, but in your probe example we also get
> a (second) serial console on uart0, where as you only want one on uart1.

...but don't we only support one console per type of device? That's what
Documentation/serial-console.txt says anyway, which means having a console on
both uart0 & uart1 does not work. I could live with having console output on
an extra UART, but that's not what I was seeing when I wrote this patch.

> So I see a few possible solutions here:
> 1) Do a new version of your patch which changes the "&&
> !of_specified_console" check to "&& (newcon == tty0 ||
> !of_specified_console)", then we would still always register tty0 (as long
> as it gets registered first, just like now) and we would not register uart0
> in your above example, note the "newcon == tty0" check in my example is
> pseudo-code. I would be happy to try out such a patch
> 2) Add a new dt property to enable the new behavior you seek
> I'm myself tending towards 1 as a better solution: treat tty0 special,
> because some existing setups rely on it always being registered as a
> console even if stdout-path is specified and otherwise always honor
> stdout-path.
> Regards,
> Hans

That does feel a little hack-ish to me though... I don't like the reliance on
probe ordering, nor special casing tty0 in general.

In any case I don't think I have the time to unpick all this at the moment, so
I suggest we go ahead with your revert for now & I'll revisit the system I was
working on when I find the time.


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