linux-kernel - Re: [PATCH 1/1 v4] pwm_bl: Add support for backlight enable regulator

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Message-ID: <20130308072300.GB5772@avionic-0098.mockup.avionic-design.de>
Date:	Fri, 8 Mar 2013 08:23:00 +0100
From:	Thierry Reding <thierry.reding@...onic-design.de>
To:	Alex Courbot <acourbot@...dia.com>
Cc:	Andrew Chew <AChew@...dia.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1 v4] pwm_bl: Add support for backlight enable regulator

On Fri, Mar 08, 2013 at 11:21:04AM +0900, Alex Courbot wrote:
> On 03/08/2013 06:07 AM, Andrew Chew wrote:
> >>From: Thierry Reding [mailto:thierry.reding@...onic-design.de]
> >>Sent: Thursday, March 07, 2013 3:27 AM
> >>To: Alex Courbot
> >>Cc: Andrew Chew; linux-kernel@...r.kernel.org
> >>Subject: Re: [PATCH 1/1 v4] pwm_bl: Add support for backlight enable
> >>regulator
> >>
> >>* PGP Signed by an unknown key
> >>
> >>On Thu, Mar 07, 2013 at 07:11:25PM +0900, Alex Courbot wrote:
> >>>On 03/07/2013 04:11 PM, Thierry Reding wrote:
> >>>>>+	bool			en_supply_enabled;
> >>>>
> >>>>This boolean can be dropped. As discussed in a previous thread, the
> >>>>pwm-backlight driver shouldn't need to know about any other uses of
> >>>>the regulator.
> >>>
> >>>Sorry for being obstinate - but I'm still not convinced we can get rid
> >>>of it. I checked the regulator code, and as you mentioned in the
> >>>previous version, calls to regulator_enable() and
> >>>regulator_disable() *must* be balanced in this driver.
> >>>
> >>>Without this variable we would call regulator_enable() every time
> >>>pwm_backlight_enable() is called (and same thing when disabling).
> >>>Now imagine the driver is asked to set the following intensities: 5,
> >>>12, then 0. You would have two calls to regulator_enable() but only
> >>>one to regulator_disable(), which would result in the enable GPIO
> >>>remaining active even though it would be shut down. Or I missed
> >>>something obvious.
> >>>
> >>>The regulator must be enabled/disabled on transitions from/to 0, and
> >>>AFAICT there is no way for this driver to detect them.
> >>
> >>Yes, that's true, but I don't think it should be solved for just this one
> >>regulator. Instead if we need to track the enable state we might as well track
> >>it for *any* resource so that the PWM isn't enabled/disabled twice either.
> >
> >That makes sense, but I'm confused due to previous comments.  The most
> >obvious way to do this seems to be to have a bool track the enable state.
> >Do you still want me to do away with this bool?  I can satisfy your very
> >last comment by keeping the bool (renaming it to something more generic)
> >and encapsulating the pwm_enable()/pwm_disable() call within.
> 
> I think that's what Thierry meant, yes.

Yes, it is. =)

> >>I expect that if the changes are split up then the board-setup code changes
> >>need to be done prior to the driver change. Using the lookup tables should
> >>make this easy because they aren't tied to the platform data and can be
> >>added independently. The patches should probably go through the same
> >>subsystem tree to take care of the dependencies.
> >>
> >>Keeping everything in one patch would work too, but it's certainly more
> >>chaotic.
> >
> >Am I supposed to handle those patches?  I'm concerned that I don't have
> >hardware to test properly, but I can give it a shot if it's my responsibility.
> 
> Yes, if you introduce incompatibilities you have the burden of
> performing the transition without breaking things at any single
> point of the git history. Since this is just about adding a dummy
> regulator, it should go fine even without testing. And in the event
> it does not, that's what linux-next is for.

Right. We'll need an Acked-by from the board/machine maintainers anyway
and if something still breaks we can always fix it after somebody's
actually done the testing.

> Make sure you also update the dts of current device tree users, as
> they will break, too.
> 
> What I don't know is if you should update all users in one big
> patch, or instead provide one patch per platform changed. Maybe
> Thierry can provide some guidance here.

I think it'd be good to split them up into per-architecture and
per-machine. Per-board would probably be too much. That'll allow the
respective maintainers to ack patches that touch their machines or
boards without having them go through all other hunks too.

Thierry

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