On Friday, 4 November 2016 14:22:17 GMT Hans de Goede wrote:

Hi,

On 04-11-16 13:30, Paul Burton wrote:

quoted

Hi Hans,

On Friday, 4 November 2016 13:11:34 GMT Hans de Goede wrote:

quoted

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.

quoted

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

Hi Hans,

Could you walk me though how you're getting that behaviour from the current 
code? I don't see how that would happen besides perhaps if drivers are probed 
in a fortunate order. Is that what you're relying upon?

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.

Thanks,
    Paul