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

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Date:   Fri, 4 Nov 2016 14:22:17 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     Paul Burton <paul.burton@...tec.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Thorsten Leemhuis <regressions@...mhuis.info>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [REGRESSION] "console: don't prefer first registered if DT
 specifies stdout-path" breaks console on video outputs of various ARM boards

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@...alhost ~]# 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@...alhost ~]# 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