Merge tag 'backlight-next-5.3' of git://git.kernel.org/pub/scm/linux/kernel/git/lee/backlight

Pull backlight updates from Lee Jones:
 "New Functionality:
   - Provide support for ACPI enumeration; gpio_backlight

  Fix-ups:
   - SPDX fixups; pwm_bl
   - Fix linear	brightness levels to include number available; pwm_bl"

* tag 'backlight-next-5.3' of git://git.kernel.org/pub/scm/linux/kernel/git/lee/backlight:
  backlight: pwm_bl: Fix heuristic to determine number of brightness levels
  backlight: gpio_backlight: Enable ACPI enumeration
  backlight: pwm_bl: Convert to use SPDX identifier

1 files changed, 8 insertions, 22 deletions

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index 20d379ac8440..2201b8c78641 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -1,8 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0-only
 /*
- * linux/drivers/video/backlight/pwm_bl.c
- *
- * simple PWM based backlight control, board code has to setup
+ * Simple PWM based backlight control, board code has to setup
  * 1) pin configuration so PWM waveforms can output
  * 2) platform_data being correctly configured
  */
@@ -191,29 +189,17 @@ int pwm_backlight_brightness_default(struct device *dev,
 				     struct platform_pwm_backlight_data *data,
 				     unsigned int period)
 {
-	unsigned int counter = 0;
-	unsigned int i, n;
+	unsigned int i;
 	u64 retval;
 
 	/*
-	 * Count the number of bits needed to represent the period number. The
-	 * number of bits is used to calculate the number of levels used for the
-	 * brightness-levels table, the purpose of this calculation is have a
-	 * pre-computed table with enough levels to get linear brightness
-	 * perception. The period is divided by the number of bits so for a
-	 * 8-bit PWM we have 255 / 8 = 32 brightness levels or for a 16-bit PWM
-	 * we have 65535 / 16 = 4096 brightness levels.
-	 *
-	 * Note that this method is based on empirical testing on different
-	 * devices with PWM of 8 and 16 bits of resolution.
+	 * Once we have 4096 levels there's little point going much higher...
+	 * neither interactive sliders nor animation benefits from having
+	 * more values in the table.
 	 */
-	n = period;
-	while (n) {
-		counter += n % 2;
-		n >>= 1;
-	}
+	data->max_brightness =
+		min((int)DIV_ROUND_UP(period, fls(period)), 4096);
 
-	data->max_brightness = DIV_ROUND_UP(period, counter);
 	data->levels = devm_kcalloc(dev, data->max_brightness,
 				    sizeof(*data->levels), GFP_KERNEL);
 	if (!data->levels)
@@ -705,5 +691,5 @@ static struct platform_driver pwm_backlight_driver = {
 module_platform_driver(pwm_backlight_driver);
 
 MODULE_DESCRIPTION("PWM based Backlight Driver");
-MODULE_LICENSE("GPL");
+MODULE_LICENSE("GPL v2");
 MODULE_ALIAS("platform:pwm-backlight");