linux-kernel - Re: custom ioctl-based interface to control LED in networking (was Re: [PATCHv2 09/10] rfkill: Userspace control for airplane mode)

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Date:	Thu, 25 Feb 2016 10:06:36 +0100
From:	Pavel Machek <pavel@....cz>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	rpurdie@...ys.net, j.anaszewski@...sung.com,
	linux-leds@...r.kernel.org,
	João Paulo Rechi Vita <jprvita@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Darren Hart <dvhart@...radead.org>,
	linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
	platform-driver-x86@...r.kernel.org, linux-api@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux@...lessm.com,
	João Paulo Rechi Vita 
	<jprvita@...lessm.com>
Subject: Re: custom ioctl-based interface to control LED in networking (was
 Re: [PATCHv2 09/10] rfkill: Userspace control for airplane mode)

On Wed 2016-02-24 14:31:33, Johannes Berg wrote:
> On Wed, 2016-02-24 at 13:14 +0100, Pavel Machek wrote:
> > 
> > Why would it need to? It could look at default triggers for the led
> > if it really wanted to.
> 
> And then it needs to change them; if anything goes wrong error recovery
> is practically impossible since the trigger information is lost
> forever.

Well, conventional way to solve this is to simply name the led
"acer::airplane"... that way it is persistent. We already do that for
display/keyboard backlights.

> It's not my position to decide which combinations some system
> integrator or userspace developer might find useful.
> 
> Even when we add parameters to a trigger (I don't see a generic way to
> do that, but please do enlighten me), you're now ignoring the issue of
> the userspace controlled GSM modem...

Take a look at the blinking triggers.

> > > Really what you have here is a concept of "airplane mode", and that
> > > concept is specific to the rfkill subsystem. This happens to affect
> > > mostly an LED trigger, today, but as a concept it's something that
> > > *should* be managed within the rfkill subsystem.
> > 
> > How is that concept used outside the LEDs? What semantics does
> > "airplane mode" have? You tried to explain "airplane mode" is not
> > well defined up in this thread...
> 
> I'd say it's well-defined for any given set of system software, so if
> e.g. NetworkManager decides to define it one way, and connman another
> way, there's a definition but the kernel need not interfere with it.

So... the LED changes meaning during boot? That's surely not a nice
solution.

So... you rather store bit in a kernel with unclear semantics,
explaining "oh I need to leave the flexibility to userland"? Sorry,
that's just ugly.

> > I'm not saying that. I'm saying that LED maintainers should be Cced,
> > to keep the interfaces consistent.
> 
> I pretty much have to read it that way, since the LED API is in no way
> impacted by these changes. Here's a new trigger, with some magic inner
> working. No impact on the LED API.

New LED trigger and new ioctl for LED control... not matching how LEDs
are normally controlled.

Best regards,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html