HID: wacom: Improve behavior of non-standard LED brightness values

Assigning a non-standard brightness value to an LED can cause the value
to slowly drift downward over time as the effects of integer division
accumulate. Each time that an LED is triggered, a series of set and get
calls occur. For example, if we assume a tablet with max_hlv = 100, then
when brightness is set to "200" through sysfs, the hlv value written to
hardware will be `200*100/255 = 78`. If the LED trigger is later activated,
the hlv value will be used to determine the brightness: `78*255/100 = 198`.
This lower brightness then used to set the brightness of the next LED.
However, `198*100/255 = 77`, so the next LED ends up slightly dimmer.
Each subsequent trigger activation will cause the brightness to continue
drifting down until we reach a point where the result of integer divsion
does not introduce any new error.

This commit corrects the issue by being more careful about how we handle
scaling between the two ranges (0..max_{h,l}lv) and (0..LED_FULL).

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>

diff --git a/drivers/hid/wacom.h b/drivers/hid/wacom.h
index 6f1443999d1d9b26da2b02af1008de0f47c0453f..1deacb4568cb921f34584cee641ae3f2d45424d0 100644 (file)
--- a/drivers/hid/wacom.h
+++ b/drivers/hid/wacom.h
@@ -218,6 +218,14 @@ static inline __u32 wacom_s32tou(s32 value, __u8 n)
        return value & (1 << (n - 1)) ? value & (~(~0U << n)) : value;
 }
 
+static inline u32 wacom_rescale(u32 value, u32 in_max, u32 out_max)
+{
+       if (in_max == 0 || out_max == 0)
+               return 0;
+       value = clamp(value, 0, in_max);
+       return DIV_ROUND_CLOSEST(value * out_max, in_max);
+}
+
 extern const struct hid_device_id wacom_ids[];
 
 void wacom_wac_irq(struct wacom_wac *wacom_wac, size_t len);

diff --git a/drivers/hid/wacom_sys.c b/drivers/hid/wacom_sys.c
index 9843b52bd017a02a0dfdac4f5b7e2e29062b26e3..9652288c35712de2db223d2e222c2c6c07cd82f5 100644 (file)
--- a/drivers/hid/wacom_sys.c
+++ b/drivers/hid/wacom_sys.c
@@ -1302,10 +1302,10 @@ enum led_brightness wacom_leds_brightness_get(struct wacom_led *led)
        struct wacom *wacom = led->wacom;
 
        if (wacom->led.max_hlv)
-               return led->hlv * LED_FULL / wacom->led.max_hlv;
+               return wacom_rescale(led->hlv, wacom->led.max_hlv, LED_FULL);
 
        if (wacom->led.max_llv)
-               return led->llv * LED_FULL / wacom->led.max_llv;
+               return wacom_rescale(led->llv, wacom->led.max_llv, LED_FULL);
 
        /* device doesn't support brightness tuning */
        return LED_FULL;
@@ -1337,8 +1337,8 @@ static int wacom_led_brightness_set(struct led_classdev *cdev,
                goto out;
        }
 
-       led->llv = wacom->led.llv = wacom->led.max_llv * brightness / LED_FULL;
-       led->hlv = wacom->led.hlv = wacom->led.max_hlv * brightness / LED_FULL;
+       led->llv = wacom->led.llv = wacom_rescale(brightness, LED_FULL, wacom->led.max_llv);
+       led->hlv = wacom->led.hlv = wacom_rescale(brightness, LED_FULL, wacom->led.max_hlv);
 
        wacom->led.groups[led->group].select = led->id;