pwm_bl: Fix overflow condition
Subsystems (3)
PATCH 1/1 pwm_bl: Fix overflow condition
This fixes and overflow condition that happens with a high value of brightness-levels-scale by using a 64-bit variable. The issue would prevent a range of higher brightness levels from being set.
--- drivers/video/backlight/pwm_bl.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)
drivers/video/backlight/pwm_bl.c +5 -2
@@ -88,14 +88,17 @@ static void pwm_backlight_power_off(struct pwm_bl_data *pb)static int compute_duty_cycle (struct pwm_bl_data *pb,int brightness) {unsigned int lth = pb->lth_brightness; -int duty_cycle; +s64 duty_cycle;if (pb->levels) duty_cycle = pb->levels[brightness];else duty_cycle = brightness; -return (duty_cycle * (pb->period - lth) / pb->scale) + lth; + duty_cycle *= pb->period - lth; +do_div (duty_cycle, pb->scale); + +return duty_cycle + lth; }static int pwm_backlight_update_status (struct backlight_device *bl)
Signature · 1 line
2.14.1.342.g6490525c54-goog
Quoted 21 lines · Derek Basehore
This fixes and overflow condition that happens with a high value of brightness-levels-scale by using a 64-bit variable. The issue would prevent a range of higher brightness levels from being set.
Signed-off-by Derek Basehore <[email protected]>--- drivers/video/backlight/pwm_bl.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)drivers/video/backlight/pwm_bl.c +1
-1@@ -88,14 +88,17 @@ static void pwm_backlight_power_off(struct pwm_bl_data *pb)static int compute_duty_cycle (struct pwm_bl_data *pb,int brightness) {unsigned int lth = pb->lth_brightness; -int duty_cycle; +s64 duty_cycle;if (pb->levels)
drivers/video/backlight/pwm_bl.c static void pwm_backlight_power_off(struct pwm_bl_data *pb) · line 102
Quoted 10 lines
drivers/video/backlight/pwm_bl.c +4
-1duty_cycle = pb->levels[brightness]; else duty_cycle = brightness; -return (duty_cycle * (pb->period - lth) / pb->scale) + lth; + duty_cycle *= pb->period - lth; +do_div (duty_cycle, pb->scale); + +return duty_cycle + lth; }
I don't think your commit message accurately describes the change here. The overflow that you're preventing might happen with a large value of pb->period (or rather, in combination with a large value of duty_cycle) but it's unrelated to pb->scale.
Also, the semantics of do_div() are that it takes an unsigned dividend, so your duty_cycle should be a u64.
Thierry
Quoted 26 lines · Thierry Reding
Quoted 25 lines · Derek Basehore
This fixes and overflow condition that happens with a high value of brightness-levels-scale by using a 64-bit variable. The issue would prevent a range of higher brightness levels from being set.
Signed-off-by Derek Basehore <[email protected]>--- drivers/video/backlight/pwm_bl.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)drivers/video/backlight/pwm_bl.c +1
-1@@ -88,14 +88,17 @@ static void pwm_backlight_power_off(struct pwm_bl_data *pb)static int compute_duty_cycle (struct pwm_bl_data *pb,int brightness) {unsigned int lth = pb->lth_brightness; -int duty_cycle; +s64 duty_cycle;if (pb->levels) duty_cycle = pb->levels[brightness];else duty_cycle = brightness;
Quoted 11 lines
drivers/video/backlight/pwm_bl.c static void pwm_backlight_power_off(struct pwm_bl_data *pb) · line 102
Quoted 6 lines
drivers/video/backlight/pwm_bl.c +4
-1-return (duty_cycle * (pb->period - lth) / pb->scale) + lth; + duty_cycle *= pb->period - lth; +do_div (duty_cycle, pb->scale); + +return duty_cycle + lth; }I don't think your commit message accurately describes the change here. The overflow that you're preventing might happen with a large value of
pb->period(or rather, in combination with a large value ofduty_cycle) but it's unrelated topb->scale.
I'm referring to the of property brightness-levels-scale. If there aren't levels defined in a DTS, duty_cycle can be up to this value. I'll change the CM to describe what's happening based on the variable names from the function instead.
Quoted 3 lines
Also, the semantics of
do_div()are that it takes an unsigned dividend, so yourduty_cycleshould be a u64.
I'll change it.
Quoted 2 lines
Thierry
Quoted 40 lines · dbasehore .
Quoted 26 lines · Thierry Reding
Quoted 25 lines · Derek Basehore
This fixes and overflow condition that happens with a high value of brightness-levels-scale by using a 64-bit variable. The issue would prevent a range of higher brightness levels from being set.
Signed-off-by Derek Basehore <[email protected]>--- drivers/video/backlight/pwm_bl.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)drivers/video/backlight/pwm_bl.c +1
-1@@ -88,14 +88,17 @@ static void pwm_backlight_power_off(struct pwm_bl_data *pb)static int compute_duty_cycle (struct pwm_bl_data *pb,int brightness) {unsigned int lth = pb->lth_brightness; -int duty_cycle; +s64 duty_cycle;if (pb->levels) duty_cycle = pb->levels[brightness];else duty_cycle = brightness;Quoted 11 lines
drivers/video/backlight/pwm_bl.c static void pwm_backlight_power_off(struct pwm_bl_data *pb) · line 102
Quoted 6 lines
drivers/video/backlight/pwm_bl.c +4
-1-return (duty_cycle * (pb->period - lth) / pb->scale) + lth; + duty_cycle *= pb->period - lth; +do_div (duty_cycle, pb->scale); + +return duty_cycle + lth; }I don't think your commit message accurately describes the change here. The overflow that you're preventing might happen with a large value of
pb->period(or rather, in combination with a large value ofduty_cycle) but it's unrelated topb->scale.
Quoted 1 line
I'm referring to the of property brightness-levels-scale.
I don't see a brightness-levels-scale property in either the DT bindings or the pwm-backlight driver. Do you have any pointers?
Thierry