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A particular sequence of brightness and bl_power set from sysfs shows abnormal behavior

Question asked by Mohsin Kazmi on Feb 19, 2014

Hi All,

          I am facing a backlight issue on imx6q sabreauto board with 10'' LVDS from boundary devices. Details of the issue and analysis are given below:

 

Execute the following commands and notice the change in backlight of attached LVDS:

 

### command 1 ### root@mx6q:/sys/devices/pwm_bl_lvds1.25/backlight/pwm_bl_lvds1.25# echo 7 > bl_power 
### command 2 ### root@mx6q:/sys/devices/pwm_bl_lvds1.25/backlight/pwm_bl_lvds1.25# echo 0 > bl_power
### command 3 ### root@mx6q:/sys/devices/pwm_bl_lvds1.25/backlight/pwm_bl_lvds1.25# echo 0 > brightness
### command 4 ### root@mx6q:/sys/devices/pwm_bl_lvds1.25/backlight/pwm_bl_lvds1.25# echo 7 > bl_power 
### command 5 ### root@mx6q:/sys/devices/pwm_bl_lvds1.25/backlight/pwm_bl_lvds1.25# echo 0 > brightness 
### command 6 ### root@mx6q:/sys/devices/pwm_bl_lvds1.25/backlight/pwm_bl_lvds1.25# cat max_brightness > brightness 
### command 7 ### root@mx6q:/sys/devices/pwm_bl_lvds1.25/backlight/pwm_bl_lvds1.25# echo 0 > bl_power

 

It is expected at this point that backlight should turn ON at full intensity, but it remains off. Note that actual_brightness value here is also 7.

 

root@mx6q:/sys/devices/pwm_bl_lvds1.25/backlight/pwm_bl_lvds1.25# cat actual_brightness 7

 

If we only skip 5th command from the sequence of 7 commands given in the 1st code block, output is as expected. i.e. backlight turns on at maximum brightness.

 

 

I have observed the following behavior:

root@mx6q:/sys/devices/pwm_bl_lvds0.30/backlight/pwm_bl_lvds0.30# echo 0 > brightness

root@mx6q:/sys/devices/pwm_bl_lvds0.30/backlight/pwm_bl_lvds0.30# echo 1 > bl_power

root@mx6q:/sys/devices/pwm_bl_lvds0.30/backlight/pwm_bl_lvds0.30# echo 0 > brightness

root@mx6q:/sys/devices/pwm_bl_lvds0.30/backlight/pwm_bl_lvds0.30# echo 7 > brightness

root@mx6q:/sys/devices/pwm_bl_lvds0.30/backlight/pwm_bl_lvds0.30# echo 0 > bl_power

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It didn't display.

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root@mx6q:/sys/devices/pwm_bl_lvds0.30/backlight/pwm_bl_lvds0.30# echo 7 > brightness

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It displayed correctly.

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I have debugged both stack and driver for backlight and pwm subsystems. But I couldn't find any issue. I have added additional debug prints but they shows the expected values and result while hardware is not working properly as it shows undesired state on particular sequence.

 

Can anybody help me in this regard?

 

Thanks in Advance,

Moshin Kazmi

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