linux-kernel - Re: [PATCH v3] printk: add halt_delay parameter for printk delay in halt phase

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Message-ID: <20090608081439.GB6372@elte.hu>
Date:	Mon, 8 Jun 2009 10:14:39 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Dave Young <hidave.darkstar@...il.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3] printk: add halt_delay parameter for printk delay
	in halt phase


* Dave Young <hidave.darkstar@...il.com> wrote:

> Add a halt_delay module parameter in printk.c used to read the 
> printk messages in halt/poweroff/restart phase, delay each printk 
> messages by halt_delay milliseconds. It is useful for debugging if 
> there's no other way to dump kernel messages that time.
> 
> The halt_delay max value is 65535, default value is 0, change it 
> by:
> 
> echo xxx > /sys/module/printk/parameters/halt_delay
> 
> Signed-off-by: Dave Young <hidave.darkstar@...il.com>
> ---
> Documentation/kernel-parameters.txt |    5 +++++
> kernel/printk.c                     |   17 +++++++++++++++++
> 2 files changed, 22 insertions(+)
> 
> --- linux-2.6.orig/kernel/printk.c	2009-06-08 13:55:35.000000000 +0800
> +++ linux-2.6/kernel/printk.c	2009-06-08 13:56:23.000000000 +0800
> @@ -250,6 +250,22 @@ static inline void boot_delay_msec(void)
>  }
>  #endif
> 
> +/* msecs delay after each halt/poweroff/restart phase printk,
> + unsigned short is enough for delay in milliseconds */
> +static unsigned short halt_delay;
> +
> +static inline void halt_delay_msec(void)
> +{
> +	if (unlikely(halt_delay == 0 || !(system_state == SYSTEM_HALT
> +				|| system_state == SYSTEM_POWER_OFF
> +				|| system_state == SYSTEM_RESTART)))
> +		return;

This is a tiny bit ugly (and goes into the vprintf path) but i can 
see no other way either - a system_state > SYSTEM_RUNNING check 
would needlessly include the suspend-to-disk state (which we dont 
want to include here).

In theory we could turn system_state into a bitmask and have a 
print_delay_mask check instead of these flags ... but that is a far 
wider change and i'm not sure it's a net step forwards.

I've applied your patch to tip:core/printk with small edits to the 
changelog - Linus & Andrew is Cc:ed, should they have any 
objections.

	Ingo
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