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 04-11-16 13:30, Paul Burton wrote:
> Hi Hans,
>
> On Friday, 4 November 2016 13:11:34 GMT Hans de Goede wrote:
> > Hi All,
> >
> > While booting 4.9-rc# for the first time on an Allwinner A33 tablet,
> > I noticed that after u-boot the LCD display stayed black. It turns out
> > that there was an issue which caused X to never get up, and all kernel
> > (and other startup) messages prior to that only went to ttyS0 which
> > consists of 2 tiny testpads on the PCB with this tablet.
> >
> > The same issue will also happen on any ARM boards which have a HDMI or
> > composite video output and which use a stdout-path pointing to their
> > serial console. I think this will e.g. also impact the Raspberry Pi,
> > I know for certain that this will impact the 99 different Allwinnner
> > boards currently supported by mainline u-boot + the mainline kernel.
> >
> > This is a behavior changes from previous kernels and I consider this
> > a regression. Thus I propose to revert the commit in question, for more
> > info here is a partial copy of the commit message of the proposed revert:
> >
> > The reverted commit changes existing behavior on which many ARM boards
> > rely. Many ARM small-board-computers, like e.g. the Raspberry Pi have
> > both a video output and a serial console. Depending on whether the user
> > is using the device as a more regular computer; or as a headless device
> > we need to have the console on either one or the other.
> >
> > Many users rely on the kernel behavior of the console being present on
> > both outputs, before the reverted commit the console setup with no
> > console= kernel arguments on an ARM board which sets stdout-path in dt
> > would look like this:
> >
> > [root@localhost ~]# cat /proc/consoles
> > ttyS0                -W- (EC p a)    4:64
> > tty0                 -WU (E  p  )    4:1
> >
> > Where as after the reverted commit, it looks like this:
> >
> > [root@localhost ~]# cat /proc/consoles
> > ttyS0                -W- (EC p a)    4:64
> >
> > This commit reverts commit 05fd007e4629 ("console: don't prefer first
> > registered if DT specifies stdout-path") restoring the original behavior.
> >
> > Regards,
> >
> > Hans
>
> Ugh... so the devices you're talking about rely upon set stdout-path in their
> device tree but effectively rely upon us ignoring it?

No they rely on the kernel using stdout-path as an extra console while keeping
tty0 as console, not ignoring it. This how stdout-path has always worked
(at least as long as the Allwinner boards have used it, which has been
 2 - 3 years now).

If you want only the console specified by stdout-path you can get this by
specifying it with console=... on the kernel cmdline.

> If that's the case then I guess reverting is probably the best option, but it
> does restore us to a position where we honor stdout-path for earlycon & then
> essentially ignore it for the proper kernel console. That seems pretty bust to
> me...

We do not ignore it, we use both the tty pointed to by stdout-path and tty0.

Regards,

Hans