[PATCH] backlight: add DDC/CI brightness driver

[MiÅosz RachwaÅ]

This patch contains driver for exposing VESA DDC/CI MCCS brightness
settings via kernel backlight API. This allows to control monitor
brightness using standard software which uses /sys/class/backlight/
interface, same as for laptop backlight drivers.
Because some monitors don't correctly implement DDC/CI standard module
parameter allows to override maximum brightness value reported by
monitor.
This module doesn't support autodetection, so driver must be manually
instantiated on monitor IÂC bus under 0x37 address.

Signed-off-by: MiÅosz RachwaÅ <me@xxxxxxxxx>
---
 drivers/video/backlight/Kconfig    |   7 ++
 drivers/video/backlight/Makefile   |   1 +
 drivers/video/backlight/ddcci_bl.c | 128 +++++++++++++++++++++++++++++++++++++
 3 files changed, 136 insertions(+)
 create mode 100644 drivers/video/backlight/ddcci_bl.c

diff --git a/drivers/video/backlight/Kconfig b/drivers/video/backlight/Kconfig
index 5ffa4b4e..cce62832 100644
--- a/drivers/video/backlight/Kconfig
+++ b/drivers/video/backlight/Kconfig
@@ -460,6 +460,13 @@ config BACKLIGHT_BD6107
 	help
 	  If you have a Rohm BD6107 say Y to enable the backlight driver.
 
+config BACKLIGHT_DDCCI
+	tristate "VESA DDC/CI MCCS brightness driver"
+	depends on BACKLIGHT_CLASS_DEVICE && I2C
+	default m
+	help
+	  This supports brightness control via VESA DDC/CI MCCS.
+
 endif # BACKLIGHT_CLASS_DEVICE
 
 endif # BACKLIGHT_LCD_SUPPORT
diff --git a/drivers/video/backlight/Makefile b/drivers/video/backlight/Makefile
index 16ec534c..2e5352aa 100644
--- a/drivers/video/backlight/Makefile
+++ b/drivers/video/backlight/Makefile
@@ -30,6 +30,7 @@ obj-$(CONFIG_BACKLIGHT_CARILLO_RANCH)	+= cr_bllcd.o
 obj-$(CONFIG_BACKLIGHT_CLASS_DEVICE)	+= backlight.o
 obj-$(CONFIG_BACKLIGHT_DA903X)		+= da903x_bl.o
 obj-$(CONFIG_BACKLIGHT_DA9052)		+= da9052_bl.o
+obj-$(CONFIG_BACKLIGHT_DDCCI)		+= ddcci_bl.o
 obj-$(CONFIG_BACKLIGHT_EP93XX)		+= ep93xx_bl.o
 obj-$(CONFIG_BACKLIGHT_GENERIC)		+= generic_bl.o
 obj-$(CONFIG_BACKLIGHT_GPIO)		+= gpio_backlight.o
diff --git a/drivers/video/backlight/ddcci_bl.c b/drivers/video/backlight/ddcci_bl.c
new file mode 100644
index 00000000..c314cbc8
--- /dev/null
+++ b/drivers/video/backlight/ddcci_bl.c
@@ -0,0 +1,128 @@
+/*
+ * VESA DDC/CI MCCS brightness driver
+ *
+ * Copyright (C) 2017 MiÅosz RachwaÅ <me@xxxxxxxxx>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version.
+ */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/i2c.h>
+#include <linux/backlight.h>
+#include <linux/delay.h>
+
+static int maxbr;
+module_param(maxbr, int, 0644);
+MODULE_PARM_DESC(maxbr, "Override maximum brightness value specified by monitor");
+
+static int ddcci_update_status(struct backlight_device *bl)
+{
+	struct i2c_client *client = bl_get_data(bl);
+
+	char buf[] = { 0x51, 0x84, 0x03, 0x10, 0x00, 0x00, 0x00 };
+	buf[4] = bl->props.brightness >> 8;
+	buf[5] = bl->props.brightness;
+	buf[6] = (client->addr << 1) ^ 0xC6 ^ buf[4] ^ buf[5];
+
+	i2c_master_send(client, buf, 7);
+	msleep(50);
+
+	return 0;
+}
+
+static int ddcci_read(struct i2c_client *client, struct backlight_properties *props, int init)
+{
+	int i;
+	char xor;
+
+	char buf[11] = { 0x51, 0x82, 0x01, 0x10, 0x00 };
+	buf[4] = (client->addr << 1) ^ 0xC2;
+
+	i2c_master_send(client, buf, 5);
+	msleep(40);
+	i2c_master_recv(client, buf, 11);
+
+	if (buf[3] != 0)
+		goto fail;
+
+	xor = 0x50;
+	for (i = 0; i < 11; i++)
+		xor ^= buf[i];
+
+	if (xor)
+		goto fail;
+
+	if (init) {
+		if (maxbr)
+			props->max_brightness = maxbr;
+		else
+			props->max_brightness = (buf[6] << 8) | buf[7];
+	}
+	props->brightness = (buf[8] << 8) | buf[9];
+
+	return 0;
+
+fail:
+	dev_err(&client->dev, "failed to read brightness");
+	return -1;
+}
+
+static int ddcci_get_brightness(struct backlight_device *bl)
+{
+	struct i2c_client *client = bl_get_data(bl);
+
+	ddcci_read(client, &bl->props, 0);
+	return bl->props.brightness;
+}
+
+static const struct backlight_ops ddcci_ops = {
+	.update_status = ddcci_update_status,
+	.get_brightness = ddcci_get_brightness
+};
+
+static int ddcci_probe(struct i2c_client *client, const struct i2c_device_id *id)
+{
+	struct backlight_properties props;
+	int retry;
+	char name[20];
+
+	memset(&props, 0, sizeof(struct backlight_properties));
+	props.type = BACKLIGHT_FIRMWARE;
+	snprintf(name, 20, "ddcci_%s", dev_name(&client->dev));
+
+	for (retry = 0; retry < 3; retry++) {
+		if (ddcci_read(client, &props, 1) >= 0) {
+			devm_backlight_device_register(&client->dev, name, &client->dev, client, &ddcci_ops, &props);
+			return 0;
+		}
+	}
+
+	return -1;
+}
+
+static int ddcci_remove(struct i2c_client *client)
+{
+	return 0;
+}
+
+static struct i2c_device_id ddcci_idtable[] = {
+	{ "ddcci_bl", 0 },
+	{}
+};
+MODULE_DEVICE_TABLE(i2c, ddcci_idtable);
+
+static struct i2c_driver ddcci_driver = {
+	.driver = { .name = "ddcci_bl" },
+	.id_table = ddcci_idtable,
+	.probe = ddcci_probe,
+	.remove = ddcci_remove
+};
+module_i2c_driver(ddcci_driver);
+
+MODULE_AUTHOR("MiÅosz RachwaÅ <me@xxxxxxxxx>");
+MODULE_DESCRIPTION("VESA DDC/CI MCCS brightness driver");
+MODULE_LICENSE("GPL");
-- 
2.11.0