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

[Jean-Jacques Hiblot]

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:
> > >
> > > +-----------+
> > > |ÂÂÂÂÂÂÂÂÂÂ |
> > > | Regulator |
> > > |ÂÂÂÂÂÂÂÂÂÂ +------------------------+
> > > |ÂÂÂÂÂÂÂÂÂÂ |ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |
> > > +-----------+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ __|__
> > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ \ÂÂ /
> > >  ÂÂÂÂÂÂÂÂ +---------------------+ÂÂÂ \ / led
> > >  ÂÂÂÂÂÂÂÂ |ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |ÂÂÂÂ V
> > >  ÂÂÂÂÂÂÂÂ |ÂÂÂ Led DriverÂÂÂÂÂÂ |ÂÂ --+--
> > >  ÂÂÂÂÂÂÂÂ |ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |ÂÂÂÂ |
> > >  ÂÂÂÂÂÂÂÂ |ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |ÂÂÂÂ |
> > >  ÂÂÂÂÂÂÂÂ |ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ +----------+
> > >  ÂÂÂÂÂÂÂÂ |ÂÂÂÂÂÂÂÂÂÂÂÂÂ \ÂÂÂÂÂ |
> > >  ÂÂÂÂÂÂÂÂ |ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ \ÂÂÂÂ |
> > >  ÂÂÂÂÂÂÂÂ |ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ +ÂÂÂ |
> > >  ÂÂÂÂÂÂÂÂ |ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |ÂÂÂ |
> > >  ÂÂÂÂÂÂÂÂ +---------------------+
> > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |
> > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ +--+--+
> > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ///////
> > >
> > >
> > > 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.