currently i write an driver module which offers some entries in the sysfs. I read a lot through the driver source tree and the internet. I found two approches where the sysfs_create_group() is called:
a) most commonly: In the probe() function of the Driver. Like adviced here
How to attach file operations to sysfs attribute in platform driver?
Random Thing i looked at:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/rtc/rtc-ds1307.c#n1580
b) In the driver struct.
http://kroah.com/log/blog/2013/06/26/how-to-create-a-sysfs-file-correctly/
I know, Greg KH is a very well known developer. So i tried to follow his advice. In the bla_show()/bla_store() functions i tried to get my Driver private data, but my printk()'s showed much different addresses than i printed in the probe() function. My private data is (null). Which is ofc wrong.
When i use approch a) it works as expected, but as Greg KH suggests it is wrong too. I see it a lot in the stable tree in different drivers. Greg writes, the userspace already got the notification that there is a new device, but the LDD3 book states, that the probe function is there to determine if the device is present.
To sum my question up:
Why get the userspace notified about it, even when the kernel doesnt know if it can handle it?
Where is the right place to call sysfs_create_group()? Is it a) or b) ?
LDD3: https://static.lwn.net/images/pdf/LDD3/ch14.pdf
PDF page 24
probe is a function called to query the existence of a specific device
(and whether this driver can work with it), remove is called when the
device is removed from the system,and shutdown is called at shutdown
time to quiesce the device.
I am more confused than before .....
Best Regards
Georg
A device driver is a program that controls a particular type of device that is attached to your computer.
Platform devices are inherently not discoverable, i.e. the hardware cannot say "Hey! I'm present!" to the software. So for these kind of device we need a driver which call as Platform drivers. Drivers provide probe() and remove() methods.
struct platform_driver {
int (*probe)(struct platform_device *);
int (*remove)(struct platform_device *);
.
.
struct device_driver driver;// this file has 2 parameter name or owner.
};
probe() should in general verify that the specified device hardware
actually exists. First we register our driver. Once it found device then it'll call driver probe. It is using name for searching a device.
Ans : your device is available then you need sysfs entry for communication (To the user space). so conceptually you need to define you sysfs entry in probe.
sys_notify function on your attribute and it will cause your userspace code to wake up. It will trigger when sysfs is available for userspace. It just avoiding a blocking call. When kernel does not have sysfs then it'll not notify userspace.
sysfs is a virtual file system provided by the Linux kernel that exports information about various kernel subsystems, hardware devices, and associated device drivers from the kernel's device model to user space through virtual files. When your device is available then you need this entry to export your information.
Related
I have written a Linux driver that is contained in a module. The probe() function sets up the hardware and creates a device file for userspace usage with device_create(). The device_create() is the last thing in the probe function.
I load the kernel module through the init_module syscall. It seems to me that the probe function is run as a callee of the syscall, i.e. that the probe function is done once the syscall returns to userspace. However I am not sure about this.
Sometimes, right after module insertion, the device file that is created through device_create is not there yet.
What is the correct way to wait for the driver has probed and all the effects of the probe such as created device files are visible?
A possible solution is waiting for node creation events in /dev by "inotify" API, provided that you can foresee the name. Here the official description and an example: https://man7.org/linux/man-pages/man7/inotify.7.html
I have seen in pci device drivers this line
module_pci_driver(cp_driver);
But in other pci device drivers this like
module_init(rtl8139_init_module);
both lines found at the end of driver's .c file in different drivers
What I know is: I can create a pci device driver with __init but I can also create a pci device driver without __init
i.e.
[Realtek Ethernet has two drivers in Linux source
1) 139cp.c (without __init)
2) 8139too.c with __init].
I assume that the main difference between the two is simply that if I have to use a pci device driver right after loading of the driver module with insmod command so I use an implementation of a device driver with __init.
Question
On the contrary, if I just want to load the pci device driver but not use it, Then should I create a pci device driver with module_pci_driver() (So no need to add __init)? And what does it do(module_pci_driver)? How its different from pci driver with __init
I like to know I may have a misconception anyone please clarify. Also does the probe function of both type of drivers will run when I load the driver with insmod command? When? if yes than what's the difference since most configuring of device is done in proble function
To sum up
When initialization happens in driver with module_pci_driver(cp_driver); since they dont have __init implemented. What command used
module_pci_driver() is a helper meant for drivers that don't do anything special in their module init and exit (i.e. modules whose init and exit functions just register/unregister).
It generates the init and exit functions for you, reducing some boilerplate.
In this specific case, the 8139too driver might do something in addition to registering the driver (in this case, logging the driver name), so isn't using module_pci_driver()
The probe function is called for existing or new devices that match the ID table and aren't already owned (see How To Write Linux PCI Drivers for details).
It returns a value indicating if the driver will be taking ownership of the device (either 0 or an error code).
I have recently been reading about Linux Driver and Device Model. I wanted to understand how the following works in linux in the driver subsystem. So lets say my device tree looks as follows
To be concrete lets assume Bus1 is a PCI bus, Bus2 is ISA and Bus3 is USB. Buses are connected with each other using bridges.
Linux will identify this device tree through the enumeration process and through enumeration and probing mechanism suitable usb driver for terminal device would be identified.
Now lets assume a Tx operation to this terminal device. The terminal device usb driver would end up doing a urb_submit(dev, write_buffer).
My question is in order for the URB to reach the terminal device, theoretically it would have to be enveloped in Bus2 and Bus1 envelopes. So theoretically speaking the outgoing packet has to look something like this
So at some after the urb_submit happens in the driver, does some kernel code walk up the device tree structure and invoke the bus drivers in order (bus 2 driver and then bus 1 driver) to create this envelope structure.
Can anyone point me to the code in the linux kernel where this happens? I tried to follow urb_submit but could not figure this out.
Thanks a bunch!
For device drivers that have memory-mapped registers, the device's private data struct contains the addresses assigned by enumeration for each device and they directly write to the registers.
For device drivers that go through layers, as you describe above, the enumeration tells each driver who its parent is, and the driver saves this in its private data when it is instantiated. Then the read or write call of the driver formats the request properly for the device type and invokes the read or write method of the parent driver. For example, a USB disk read might call the read method in the file_operations block of its parent which would be https://elixir.bootlin.com/linux/v4.19.107/source/drivers/usb/core/devio.c#L2614
I'm new in developing Linux driver.
Traditionally, If i want to create char device, I just need to implement read write and ioctl functions, and register it by register_chrdev_region with corresponding Major device ID. And then use mknod to create a device file of that Major device ID. Then call read write open in user space will then call the corresponding functions by kernel.
But now i'm studying Linux device model and sys filesystem, which is added in kernel 2.6. I'm told if possible, Don't use /dev filesystem, as linux now has a good device model to handle device and driver.This confused me, so I'll summarize my confusion into some questions:
How do I create a char device in /sys? To be more specific, how do I create a null device just like /dev/null?
How do I call my char device driver function from userspace?
I heard that udev is based on sys filesystem to create device file in /dev. Why? Since I'm told "If possible, Don't use /dev filesystem", why does udev use /sys to create file in /dev?
Does a file in sys have any conceptions like char device file or block device?
In /dev, open write from user space will finally map to functions in file operation structure defined by me, then what functions will be called when I open or write to files in /ssy?
Without context your statement about /dev is not clear. Anyway:
You cannot create char devices on sysfs. The main purpose of sysfs is to export information and allow the user to adjust single values (just navigate under /sys/ for some examples). Char devices usually does much complicated things.
If you mean how you call your driver's open,read,write,ioctl, ... well, by doing open(2), read(2), write(2), ioctl(2) (look at man pages of these commands)
when a device appear, the kernel create a directory under /sys. For example take a look at ls /sys/bus/usb/devices/. All that directories are created when an USB device appear. You can try by plug/unplug USB devices. udev put an eye on sysfs to detect new devices and according to the information from sysfs it create the device under /dev. This happens when the driver, somehow, calls device_add(). Often this functions is invoked by other register functions like: device_create, device_register, or other from other sub-systems.
the idea of sysfs is to provide information about the devices and drivers loaded. So you can change device, bus and driver options. Or manually attach a device to a module
Actually, behind the sysfs attributes there is a set of file_operation, where open, read and write are managed by the kernel and not by your driver. In order to create a sysfs attribute you have to provide the pair of function show and store to read/write something from/to the driver. Then the kernel will route the requests to your correct attribute
I am a linux newbie, trying to understand Linux Device Model. I had been going through Linux 3.1.6 code base, particularly the driver part and found that
some of the drivers were using (for example i2c-bus device : linux-3.1.6/drivers/i2c/i2c-dev.c) *register_chrdev()* and
few others (for example pci bus : linux-3.1.6/drivers/pci/bus.c) were using *device_register()*.
My question is when to use register_chrdev (yes, I know its for a character device, but why not use device_register) and device_register ?
Does that depend on where does the driver developer wants his device/driver to be listed down, like devfs vs sysfs ? Or the interface exposed to the user space to access the device ?
One function registers a character device association (hooking up major:minors to your function), the other just creates an abstract device object (only), so to speak. The two are complementary. The device object is used for the generation of an event so that udev can, if there is also a cdev association registered, create a node in /dev. (Compare with, for example, drivers/char/misc.c.)
See when you register a device as a character device specifically then following thing happens:
Major Number is given in accordance. If you use any device depending on functionality whose registration is based on character device (like tty, input etc), then those will have their respective major number. Thats why its said that dont assign major number statically if not sure.
And
There are certain file operations which correspond to operations that could be performed on char devices only.
Do ask if any query.