linux-kernel - Re: [PATCH v5 3/3] leds: Add control of the voltage/current regulator to the LED core

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Date:   Wed, 16 Oct 2019 10:13:54 +0200
From:   Jean-Jacques Hiblot <jjhiblot@...com>
To:     Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Daniel Thompson <daniel.thompson@...aro.org>
CC:     Pavel Machek <pavel@....cz>, <linux-leds@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <dmurphy@...com>,
        <tomi.valkeinen@...com>
Subject: Re: [PATCH v5 3/3] leds: Add control of the voltage/current regulator
 to the LED core


On 14/10/2019 20:48, Jacek Anaszewski wrote:
> On 10/14/19 2:38 PM, Daniel Thompson wrote:
>> On Mon, Oct 14, 2019 at 12:49:07PM +0200, Jean-Jacques Hiblot wrote:
>>> On 13/10/2019 14:09, Pavel Machek wrote:
>>>> Hi!
>>>>
>>>>> I must say I'm not a big fan of this change.
>>>>> It adds a bunch of code to the LED core and gives small
>>>>> functionality in a reward. It may also influence maximum
>>>>> software blinking rate, so I'd rather avoid calling
>>>>> regulator_enable/disable when timer trigger is set.
>>>>>
>>>>> It will of course require more code.
>>>>>
>>>>> Since AFAIR Pavel was original proponent of this change then
>>>>> I'd like to see his opinion before we move on to discussing
>>>>> possible improvements to this patch.
>>>> Was I?
>>>>
>>>> Okay, this series looks quite confusing to me. First, 1/3 looks
>>>> "interesting" (would have to analyze it way more).
>>>>
>>>> Second, 3/3... So we have a LED driver _and_ a regulator? So yes, the
>>>> chip driving a LED is usually ... voltage/current regulator. What is
>>>> second regulator doing there? Is that a common setup?
>>> This is quite common with current-sink LED drivers.
>>>
>>> The setup looks like this:
>>>
>>> +-----------+
>>> |           |
>>> | Regulator |
>>> |           +------------------------+
>>> |           |                        |
>>> +-----------+                      __|__
>>>                                     \   /
>>>           +---------------------+    \ / led
>>>           |                     |     V
>>>           |    Led Driver       |   --+--
>>>           |                     |     |
>>>           |                     |     |
>>>           |                +----------+
>>>           |              \      |
>>>           |               \     |
>>>           |                +    |
>>>           |                |    |
>>>           +---------------------+
>>>                            |
>>>                         +--+--+
>>>                         ///////
>>>
>>>
>>> Only the regulator usually does not supply only one LED.
>> Given questions have been raised about the complexity of the change I
>> wondered whether, for a system with this topology, the regulator
>> could/should be enabled when the LED driver driver probes?
> And this is how are doing that now.
>
> Moreover, just after seeing your ASCII art it has become obvious to me
> that we can't simply do regulator_disable() while setting brightness to
> LED_OFF since it can result in powering off the LED controller, which
> would need to be reconfigured on next transition to
> brightness > LED_OFF.

That is a problem only if the LED driver is powered by the same 
regulator, which is not the case in the diagram.

This series make sense only for boards where LEDs have a dedicated 
voltage rail or can be modeled this way.

My use case is a LED panel driven by a LED current-sink controller that 
uses both a PWM-style control for brightness AND a active-low enable 
pin. If the enable pin is not HIGH, the panel is never completely dark 
even if the LED brightness is set to 0. I modeled this switch with a 
fixed-voltage regulator  which is a common way of doing it (it is after 
all how this things is done inside the panel).

JJ
> It looks that the idea is flawed I'm afraid.