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 <[e-mail]>
Signed-off-by: Jiri Kosina <[e-mail]>

drivers/hid/wacom.h

···
 	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);

drivers/hid/wacom_sys.c

···
 	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;
···
 		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;