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authorArun Murthy <arun.murthy@stericsson.com>2010-11-11 14:05:28 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2010-11-12 07:55:33 -0800
commitfef7764f8bca9d603a8a51dcb522db97739a33c2 (patch)
treecd3a3d97e729cf814c924e3dd5bd81aca6a3ace4 /drivers/video
parenta1025e224c518dceb342d0cc54e5513c6476f60c (diff)
backlight: add low threshold to pwm backlight
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: Arun Murthy <arun.murthy@stericsson.com> Acked-by: Linus Walleij <linus.walleij@stericsson.com> Acked-by: Richard Purdie <rpurdie@linux.intel.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'drivers/video')
-rw-r--r--drivers/video/backlight/pwm_bl.c7
1 files changed, 6 insertions, 1 deletions
diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index 550443518891..21866ec69656 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -25,6 +25,7 @@ struct pwm_bl_data {
struct pwm_device *pwm;
struct device *dev;
unsigned int period;
+ unsigned int lth_brightness;
int (*notify)(struct device *,
int brightness);
};
@@ -48,7 +49,9 @@ static int pwm_backlight_update_status(struct backlight_device *bl)
pwm_config(pb->pwm, 0, pb->period);
pwm_disable(pb->pwm);
} else {
- pwm_config(pb->pwm, brightness * pb->period / max, pb->period);
+ brightness = pb->lth_brightness +
+ (brightness * (pb->period - pb->lth_brightness) / max);
+ pwm_config(pb->pwm, brightness, pb->period);
pwm_enable(pb->pwm);
}
return 0;
@@ -92,6 +95,8 @@ static int pwm_backlight_probe(struct platform_device *pdev)
pb->period = data->pwm_period_ns;
pb->notify = data->notify;
+ pb->lth_brightness = data->lth_brightness *
+ (data->pwm_period_ns / data->max_brightness);
pb->dev = &pdev->dev;
pb->pwm = pwm_request(data->pwm_id, "backlight");

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