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

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Date:	Wed, 6 Mar 2013 17:37:42 +0900
From:	Alex Courbot <acourbot@...dia.com>
To:	Thierry Reding <thierry.reding@...onic-design.de>
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 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.

Alex.

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