I have read that driver core sends uevents through netlink from sysfs to udev or some daemons which are listening to it and thenafter corresponding driver is recongnised by udev and loads it. But how this sysfs entries are created corresponding to a device which is hotplugged? .
Thanks in advance for your response.
Drivers create the sysfs entries when they initialize with the major/minor number that they've either gotten or hard coded to use. The file 'uevent' file in each of those device folder allow the daemon to cause the events to be sent in through the netlink socket.
A value of "add" being written into the 'uevent' file will cause all the add events to get generated and thus you can udev or whatever daemon is running (with the appropriate privileges) can create the /dev entries.
Look at this pending hotplug.txt file to see how that works.
Related
I am working on a Linux device driver for a piece of hardware that relies on some configuration specific to each implementation. On the first boot-up, we require a user-space application to generate this configuration data through a calibration process. Naturally, I would store this configuration data in a file and read this file when the driver is loaded or the hardware is configured.
Reading from a file from inside kernel-space, however, is highly discouraged.
All user-space/kernel-space interaction should occur through the sysfs interface. This confuses me a little bit for two reasons:
The sysfs file-system is virtual, so every time the kernel is booted, the user (or user-space application) would have to write the configuration data to the sysfs filesystem for the kernel to use it.
The driver probe function is run long before a user-space application would have the the ability to write to sysfs.
Are my assumptions about sysfs correct? How would I be able to get this configuration data to the kernel without having to rely on a user-space application to write it to sysfs?
If use of the ramdisk is possible in your case, you could do the configuration in some script in this ramdisk, like for example:
# insmod module.ko
# cat config.file > /sys/sys_file
Such a solution will allow you to configure quite early your kernel module.
I am a newbie to linux kernel. While interfacing with the external device, I made the required changes in bsp file. The bsp file device name and platform_driver name should match in order to invoke probe(). I have heard that probe will check whether h/w exists and probe() will create device file (/dev/rtc1) for a new external device. I am not sure about the functionality of the probe(). Can someone explain me how device file is created in this context?Thank you in advance!
probe() is a callback function that gets called, irrespective of h/w presence, when the bus_match_driver() routine returns success. The probe function is called when a device is installed that the driver-core thinks it should handle; the probe function should perform checks on the information passed to it about the device and decide whether the driver is really appropriate for that device. A device file is created either manually by mknod or automatically by udev and is not directly related to probe function. The book https://lwn.net/Kernel/LDD3/ (highly recommended) has all the details regarding creating a device node.
I am first time trying debugfs to transfer data from the kernel to the user space. Please suggest how to proceed in this regard.
I do not want to use tty driver. It is just for practice from my side. Actually i am trying to write my kernel driver for USART where interrupt data will be pushed to user-space application using debugfs ring buffer.
Long before i found a post by a fellow Stackexchange friend to his GIT repository -- where he had implemented debugfs as ring buffer ... but that link is lost to me... so not able to find some refrence to proced in this regard.
Please suggest.
i know few links which i tried from eugene :---
ioctl vs netlink vs memmap to communicate between kernel space and user space
writing data to debugfs --- from a device driver
http://code.google.com/p/kernel-strider/source/browse/sources/core/resolve_ip.c
I think you should use netlink .
Netlink socket is a special IPC used for transferring information
between kernel and user-space processes. It provides a full-duplex
communication link between the two by way of standard socket APIs for
user-space processes and a special kernel API for kernel modules.
I have received a driver (PCAN driver for PCI card, using rtdm), that creates /dev/pcan0 and /dev/pcan1 is compiled as a netdev driver.
There are many facilities that come with this driver, but they are all targeted at a user-level program reading CAN messages. What I need however is to read these messages from a kernel module. The PCAN driver doesn't export any variables/functions which means it doesn't provide a kernel-level API for me to use.
I looked briefly at the code and reading from the /dev device and writing to it don't use copy_from_user or copy_to_user. Therefore, I thought it should be safe for me to open the /dev/pcan0 from my kernel module and read from it.
Now my question is, how do I open/read from a /dev device from a kernel module?
P.S. I want to read from the CAN bus from an RTAI real-time thread, do you think that would cause a problem (for example every read passing through linux kernel and thus breaking real-time conditions?)
You can use syscalls directly from kernel space: sys_open(), sys_read(), sys_close().
There's Linuxjournal article about that.
P/S: copy_from_user() works perfectly with kernel-space addresses.
Given that I was using RTDM, there were two options:
Using RTDM direct functions, such rt_dev_open, rt_dev_read etc
This is not implemented in the current version of pcan driver
Using RTDM ioctl
This was the solution and it worked
I'm trying to learn about device drivers on Linux Kernel, for that I've created three modules with:
A bus type
A device driver
A fake device that does nothing now, only is registered
Everything works fine, I can load the bus, the driver and the module that creates the device. Everything appears on sysfs, including the link between the device and the device driver that indicates that they are binded.
And when the driver and device are loaded, I can see using udevadm monitor that also some events are provoked:
KERNEL[1275564332.144997] add /module/bustest_driver (module)
KERNEL[1275564332.145289] add /bus/bustest/drivers/bustest_example (drivers)
UDEV [1275564332.157428] add /module/bustest_driver (module)
UDEV [1275564332.157483] add /bus/bustest/drivers/bustest_example (drivers)
KERNEL[1275564337.656650] add /module/bustest_device (module)
KERNEL[1275564337.656817] add /devices/bustest_device (bustest)
UDEV [1275564337.658294] add /module/bustest_device (module)
UDEV [1275564337.664707] add /devices/bustest_device (bustest)
But after everything, the device doesn't appear on hal. What else need a device to be seen by hal?
Everything seems to be ok with the device, the problem is that Hal needs a handler for each subsystem (the list of handlers can be found in hald/linux/device.c), and obviously, hal doesn't support bustest, the subsystem invented for this case.
If the bus is registered with the name "pseudo" instead of "bustest", hal uses a set of handlers defined for fake devices to initialize the database entry, registers it and send a DeviceAdded event.