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 12:30:35 +0000
From:   Paul Burton <paul.burton@...tec.com>
To:     Hans de Goede <hdegoede@...hat.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 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?

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...

Thanks,
    Paul
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