linux-kernel - Re: [PATCH] ARM: early_printk: respect keep parameter

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Date:   Tue, 17 Jul 2018 22:31:53 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:     Russell King <linux@...linux.org.uk>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: early_printk: respect keep parameter

On 17/07/2018 16:16:44-0400, Nicolas Pitre wrote:
> On Wed, 11 Jul 2018, Alexandre Belloni wrote:
> 
> > Parse and use the keep parameter of earlyprintk to avoid having the kernel
> > output stop with:
> > 
> > Console: colour dummy device 80x30
> > console [tty0] enabled
> > bootconsole [earlycon0] disabled
> > 
> > This happens because once tty0, the virtual terminal console is registered,
> > it is selected as the default console, disregarding an existing stdout-path
> > DT property and the early console is shutdown before the console driver is
> > probed.
> > 
> > Signed-off-by: Alexandre Belloni <alexandre.belloni@...tlin.com>
> 
> Why can't you have a real console driver that takes over at that point 
> which respects whatever DT property you give it?
> 

See Russell's analysis here:

https://www.spinics.net/lists/linux-serial/msg25363.html


> The early_printk code is very raw and inefficient, and it shouldn't be 
> used for anything but the debugging of early boot hangs.
> 

The issue I had is that he pinctrl driver wasn't probing so the real
console driver could never probe. I think this fits the use case.

Also, atmel_serial doesn't yet have earlycon support.

> > ---
> >  arch/arm/kernel/early_printk.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/arch/arm/kernel/early_printk.c b/arch/arm/kernel/early_printk.c
> > index 9257736ec9fa..bbb10150b11a 100644
> > --- a/arch/arm/kernel/early_printk.c
> > +++ b/arch/arm/kernel/early_printk.c
> > @@ -42,6 +42,9 @@ static struct console early_console_dev = {
> >  
> >  static int __init setup_early_printk(char *buf)
> >  {
> > +	if (buf && strstr(buf, "keep"))
> > +		early_console_dev.flags &= ~CON_BOOT;
> > +
> >  	early_console = &early_console_dev;
> >  	register_console(&early_console_dev);
> >  	return 0;
> > -- 
> > 2.18.0
> > 
> > 
> 

-- 
Alexandre Belloni, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com