Re: [PATCH] backlight: add DDC/CI brightness driver

From: Daniel Thompson
Date: Mon Feb 13 2017 - 07:12:33 EST

On 21/01/17 18:31, MiÅosz RachwaÅ wrote:
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.

This driver is well written and very clear but I'm afraid I have a few high level concerns about this driver.

Firstly, have you discussed this driver with the DRM developers? DDC/CI is closely related to EDID (monitor usually implemented both, same I2C bus, etc). It seems odd for this driver to be standalone rather than integrated into the DRM connector model (allowing it to probe automatically).

Secondly, it assumes the Luminance VCD is the best way to control the backlight (ignoring both the current and legacy backlight controls). I'm not totally sure that falling back to luminance is sensible for a backlight control but it is certainly wrong to go directly to it, ignoring the other VCPs.

Finally, I'm also not sure about they way the maxbr module parameter works because it makes a per-device configuration setting and applies it system-wide.


Daniel.


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");

Hmnnn... module parameters are

+
+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");