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

From: MiÅosz RachwaÅ
Date: Thu Aug 10 2017 - 09:02:54 EST

On 13.02.2017 13:12, Daniel Thompson wrote:
> 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).

Yes, it should be somehow integrated into DRM EDID probing, but I haven't
looked into that code yet. But I think that this driver should stay mostly
unchanged, and DRM code should only do probing of this driver and pass
fb_info* so that it is possible to check relation of framebuffer and
backlight device via backlight_ops.check_fb, yes?
Or whole driver should be moved into DRM code?

> 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.

Oops, I used VCD that worked on my monitor and forgotten about the others.
It is probably necessary to query and parse capability string to get
supported VCP codes.
But anyway in MCCS specifiaction the only other code that seems revelant
is 0x6B "backlight level: white", what are other "current and legacy backlight controls"?

> 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.

maxbr is written to backlight_properties.max_brightness during probing,
so it could be changed between device instantiations and each instance
would get their own max_brightness value. Ugly solution, but I don't had
any other idea as i2c probe function doesn't have any extra arguments.
Though while that approach was doable with driver instantiatiated by
userspace sysfs interface it won't be with automatic probing by DRM.
I think alternative solution is passing quirk list with values for specific
monitor models, or maybe just hardcoded quirk list.

>
>
> 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");
>>
>