Messages in this thread Patch in this message |  | | From | Yusuf Khan <> | Subject | [PATCH v10 3/3] drivers: ddcci: add drivers for DDCCI | Date | Sun, 3 Apr 2022 16:08:51 -0700 |
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This patch adds Documentation for the DDCCI drivers.
Signed-off-by: Yusuf Khan <yusisamerican@gmail.com> Signed-off-by: Christoph Grenz <christophg+lkml@grenz-bonn.de> --- Documentation/ABI/testing/sysfs-driver-ddcci | 57 +++++++++ Documentation/driver-api/ddcci.rst | 122 +++++++++++++++++++ 2 files changed, 179 insertions(+) create mode 100644 Documentation/ABI/testing/sysfs-driver-ddcci create mode 100644 Documentation/driver-api/ddcci.rst
diff --git a/Documentation/ABI/testing/sysfs-driver-ddcci b/Documentation/ABI/testing/sysfs-driver-ddcci new file mode 100644 index 000000000000..19f77ccf3ed0 --- /dev/null +++ b/Documentation/ABI/testing/sysfs-driver-ddcci @@ -0,0 +1,57 @@ +What: /sys/bus/ddcci/ddcci<I²C bus number>i<hex address> +Date: March 2022 +KernelVersion: 5.18 +Contact: Christoph Grenz <christophg+lkml@grenz-bonn.de> +Description: This file is a user interface for an internal + dependent device on the I2C bus, it exports the same + information as the master device(/sys/bus/ddcci/ + ddcci<I²C bus number>) that is referenced in this + document. + +What: /sys/bus/ddcci/ddcci<I²C bus number>e<hex address> +Date: March 2022 +KernelVersion: 5.18 +Contact: Christoph Grenz <christophg+lkml@grenz-bonn.de> +Description: This file is a user interface for an external + dependent device on the I2C bus, it exports the same + information as the master device(/sys/bus/ddcci/ + ddcci<I²C bus number>) that is referenced in this + document. + +What: /sys/bus/ddcci/ddcci<I²C bus number> +Date: March 2022 +KernelVersion: 5.18 +Contact: Christoph Grenz <christophg+lkml@grenz-bonn.de> +Description: This file provides the user interface for the + master device on the I2C bus. It exports the following + peices of information: + - idProt + ACCESS.bus protocol supported by the device. Usually + "monitor". + + - idType + ACCESS.bus device subtype. Usually "LCD" or "CRT". + + - idModel + ACCESS.bus device model identifier. Usually a + shortened form of the device model name. + + - idVendor + ACCESS.bus device vendor identifier. Empty if the + Identification command is not supported. + + - idModule + ACCESS.bus device module identifier. Empty if the + Identification command is not supported. + + - idSerial + 32 bit device number. A fixed serial number if it's + positive, a temporary serial number if negative and zero + if the Identification command is not supported. + + - modalias + A combined identifier for driver selection. It has the form: + ddcci:<idProt>-<idType>-<idModel>-<idVendor>-<idModule>. + All non-alphanumeric characters (including whitespace) + in the model, vendor or module parts are replaced by + underscores to prevent issues with software like systemd-udevd. diff --git a/Documentation/driver-api/ddcci.rst b/Documentation/driver-api/ddcci.rst new file mode 100644 index 000000000000..2b7de1ac2656 --- /dev/null +++ b/Documentation/driver-api/ddcci.rst @@ -0,0 +1,122 @@ +.. SPDX-License-Identifier: GPL-2.0-or-later + +============== +DDC/CI +============== + +1. Introduction +=============== +DDC/CI is a control protocol for monitor settings supported by most +monitors since about 2005. It is based on ACCESS.bus (an early USB predecessor). +This could be used to create drivers that communicate with the DDCCI component, +see ddcci-backlight for an example. + +2. sysfs interface +================== +Each detected DDC/CI device gets a directory in /sys/bus/ddcci/devices. +The main device on a bus is named ddcci[I²C bus number]. +Internal dependent devices are named ddcci[I²C bus number]i[hex address] +External dependent devices are named ddcci[I²C bus number]e[hex address] +There the following files export information about the device: + +capabilities +The full ACCESS.bus capabilities string. It contains the protocol, +type and model of the device, a list of all supported command +codes, etc. See the ACCESS.bus spec for more information. + +- idProt +ACCESS.bus protocol supported by the device. Usually "monitor". + +- idType +ACCESS.bus device subtype. Usually "LCD" or "CRT". + +- idModel +ACCESS.bus device model identifier. Usually a shortened form of the +device model name. + +- idVendor +ACCESS.bus device vendor identifier. Empty if the Identification command +is not supported. + +- idModule +ACCESS.bus device module identifier. Empty if the Identification command +is not supported. + +- idSerial +32 bit device number. A fixed serial number if it's positive, a temporary +serial number if negative and zero if the +Identification command is not supported. + +- modalias +A combined identifier for driver selection. It has the form: +ddcci:<idProt>-<idType>-<idModel>-<idVendor>-<idModule>. +All non-alphanumeric characters (including whitespace) in the model, +vendor or module parts are replaced by underscores to prevent issues +with software like systemd-udevd. + +3. Character device interface +============================= +For each DDC/CI device a character device in +/dev/bus/ddcci/[I²C bus number]/ is created, +127 devices are assigned in total. + +The main device on the bus is named display. + +Internal dependent devices are named i[hex address] + +External dependent devices are named e[hex address] + +These character devices can be used to issue commands to a DDC/CI device +more easily than over i2c-dev devices. They should be opened unbuffered. +To send a command just write the command byte and the arguments with a +single write() operation. The length byte and checksum are automatically +calculated. + +To read a response use read() with a buffer big enough for the expected answer. + +NOTE: The maximum length of a DDC/CI message is 32 bytes. + +4. ddcci-backlight (monitor backlight driver) +============================================= +[This is not specific to the DDC/CI backlight driver, if you already dealt with +backlight drivers, skip over this.] + +For each monitor that supports accessing the Backlight Level White +or the Luminance property, a backlight device of type "raw" named like the +corresponding ddcci device is created. You can find them in /sys/class/backlight/. +For convenience a symlink "ddcci_backlight" on the device associated with the +display connector in /sys/class/drm/ to the backlight device is created, as +long as the graphics driver allows to make this association. + +5. Limitations +============== + +-Dependent devices (sub devices using DDC/CI directly wired to the monitor, +like Calibration devices, IR remotes, etc.) aren't automatically detected. +You can force detection of external dependent devices by writing +"ddcci-dependent [address]" into /sys/bus/i2c/i2c-?/new_device. + +There is no direct synchronization if you manually change the luminance +with the buttons on your monitor, as this can only be realized through polling +and some monitors close their OSD every time a DDC/CI command is received. + +Monitor hotplugging is not detected. You need to detach/reattach the I²C driver +or reload the module. + +6. Debugging +============ +Both drivers use the dynamic debugging feature of the Linux kernel. +To get detailed debugging messages, set the dyndbg module parameter. +If you want to enable debugging permanently across reboots, create a file +/etc/modprobe.d/ddcci.conf containing lines like the following before loading the modules: + +options ddcci dyndbg +options ddcci-backlight dyndbg + +7. Origin +============ +This driver originally came from Christoph Grenz in DKMS form here: +https://gitlab.com/ddcci-driver-linux/ddcci-driver-linux +with multiple backups available on the wayback machine. It also +inlcudes a example program for the usage of this driver in +userland. -- 2.25.1
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