linux-kernel - Re: [PATCH] clockevents: Add (missing) default case for switch blocks

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Date:	Fri, 20 Feb 2015 09:48:07 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linaro-kernel@...ts.linaro.org,
	Kevin Hilman <khilman@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	linaro-networking@...aro.org, Steven Miao <realmz6@...il.com>,
	Mark Salter <msalter@...hat.com>,
	Michal Simek <monstr@...str.eu>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Ley Foon Tan <lftan@...era.com>,
	Jonas Bonn <jonas@...thpole.se>,
	"David S. Miller" <davem@...emloft.net>,
	Jeff Dike <jdike@...toit.com>,
	Guan Xuetao <gxt@...c.pku.edu.cn>
Subject: Re: [PATCH] clockevents: Add (missing) default case for switch blocks

On Fri, Feb 20, 2015 at 09:38:42AM +0100, Ingo Molnar wrote:
> 
> * Viresh Kumar <viresh.kumar@...aro.org> wrote:
> 
> > Many clockevent drivers are using a switch block for handling modes in their
> > ->set_mode() callback. Some of these do not have a 'default' case and adding a
> > new mode in the 'enum clock_event_mode', starts giving following warnings for
> > these platforms about unhandled modes (e.g. XXX).
> > 
> > 	warning: enumeration value ‘XXX’ not handled in switch [-Wswitch]
> > 
> > This patch adds default cases for them.
> > 
> > In order to keep things simple, add following to the switch blocks:
> > 
> > 	default:
> > 		break;
> > 
> > This can lead to different behavior for individual cases.
> > 
> > 1) Some of the drivers don't have any special stuff in their ->set_mode()
> >    callback before or after the switch blocks. And so this default case would
> >    simply return for them without any updates to the clockevent device.
> > 
> > 2) But in some cases, the clockevent device is disabled/stopped as soon as we
> >    enter the ->set_mode() callback and is enabled within the switch block or
> >    after it. And the clockevent device *may* stay disabled for default case.
> 
> So this whole approach looks fragile for several reasons:
> 
>    - 'mode setting' callbacks are just bad by design
>      because they mix several functions into the same entry
>      point, complicating the handler functions 
>      unnecessarily. We should reduce complexity, not expand 
>      on it.
> 
>    - now by adding 'default' you hide from drivers the
>      ability to easily discover whether it has been updated
>      to some new core clockevents mode setting feature or
>      not.

So this patch was a follow on from bd624d75db21 ("clockevents: Introduce
mode specific callbacks").

That patch changes the set_mode() interface; and provides per mode
functions.

New (and updated) drivers should not use ->set_mode() anymore, but it
was felt that we do not want to go do flag day changes.

And it allows for adding optional modes; not every driver needs to go
implement all mode functions if there is a sane default action.

But it does mean we need to be able to add values to the enum.

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