auxdisplay: vk2c21: add sysfs brightness control

Expose a `brightness` sysfs attribute that drives the chip's IVASET
register (0x8A):

  - "default" (the boot-time state): IVASET=0x0F, IVA off, bias driven
    by the board's external VR resistor (same as before this change).
  - 0..15:                          IVASET=0x3F..0x30 (IVA on, SEG mode);
                                    15 = brightest (1.000 x VDD), 0 =
                                    dimmest (0.529 x VDD at 1/3 bias).

SEG mode (0x30..0x3F) is used rather than VLCD-output mode (0x10..0x1F)
because the R58X-Pro ties the VLCD pin to VDD through an external
resistor: in VLCD-output mode the external circuit dominates the chip's
internal driver and the brightness register has no visible effect. In
SEG mode the chip ignores the VLCD pin and generates the bias voltage
purely internally, so the 16 levels actually take effect.

Probe-time behaviour is unchanged: init still sends IVA=0x0F, and the
attribute reads back as "default" until userspace writes a number.

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

@@ -41,7 +41,23 @@
 #define VK2C21_SYS_OFF_LCD_OFF	0x00	/* System + LCD off */
 #define VK2C21_FRAME_80HZ	0x00	/* 80 Hz frame rate */
 #define VK2C21_BLINK_OFF	0x00	/* Blinking off     */
-#define VK2C21_IVA_DEFAULT	0x0F	/* VLCD selected, IVA off, R1 */
+#define VK2C21_IVA_DEFAULT	0x0F	/* VLCD selected, IVA off, R1 (external VR) */
+
+/*
+ * 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_ON_BASE	0x30
+#define VK2C21_BRIGHTNESS_MAX	15
+#define VK2C21_BRIGHTNESS_DEFAULT	(-1)	/* IVA off, external VR in use */
 
 /* Display RAM has 10 addressable bytes (SEG/COM matrix) */
 #define VK2C21_RAM_SIZE		10
@@ -185,6 +201,7 @@ struct vk2c21_data {
 	unsigned int		half_period_ns;
 	u8			dispram[VK2C21_RAM_SIZE];
 	char			display_text[VK2C21_MAX_DIGITS + 1];
+	int			brightness;	/* -1 = IVA off / "default", 0..15 = IVA level */
 };
 
 /* ------------------------------------------------------------------ */
@@ -414,9 +431,51 @@ static ssize_t clear_store(struct device *dev,
 }
 static DEVICE_ATTR_WO(clear);
 
+static ssize_t brightness_show(struct device *dev,
+				struct device_attribute *attr, char *buf)
+{
+	struct vk2c21_data *d = dev_get_drvdata(dev);
+
+	if (d->brightness == VK2C21_BRIGHTNESS_DEFAULT)
+		return sysfs_emit(buf, "default\n");
+	return sysfs_emit(buf, "%d\n", d->brightness);
+}
+
+static ssize_t brightness_store(struct device *dev,
+				 struct device_attribute *attr,
+				 const char *buf, size_t count)
+{
+	struct vk2c21_data *d = dev_get_drvdata(dev);
+	int ret, level;
+	u8 iva;
+
+	if (sysfs_streq(buf, "default")) {
+		level = VK2C21_BRIGHTNESS_DEFAULT;
+		iva = VK2C21_IVA_DEFAULT;
+	} else {
+		ret = kstrtoint(buf, 0, &level);
+		if (ret)
+			return ret;
+		if (level < 0 || level > VK2C21_BRIGHTNESS_MAX)
+			return -EINVAL;
+		/* Invert: userspace 15 = brightest = IVA level 0 = 0x30 */
+		iva = VK2C21_IVA_ON_BASE + (VK2C21_BRIGHTNESS_MAX - level);
+	}
+
+	mutex_lock(&d->lock);
+	ret = vk2c21_send_cmd(d, VK2C21_IVASET, iva);
+	if (!ret)
+		d->brightness = level;
+	mutex_unlock(&d->lock);
+
+	return ret ? ret : count;
+}
+static DEVICE_ATTR_RW(brightness);
+
 static struct attribute *vk2c21_sysfs_attrs[] = {
 	&dev_attr_display.attr,
 	&dev_attr_clear.attr,
+	&dev_attr_brightness.attr,
 	NULL,
 };
 
@@ -445,6 +504,7 @@ static int vk2c21_probe(struct platform_device *pdev)
 		return -ENOMEM;
 
 	d->dev = dev;
+	d->brightness = VK2C21_BRIGHTNESS_DEFAULT;
 	mutex_init(&d->lock);
 	platform_set_drvdata(pdev, d);