How to use linux device model and /sys filesystem?

Question

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:

1. How do I create a char device in /sys? To be more specific, how do I create a null device just like /dev/null?
2. How do I call my char device driver function from userspace?
3. 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?
4. Does a file in sys have any conceptions like char device file or block device?
5. 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?

Answer 1

Without context your statement about /dev is not clear. Anyway:

1. 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.

2. 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)

3. 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.

4. 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

5. 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