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Subject[PATCH 2/2] backlight: pwm_bl: add configurable delay between re-enabling PWM and switching backlight power on
Date
When switching the backlight on, the LCD may need some time to adjust
to the configured PWM duty cycle. Add a configurable delay between
configuring the PWM and enabling the backlight regulator to account
for this.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
---
.../bindings/leds/backlight/pwm-backlight.txt | 4 ++++
drivers/video/backlight/pwm_bl.c | 22 +++++++++++++++++++---
2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
index 764db86..95594c3 100644
--- a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
+++ b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
@@ -17,6 +17,10 @@ Optional properties:
"pwms" property (see PWM binding[0])
- enable-gpios: contains a single GPIO specifier for the GPIO which enables
and disables the backlight (see GPIO binding[1])
+ - turn-on-delay-ms: delay in milliseconds between configuring the PWM
+ and switching PWM on. This may be required to eliminate
+ flicker when switching the PWM on after it has been
+ disabled.

[0]: Documentation/devicetree/bindings/pwm/pwm.txt
[1]: Documentation/devicetree/bindings/gpio/gpio.txt
diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index 921f322..9578f65 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -23,6 +23,7 @@
#include <linux/pwm_backlight.h>
#include <linux/regulator/consumer.h>
#include <linux/slab.h>
+#include <linux/delay.h>

struct pwm_bl_data {
struct pwm_device *pwm;
@@ -35,6 +36,7 @@ struct pwm_bl_data {
struct gpio_desc *enable_gpio;
unsigned int scale;
bool legacy;
+ int turn_on_delay_ms;
int (*notify)(struct device *,
int brightness);
void (*notify_after)(struct device *,
@@ -52,6 +54,17 @@ static void pwm_backlight_power_on(struct pwm_bl_data *pb, int brightness)

pwm_enable(pb->pwm);

+ if (pb->turn_on_delay_ms > 0) {
+ dev_dbg(pb->dev, "Sleeping %u..%uµs\n",
+ pb->turn_on_delay_ms * 1000,
+ pb->turn_on_delay_ms * 1100);
+ if (pb->turn_on_delay_ms > 20)
+ msleep(pb->turn_on_delay_ms);
+ else
+ usleep_range(pb->turn_on_delay_ms * 1000,
+ pb->turn_on_delay_ms * 1100);
+ }
+
err = regulator_enable(pb->power_supply);
if (err < 0)
dev_err(pb->dev, "failed to enable power supply\n");
@@ -108,8 +121,9 @@ static int pwm_backlight_update_status(struct backlight_device *bl)
duty_cycle = compute_duty_cycle(pb, brightness);
pwm_config(pb->pwm, duty_cycle, pb->period);
pwm_backlight_power_on(pb, brightness);
- } else
+ } else {
pwm_backlight_power_off(pb);
+ }

if (pb->notify_after)
pb->notify_after(pb->dev, brightness);
@@ -264,9 +278,9 @@ static int pwm_backlight_probe(struct platform_device *pdev)
pb->scale = data->levels[i];

pb->levels = data->levels;
- } else
+ } else {
pb->scale = data->max_brightness;
-
+ }
pb->notify = data->notify;
pb->notify_after = data->notify_after;
pb->check_fb = data->check_fb;
@@ -329,6 +343,8 @@ static int pwm_backlight_probe(struct platform_device *pdev)
goto err_alloc;
}

+ of_property_read_u32(node, "turn-on-delay-ms", &pb->turn_on_delay_ms);
+
dev_dbg(&pdev->dev, "got pwm for backlight\n");

/*
--
2.1.4
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