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

[Hans de Goede]

Hi,

On 07-11-16 18:16, Paul Burton wrote:
> 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:
> > >
> > > https://patchwork.kernel.org/patch/9263753/
> > >
> > > 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.

Ah yes, you're probably right about that.

> > 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.

Given that we've just got a "me too" reply from the ppc side of things,
it seems that in reality people have been relying on probe ordering here
for a long time now. IMHO tty0 is special, so it does make sense that it
always gets probed first (*) and it does make sense to handle it special.

*) Even though that seems to be more how things work (an implicit thing)
rather then explicit.

> 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.

Ok.

Regards,

Hans