auxdisplay: vk2c21: drive brightness in SEG mode, not VLCD-output mode

Writing IVASET in VLCD-output mode (0x10..0x1F) has no visible effect on
the R58X-Pro: the board ties the VLCD pin to VDD through an external
resistor, so the chip's internal driver is overpowered by the external
circuit and the bias voltage stays where the resistor sets it.

Move the IVA-on range to 0x30..0x3F (SEG mode). In that mode the chip
ignores the VLCD pin entirely and generates the bias voltage internally,
so the 16 brightness levels actually take effect.

Signed-off-by: Ricardo Pardini <ricardo@pardini.net>

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: rpardini

drivers/auxdisplay/lcd-vk2c21.c

@@ -44,13 +44,18 @@
 #define VK2C21_IVA_DEFAULT	0x0F	/* VLCD selected, IVA off, R1 (external VR) */
 
 /*
- * IVA on, VLCD-output mode: 16 internal bias-voltage levels.
- *   0x10 = level 0  = 1.000 x VDD (highest contrast / "brightest")
- *   0x1F = level 15 = 0.529 x VDD (lowest contrast / "dimmest")
+ * IVA on, SEG mode: 16 internal bias-voltage levels.
+ *   0x30 = level 0  = 1.000 x VDD (highest contrast / "brightest")
+ *   0x3F = level 15 = 0.529 x VDD (lowest contrast / "dimmest")
+ * SEG mode (vs. VLCD-output mode at 0x10..0x1F) is used because the R58X-Pro
+ * has an external resistor on the VLCD pin: in VLCD-output mode the external
+ * resistor dominates the chip's internal driver and the brightness register
+ * has no visible effect. SEG mode tells the chip to ignore the VLCD pin and
+ * generate the bias voltage purely internally.
  * Userspace brightness is inverted (0 = dimmest, 15 = brightest) so that
  * higher numbers look brighter, matching the usual convention.
  */
-#define VK2C21_IVA_VLCD_ON_BASE	0x10
+#define VK2C21_IVA_ON_BASE	0x30
 #define VK2C21_BRIGHTNESS_MAX	15
 #define VK2C21_BRIGHTNESS_DEFAULT	(-1)	/* IVA off, external VR in use */
 
@@ -453,8 +458,8 @@ static ssize_t brightness_store(struct device *dev,
 			return ret;
 		if (level < 0 || level > VK2C21_BRIGHTNESS_MAX)
 			return -EINVAL;
-		/* Invert: userspace 15 = brightest = IVA level 0 = 0x10 */
-		iva = VK2C21_IVA_VLCD_ON_BASE + (VK2C21_BRIGHTNESS_MAX - level);
+		/* Invert: userspace 15 = brightest = IVA level 0 = 0x30 */
+		iva = VK2C21_IVA_ON_BASE + (VK2C21_BRIGHTNESS_MAX - level);
 	}
 
 	mutex_lock(&d->lock);