Reading about the driver-model platform, my understanding is that a platform driver is a static piece of kernel code that controls a platform device, which is a device which generally is always present in the system, as opposed to a driver composed of a kernel module, which can be loaded or removed at will and controls devices which might not always be connected. This answer seems to support that statement.
If all this is true, I expect it's what I'm looking for. I am trying to write a kernel module which, among other things, needs to communicate with an external device over I2C. While the external device isn't always part of the system, the I2C bus IS, so it ought to be controlled by a platform driver. So rather than writing code in my module to directly control the I2C, I ought to be using the platform driver. I found what I believe to be the proper platform driver for the I2C bus on my platform here, and it does use < linux/platform_device.h >, so that seems to be consistent.
However, in addition to this, when looking up I2C in general, I have seen reference to an I2C "client". From the name alone, this sounds like what I am trying to write now, a kernel module that uses the platform driver to communicate over I2C. However, in the documentation for the driver-model platform, nothing about a "client" is mentioned, so I don't know if this is correct or if it is some completely separate different model.
So how does a "client" kernel module fit into the driver-platform model for interfacing with hardware in Linux? Is my module supposed to instantiate a struct of the i2c_driver, or is that something done by the platform driver, and the module is supposed to create an interface to that?
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I am writing here because I am having some problems understanding some key concepts regarding kernel/drivers development.
First, I am going to try to describe what I want to achieve:
Develop a driver that uses the SPI to communicate receiving the payload from user space (I cannot use spidev because I pretend to transparently add some info to the payload before sending it). I want to load the module manually (insmod).
The problem I am seeing: I do not know where or how to specify my interface. If I define the interface within the device tree, is it mandatory to include the compatible field? I ask this because the driver will not be included within the kernel modules as it starts up... Maybe there is not a problem with it. In the examples, I found about spi they only speak about the BUS, Chip Select... but they do not mention MISO and MOSI, so I guess these must be defined by the device tree or maybe since you specify the bus the definition of the rest of the pins is straight forward... I would like to understand this.
Thanks in advance.
Best regards,
Fulgo.
Suppose I have a chip that has multiple options for interfacing with it, for example, the chip may have an I2C interface and a SPI interface and I can choose one of these interfaces and wire the device accordingly. How would I write a linux device driver for such device?
A chip with this behavior is ssd1307, which already has a driver implementation in the mainline linux kernel (https://github.com/torvalds/linux/blob/master/drivers/video/fbdev/ssd1307fb.c), implemented only for its I2C interface (the chip also has a 3/4 Wire SPI Interface). As you can see in the code, the probe function relies heavily on the fact that it is an I2C interface, so in order to use the SPI interface I would need to write an entirely new driver, repeating a lot of common code, or insert a lot of #ifdefs, which don't seem to be good approaches to me.
For chips providing several interface types, standard practice is to provide core driver and interface-specific frontends for it.
Several examples could be found e.g. in sound/soc/codecs/: pcm179x.c / pcm179x-i2c.c / pcm179x-spi.c and others.
This is not (yet?) done for ssd1307.
I'm trying to write a Linux character device driver for a device that just happens to communicate over I2C. The device is an Atmel microcontroller with code that provides an I2C address. It already works using the typical i2c-dev method on the Linux-side.
So now I want to replicate i2c-dev as a new driver that works specifically with this particular device, so that I can add some of my own device-specific abstraction code on top. But I'd like to trim out all the unnecessary code from i2c-dev that currently makes it generic. What can be removed in this situation?
What can be removed in this situation?
You're actually asking an XY question.
You would be better off looking at and adapting an existing I2C device driver that is already similar in required functionality, rather than hacking a special case driver for userspace access.
So now I want to replicate i2c-dev as a new driver that works specifically with this particular device, so that I can add some of my own device-specific abstraction code on top
So then you actually need to write a "Client driver" as described below (from Linux Documentation/i2c/summary):
When we talk about I2C, we use the following terms:
Bus -> Algorithm
Adapter
Device -> Driver
Client
An Algorithm driver contains general code that can be used for a whole class
of I2C adapters. Each specific adapter driver either depends on one algorithm
driver, or includes its own implementation.
A Driver driver (yes, this sounds ridiculous, sorry) contains the general
code to access some type of device. Each detected device gets its own
data in the Client structure. Usually, Driver and Client are more closely
integrated than Algorithm and Adapter.
Details are in Documentation/i2c/writing-clients.
To find a driver of similar functionality, scan the list of I2C client drivers. Note that these I2C drivers are located in the Linux source tree by their functionality (e.g. drivers/rtc/ or drivers/hwmon/) and not their interface (i.e. I2C).
I am currently reading the Linux Module Programming Guide and I have stumbled onto two terms that have confused a bit - device files and device driver. Upon goggling these terms I have come across the following-
A device driver is a piece of software that operates or controls a particular type of device.
A device file is an interface for a device driver that appears in a file system as if it were an ordinary file. In Unix-like operating systems, these are usually found under the /dev directory and are also called device nodes.
What I would like to know is -
1) Are device files an interface between user space programs and the device driver?
2) Does the program access the driver in the kernel via the appropriate device special file?
eg, when using say spidev char dev file, does that allow my userspace program to interact with spi.c and omap2_mcspi.c etc using simple read, write and ioctl calls?
One of the primary abstractions in Unix is the file (source):
Programs, services, texts, images, and so forth, are all files. Input and output devices, and generally all devices, are considered to be files, according to the system.
This lets users treat a variety of entities with a uniform set of operations, even through the implementation of those operations may be wildly different.
As you were getting at with your question, device files are the user facing side of the abstraction. This is what the user sees; a file that they can write to, read from, open, close, etc. The device drivers are the implementation of those operations.
So the user will make a call to a file operation such as write, and then the kernel will then use the device driver to carry out the operation.
Device File like /dev/spidevX.Y is a SW abstraction of a SPI device which exposes Linux low level SPI API to the userspace with syscalls (in Linux driver world known as "file operations"):
That is read(), write(), ioctl()...
spidev.c is a special kind of driver which is registered for generic SPI client(chip) devices, and it's main goal is to export Kernel low level SPI API to userspace.
There is a whole Linux SPI layer in between defined in spi.c
Device driver representing real HW SPI controller is where callbacks (hooks) are implemented and registered to the kernel as a part of spi_master (spi_controller)structure.
Here is a callback initialization for SPI message transfer:
master->transfer_one_message = atmel_spi_transfer_one_message;
everything in linux is a file.
device driver is a software used by operating system to communicate with device.
device driver makes use of device files.
I have a hardware device that consists of 3 separate chips on an I2C bus. I would like to group them together and expose them to userland as one logical device. The user would see the logical device represented by a single directory somewhere in /sys as well as the nodes you'd expect from the I2C chips under /sys/class/i2c-adapter/i2c-?/*.
One of the chips is an MCP23017 which as far as I can tell already has a driver (drivers/gpio/gpio-mcp23s08.c) and I'd like to reuse it. Another of the chips is a PCA9685 and I would like to contribute a driver for this chip that uses the PWM system in include/linux/pwm.h. The third chip is an MCU running custom firmware.
How should I structure the set of drivers? One idea I had is to register a platform driver to present the logical device, and use I2C drivers from within that. Is this a good way to go? Are there better ways?
The logical device is a motor driver board and IR receiver. I have a simple diagram of its structure.
I'm looking to create two interfaces. The first similar to /sys/class/gpio where motors can be 'exported' and then accessed via reading and writing attributes. This would be useful for shell script access and quick debugging of the mechanical parts of the system attached to the motors. The second a character device node in /dev where data can be read or written in binary format, more useful for application control.
it seems not usual design, are you sure you have access to I2C bus of all chips ?
I think you should be able to talk only to MCU, and MCU should manage other devices.
Otherwise, why MCU is there ?
However, I cannot see your diagram, perhaps link is wrong.