linux-kernel - Re: [RFC][PATCH] x86: Allow early_printk to use console style param like 115200n8

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Message-ID: <20150704110359.GA18107@gmail.com>
Date:	Sat, 4 Jul 2015 13:03:59 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	David Cohen <david.a.cohen@...ux.intel.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Alan Cox <alan@...ux.intel.com>,
	"Stuart R. Anderson" <stuart.r.anderson@...el.com>
Subject: Re: [RFC][PATCH] x86: Allow early_printk to use console style param
 like 115200n8


* Steven Rostedt <rostedt@...dmis.org> wrote:

> When I enable early_printk on a kernel, I cut and paste the console=
> input and add to earlyprintk parameter. But I notice recently that
> ktest has not been detecting triple faults. The way it detects it, is
> by seeing the kernel banner "Linux version .." with a different kernel
> version pop up. Then I noticed that early printk was no longer working
> on my console, which was why ktest was not seeing it.
> 
> I bisected it down and it was added to 4.0 with this commit:
> 
> commit ea9e9d802902 ("Specify PCI based UART for earlyprintk")

Ugh, this commit changed x86 code but was not Cc:-ed to any x86 maintainer, and 
the title was pretty misleading as well...

( That patch should have been split into at least two parts: the generic 
  earlyprintk changes affecting all modes, and the 'pciserial' enablement. Anyway, 
  that's water down the bridge. )

> because it converted the simple_strtoul() that converts the baud number into a 
> kstrtoul(). The problem with this is, I had as my baud rate, 115200n8 
> (acceptable for console=ttyS0), but because of the "n8", the kstrtoul() doesn't 
> parse the baud rate and returns an error, which sets the baud rate to the 
> default 9600. This explains the garbage on my screen.

ugh. I bet it also breaks the earlyprintk=ttyS0..,keep format?

> Now, earlyprintk= kernel parameter does not say it accepts that format. Thus, 
> one answer would simply be me changing my kernel parameters to remove the "n8" 
> since it isn't parsed anyway. But I wonder if other people run into this, and it 
> seems strange that the two consoles for serial accepts different input.

So why not revert to the known-working simple_strtoul()? I don't see this as an 
improvement:

> +		/*
> +		 * In case the input is like console with text after the baud
> +		 * rate. e.g. 115200n8. kstrtoul() will error on such input.
> +		 */
> +		for (p = s; *p && isdigit(*p); p++)
> +			;
> +		*p = 0;
> +
>  		if (kstrtoul(s, 0, &baud) < 0 || baud == 0)
>  			baud = DEFAULT_BAUD;


Over the old:

		baud = simple_strtoul(s, &e, 0);

Thanks,

	Ingo
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