Issue 689: Backlight PWM

Reported by Nikolaus Schaller, Jun 14, 2015

missing on GTA04 and BeagleBoneBlack+Panel.

Comment 1 by Nikolaus Schaller, Oct 24, 2015

There is work ongoing on LKML:

https://lkml.org/lkml/2015/10/19/495

And, we have fixed the DT to at least enable/disable the backlight 
correctly and there are stub nodes for omap-pwm for quite long time.

So as soon as omap-pwm appears mainline, we can fix the DT.

Status: Started

Comment 2 by Nikolaus Schaller, Feb 13, 2016

DT is fixed. Works on GTA04 but there are issues:
1. 100% is not 100% - the impression is that it is not as bright as 
if we permanently enable the GPIO
2. there are problems on OMAP5 with this driver

Comment 3 by Nikolaus Schaller, Apr 22, 2016

OMAP5 problems have vanished with recent upstream kernels.

Comment 4 by Nikolaus Schaller, May 11, 2016

Full level intensity problems are still there.

Comment 5 by Nikolaus Schaller, Jun 7, 2016

The intensity problem is that we run PWM at 500 Hz and 100% is not 
exactly 100%, rather 99%. This means there is still a short 500 Hz 
impulse.
This is sufficient to turn off the DC/DC backlight converter for a 
moment - and it needs 1.5 ms to restart to full voltage/current. 500 
Hz means 2 ms, i.e. only 25% of the cycle are running at full 
current at 100% brightness level.
This explains why it is consistently darker.
Reducing PWM frequency to 80 Hz does not introduce visible flicker 
but makes the 100% almost 100%.

Status: Fixed

Letux Kernel