linux-kernel - Re: [PATCH 1/1 v3] pwm_bl: Add support for backlight enable regulator

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Message-ID: <20130306101159.GA3648@avionic-0098.mockup.avionic-design.de>
Date:	Wed, 6 Mar 2013 11:11:59 +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 v3] pwm_bl: Add support for backlight enable regulator

On Wed, Mar 06, 2013 at 05:37:42PM +0900, Alex Courbot wrote:
> On 03/06/2013 04:00 PM, Thierry Reding wrote:
> >* PGP Signed by an unknown key
> >
> >On Wed, Mar 06, 2013 at 01:53:27PM +0900, Alex Courbot wrote:
> >>On 03/06/2013 11:41 AM, Andrew Chew wrote:
> >>>>>   struct pwm_bl_data {
> >>>>>   	struct pwm_device	*pwm;
> >>>>>   	struct device		*dev;
> >>>>>+	struct regulator	*en_supply;
> >>>>>+	bool			en_supply_enabled;
> >>>>
> >>>>Couldn't you use regulator_is_enabled() and get rid of en_supply_enabled?
> >>>>It would also ensure the driver performs correctly no matter what the initial
> >>>>state of the regulator is.
> >>>
> >>>Are you sure this works?  I'm concerned about the (bizarre and unlikely) case
> >>>where this supply is shared with another driver, so I use en_supply_enabled
> >>>to track the state of the supply such that I can ignore that case.
> >>
> >>You're right, consumers can share regulators and the calls to
> >>enable/disable need to be balanced. Also there is no way to check
> >>the intensity of the backlight prior to the change to detect a
> >>transition, so I guess your approach is indeed the most appropriate
> >>here.
> >
> >I think the right thing to do here is just enable the regulator when
> >the pwm-backlight driver needs it. If it is shared with other devices
> >they'll have to do the same and the reference counting should only
> >disable the regulator when there are no users.
> >
> >Tracking this via platform data won't work because platform data is
> >statically defined at compile time. So if indeed there was another user
> >of the regulator it enable/disable the regulator at any time and your
> >en_supply_enabled would be wrong.
> 
> Oh wait. I thought regulator_enable/disable calls needed to be
> balanced, is that not the case? So every consumer receives a
> different regulator handle in case of a shared regulator, which
> becomes disabled if all handles are disabled? In that case yes, we
> won't have to bother about a status variable here and balancing
> calls. Sorry for the confusion.

I think they'll receive the exact same handle to the regulator. Calls
will remain balanced as long as they are balanced in each user.

Thierry

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