What is a "USB user Space device driver for a custom device?"
A user-space device driver is a piece of software (a library or a daemon) that is used to get access to a custom device (a gadget). The difference with a kernel driver is that the user-space driver is run from the normal user-space, not from the kernel. That is, it is compiled and run as normal code just as any other program/library.
In order to do this, you need to access the low-level USB features from your program, but the Linux kernel gently provides that. Anyway, do not try to talk to the kernel directly, that is madness. Instead use a USB library, such as libusb
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Simply put, can a user space application access device drivers running in kernel mode? If I want to read from the I2C Linux character driver, can a basic C executable (extensionless) do that or do I need to develop a kernel module specifically for that task? If a basic C app can access the I2C character driver, what does that gcc makefile look like?
Yes. User space application can access the kernel driver if the driver is character device or the driver exposing the stream interfaces.
Specific to i2c, user space application can write and reads to the i2c device via sysfs api.
Refer the kernel document here https://www.kernel.org/doc/Documentation/i2c/dev-interface
I would like to implement a USB device driver in user space by means of libusb. I'm using a Linux machine supporting a USB OTG controller which is switched to device mode. The USB host is another machine which needs to communicate with my Linux machine by means of a USB vendor specific interface with a bulk in/out interface.
I would like to know if it is possible to use libusb to communicate with the USB host on the other side. Or, if libusb could be used only for host side functionalities.
If libusb can't be used is there any other way to implement the device driver in user space?
Thank you.
As of my knowledge this is not possible. (Vanilla) libusb is only for host usage because the whole process of how to use libusb can only provide this mode.
You can find an answer in the FAQ of the old libusb (before libusbx, the abandoning of the old libusb and the rename of libusbx to the new libusb):
libusb FAQ
Also there is a question on SO to this topic with some suggestions:
How to communicate with the USB Host from a Linux USB Client
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'm trying to write a PCI device driver that runs in user space. Not my idea, what the client wants. Target is an embedded Linux board that will never have more than a single user. I'm an experienced C programmer and know Linux, just not familiar with Linux driver development.
Is this really a device driver or just a library? Do I need to use the typical calls pci_register_driver, etc. or can I just access the device using fopen, and using mmap and ioperm to get to it?
Interrupts will be done using the MSI model. Also need to handle DMA transfers. The device will be streaming lots of data to the user.
There's not much info out there on this subject, LDD3 only devotes a couple of pages to it, and there's nothing else that I could find here on SO.
Thanks in advance!
If there is no driver handling the PCI card it would be possible to access it using ioperm (or iopl - depending on the address) if only port accesses are required.
Using DMA and interrupts is definitely impossible without a kernel-mode driver.
By googleing I found some text about something like a "generic kernel-mode driver" that allows writing user-mode drivers (including DMA and interrupts).
You should ask your customer which kind of kernel-mode drivers for accessing PCI cards is installed on the Linux board.
There is now a proper way to do high performance userspace PCI drivers, called vfio. There is not much documentation, but see the kernel docs http://lxr.free-electrons.com/source/Documentation/vfio.txt and the header file /usr/include/linux.vfio.h. It is available since Linux 3.6.