• Arun Murthy's avatar
    backlight: add low threshold to pwm backlight · fef7764f
    Arun Murthy authored
    The intensity of the backlight can be varied from a range of
    max_brightness to zero.  Though most, if not all the pwm based backlight
    devices start flickering at lower brightness value.  And also for each
    device there exists a brightness value below which the backlight appears
    to be turned off though the value is not equal to zero.
    
    If the range of brightness for a device is from zero to max_brightness.  A
    graph is plotted for brightness Vs intensity for the pwm based backlight
    device has to be a linear graph.
    
    intensity
    	  |   /
    	  |  /
    	  | /
    	  |/
    	  ---------
    	 0	max_brightness
    
    But pratically on measuring the above we note that the intensity of
    backlight goes to zero(OFF) when the value in not zero almost nearing to
    zero(some x%).  so the graph looks like
    
    intensity
    	  |    /
    	  |   /
    	  |  /
    	  |  |
    	  ------------
    	 0   x	 max_brightness
    
    In order to overcome this drawback knowing this x% i.e nothing but the low
    threshold beyond which the backlight is off and will have no effect, the
    brightness value is being offset by the low threshold value(retaining the
    linearity of the graph).  Now the graph becomes
    
    intensity
    	  |     /
    	  |    /
    	  |   /
    	  |  /
    	  -------------
    	   0	  max_brightness
    
    With this for each and every digit increment in the brightness from zero
    there is a change in the intensity of backlight.  Devices having this
    behaviour can set the low threshold brightness(lth_brightness) and pass
    the same as platform data else can have it as zero.
    
    [akpm@linux-foundation.org: coding-style fixes]
    Signed-off-by: default avatarArun Murthy <arun.murthy@stericsson.com>
    Acked-by: default avatarLinus Walleij <linus.walleij@stericsson.com>
    Acked-by: default avatarRichard Purdie <rpurdie@linux.intel.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    fef7764f
pwm_bl.c 4.87 KB