Re: [PATCH] Added possibility to use pwm_bl.c with percentage instead of levels

[Thierry Reding <thierry.reding@gmail.com>]

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On Fri, Jun 27, 2014 at 08:58:21AM +0200, Johannes Pointner wrote:
> I know what you mean because I have here for example a panel that only
> supports a duty cycle between 20 and 100 percent. But wouldn't I be a
> possibility to add optional nodes with minimal/maximal brightness or
> something like that?

We have something like that (lth_brightness) for non-DT and which was
supported in the first DT proposals. But again the equivalent using the
brightness-levels would be:

	brightness-levels = <0 20 21 ... 100>;

So the continuous range and low threshold are really just special cases
of what brightness-levels provides. Granted, it's somewhat tedious to
have to list ~100 values in a DT property like this, but I haven't
really heard any arguments why the range really needs to be that
fine-grained. Most of the devices that I've used have somewhere between
8 and 20 brightness levels. An exception to this are Android devices,
which do indeed seem to support truly continuous ranges.

Thierry

> 
> Hannes
> 
> 
> 2014-06-27 8:12 GMT+02:00 Thierry Reding <thierry.reding@gmail.com>:
> 
> > On Fri, Jun 27, 2014 at 07:32:20AM +0200, Johannes Pointner wrote:
> > > Hello,
> > >
> > > I'm new working on the linux device drivers, so if I made something wrong
> > > please point me into the right direction.
> > >
> > > I'd like to use the pwm_bl driver for a sitara based terminal and for
> > this
> > > I would need the possibility to set the backlight within a percentage
> > > range. The following patch should add this possibility to the pwm_bl
> > > driver. The idea is to keep backward compatibility by moving the required
> > > option brightness levels to optional. Therefore if there is no node with
> > > brightness levels the percentage levels are used.
> > >
> > > Signed-off-by: Johannes Pointner <johannes.pointner@gmail.com>
> > > ---
> > >  .../bindings/video/backlight/
> > > pwm-backlight.txt     | 11 +++---
> > >  drivers/video/backlight/pwm_bl.c                   | 46
> > > ++++++++++++----------
> > >  2 files changed, 31 insertions(+), 26 deletions(-)
> >
> > Also adding the backlight maintainers on Cc.
> >
> > This has been discussed a few times before. In fact the original device
> > tree binding had support for a continuous range of levels but that was
> > rejected during review. The reason was, as far as I remember, that the
> > number of levels and the corresponding duty cycle values is something
> > that's usually determined at system design time. Often backlights can't
> > properly light the whole surface of the panel at every level.
> >
> > That said, there's always the possibility to fake this by adding a DT
> > property with a continuous range, such as this:
> >
> >         brightness-levels = <0 1 2 ... 100>;
> >
> > Thierry
> >

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