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 : > > > 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 > > > --- > > > .../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 > >