linux-kernel - Re: [Patch -v4 4/4] Make reboot_cpuid a kernel parameter. other cpus.

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Message-ID: <20130417095501.GD3658@sgi.com>
Date:	Wed, 17 Apr 2013 04:55:01 -0500
From:	Robin Holt <holt@....com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Robin Holt <holt@....com>, Ingo Molnar <mingo@...hat.com>,
	Russ Anderson <rja@....com>, Shawn Guo <shawn.guo@...aro.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Joe Perches <joe@...ches.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Michel Lespinasse <walken@...gle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	Tejun Heo <tj@...nel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Patch -v4 4/4] Make reboot_cpuid a kernel parameter. other
 cpus.

On Tue, Apr 16, 2013 at 07:44:12AM -0500, Robin Holt wrote:
> On Tue, Apr 16, 2013 at 05:05:45AM -0700, H. Peter Anvin wrote:
> > Why not just support the existing syntax everywhere?
> 
> I have not given it much consideration, but IIRC, the other arches that
> were using reboot= were only looking for an 'h' or something like that.
> 
> I will consider making the syntax parse reboot=s#### when I get to
> the office.

Yesterday was rather disruptive and I did not get to this.  I have given
it some thought this morning.

Generally speaking, I don't like the feel of this for two reasons.  First,
having two different places that are parsing reboot=<reboot_mode> and
its related difficulty in documenting it.  Second, we lose the /sys/ file.

Some background.  First, arm.  It already has a reboot=<c>.  That <c>
gets passed to the reboot sub-arch function.  It looks like it is ignored,
but I am very uncertain of that as it is passed into an assembly routine.

Second unicore.  It parses as reboot=<c>.  That <c> gets ignored.

Now the meat of my question.  The Documentation/kernel-parameters.txt
file indicates reboot= is handled by the arch and are of the format
<reboot-mode>[,<reboot-mode>[,...]].

I suppose we could have both an arch and generic kernel handler for
__setup("reboot=",...)  where the generic kernel one just handles the
s###, but that seems really different from how everything else is done.
I could not find one instance where a both an arch and the kernel proper
both parsed the same command-line parameter.

I did not spend the time to see if having two __setup() declarations
would work.

Additionally, the __setup("... mechanism loses the nice feature the
core_param gives us in that there is a /sys/ file now available which
allows us to easily change that setting on the running system.

Are you really sure you want me to do more than I already have.  This
really feels _VERY_ wrong to me.  Maybe I misunderstood your direction.

Thanks,
Robin
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