What does cdev_add() actually do? I'm asking terms of registering a device with the kernel.
Does it add the pointer to cdev structure in some map which is indexed by major and minor number? How exactly does this happen when you say the device is added/registered with the kernel. I want to know what steps the cdev_add takes to register the device in the running kernel. We create a node for user-space using mknod command. Even this command is mapped using major and minor number. Does registration also do something similar?
cdev_add registers a character device with the kernel. The kernel maintains a list of character devices under cdev_map
static struct kobj_map *cdev_map;
kobj_map is basically an array of probes, which in this case is the list of character devices:
struct kobj_map {
struct probe {
struct probe *next;
dev_t dev;
unsigned long range;
struct module *owner;
kobj_probe_t *get;
int (*lock)(dev_t, void *);
void *data;
} *probes[255];
struct mutex *lock;
};
You can see that each entry in the list has the major and minor number for the device (dev_t dev), and the device structure (in the form of kobj_probe_t, which is a kernel object, which represents a cdev in this case). cdev_add adds your character device to the probes list:
int cdev_add(struct cdev *p, dev_t dev, unsigned count)
{
...
error = kobj_map(cdev_map, dev, count, NULL,
exact_match, exact_lock, p);
When you do an open on a device from a process, the kernel finds the inode associated to the filename of your device (via namei function). The inode has the major a minor number for the device (dev_t i_rdev), and flags (imode) indicating that it is a special (character) device. With this it can access the cdev list I explained above, and get the cdev structure instantiated for your device. From there it can create a struct file with the file operations to your cdev, and install a file descriptor in the process's file descriptor table.
This is what actually 'registering' a character device means and why it needs to be done. Registering a block device is similar. The kernel maintains another list for registered gendisks.
You can read Linux Device Driver. It is a little bit old, but the main ideas are the same. It is difficoult to explain a simple operation like cdev_add() and all the stuff around in few lines.
I suggest you to read the book and the source code. If you have trouble to navigate your source code, you can use some tag system like etags + emacs, or the eclipse indexer.
Please see the code comments here:
cdev_add() - add a char device to the system 464 *
#p: the cdev structure for the device 465 * #dev: the first device
number for which this device is responsible 466 * #count: the number
of consecutive minor numbers corresponding to this 467 *
device 468 * 469 * cdev_add() adds the device represented by #p to
the system, making it 470 * live immediately. A negative error code
is returned on failure. 471 */ `
the immediate answer to any such question is read the code. Thats what Linus say.
[edit]
the cdev_add basically adds the device to the system. What it means essentially is that after the cdev_add operation your new device will get visibility through the /sys/ file system. The function does all the necessary house keeping activities related to that particularly the kobj reference to your device will get inserted at its position in the object hierarchy. If you want to get more information about it, I would suggest some reading around /sysfs/ and struct kboj
Related
In Linux kernel modules, two different approaches can be followed when creating a struct cdev, as suggested in this site and in this answer:
First approach, cdev_alloc()
struct cdev *my_dev;
...
static int __init example_module_init(void) {
...
my_dev = cdev_alloc();
if (my_dev != NULL) {
my_dev->ops = &my_fops; /* The file_operations structure */
my_dev->owner = THIS_MODULE;
}
else
...
}
Second approach, cdev_init()
static struct cdev my_cdev;
...
static int __init example_module_init(void) {
...
cdev_init(&my_cdev, my_fops);
my_cdev.owner = THIS_MODULE;
...
}
(assuming that my_fops is a pointer to an initialized struct file_operations).
Is the first approach deprecated, or still in use?
Can cdev_init() be used also in the first approach, with cdev_alloc()? If no, why?
The second question is also in a comment in the linked answer.
Can cdev_init() be used also in the first approach, with cdev_alloc()?
No, cdev_init shouldn't be used for a character device, allocated with cdev_alloc.
At some extent, cdev_alloc is equivalent to kmalloc plus cdev_init. So calling cdev_init for a character device, created with cdev_alloc, has no sense.
Moreover, a character device allocated with cdev_alloc contains a hint that the device should be deallocated when no longer be used. Calling cdev_init for that device will clear that hint, so you will get a memory leakage.
Selection between cdev_init and cdev_alloc depends on a lifetime you want a character device to have.
Usually, one wants lifetime of a character device to be the same as lifetime of the module. In that case:
Define a static or global variable of type struct cdev.
Create the character device in the module's init function using cdev_init.
Destroy the character device in the module's exit function using cdev_del.
Make sure that file operations for the character device have .owner field set to THIS_MODULE.
In complex cases, one wants to create a character device at specific point after module's initializing. E.g. a module could provide a driver for some hardware, and a character device should be bound with that hardware. In that case the character device cannot be created in the module's init function (because a hardware is not detected yet), and, more important, the character device cannot be destroyed in the module's exit function. In that case:
Define a field inside a structure, describing a hardware, of pointer type struct cdev*.
Create the character device with cdev_alloc in the function which creates (probes) a hardware.
Destroy the character device with cdev_del in the function which destroys (disconnects) a hardware.
In the first case cdev_del is called at the time, when the character device is not used by a user. This guarantee is provided by THIS_MODULE in the file operations: a module cannot be unloaded if a file, corresponded to the character device, is opened by a user.
In the second case there is no such guarantee (because cdev_del is called NOT in the module's exit function). So, at the time when cdev_del returns, a character device can be still in use by a user. And here cdev_alloc really matters: deallocation of the character device will be deferred until a user closes all file descriptors associated with the character device. Such behavior cannot be obtained without cdev_alloc.
They do different things. The preference would be usual - prefer not to use dynamic allocation when not needed and allocate on stack when it's possible.
cdev_alloc() dynamically allocates my_dev, so it will call kfree(pointer) when cdev_del().
cdev_init() will not free the pointer.
Most importantly, the lifetime of the structure my_cdev is different. In cdev_init() case struct cdev my_cdev is bound to the containing lexical scope, while cdev_alloc() returns dynamically allocate pointer valid up until free-d.
I'm trying to develop a simple Linux kernel module that manages a bunch of sensors/actuators pinned on the GPIO of a Raspberry Pi.
The GPIO functionalities I need are quite simple: get/set pin values, receive IRQs, ...
In my code, I have a misc_device which implements the usual open, read, write and open operations. In my read operation, for instance, I'd like to get the value (high/low) of a specific GPIO pin.
Luckily, the kernel provides an interface for such GPIO operations. Actually, there are two interfaces, according to the official GPIO doc: the legacy one, which is extremely simple yet deprecated, and the new descriptor-based one.
I'd like to use the latter for my project, and I understand how to implement all I need except for one thing: the device tree stuff.
With reference to board.txt, before I can call gpiod_get_index() and later gpiod_get_value(), first I need to setup the device tree somehow like this:
foo_device {
compatible = "acme,foo";
...
led-gpios = <&gpio 15 GPIO_ACTIVE_HIGH>, /* red */
<&gpio 16 GPIO_ACTIVE_HIGH>, /* green */
<&gpio 17 GPIO_ACTIVE_HIGH>; /* blue */
power-gpios = <&gpio 1 GPIO_ACTIVE_LOW>;
};
However, I've absolutely no clue where to put that chunk of code, nor if I really need it. Mind that I have a misc device which looks like this, where aaa_fops contains the read operation:
static struct miscdevice aaa = {
MISC_DYNAMIC_MINOR, "aaa", &aaa_fops
};
Using the old deprecated interface, my problem would be solved because it doesn't require to mess with the device tree, but I'd still like to use the new one if not too complex.
I've read a bunch of documentation, both official and unofficial, but couldn't find a straight and simple answer to my issue. I tried to find an answer in the kernel source code, especially in the drivers section, but only got lost in a valley of complex and messy stuff.
The lack of working, minimal examples (WME) about kernel is significantly slowing down my learning process, just my opinion about it.
Could you please give me a WME of a simple device (preferably a misc) whose read() operation gets the value of a pin, using the new GPIO interface?
If you need more details about my code, just ask. Thanks in advance!
Note 1: I'm aware that most of my work could be done in userspace rather than kernelspace; my project is for educational purposes only, to learn the kernel.
Note 2: I choose a misc device because it's simple, but I can switch to a char device if needed.
... first I need to setup the device tree somehow like this:
...
However, I've absolutely no clue where to put that chunk of code
Device Tree nodes and properties should not be called "code".
Most devices are connected to a peripheral bus, so device nodes typically are child nodes of the peripheral bus node.
Could you please give me a WME of a simple device
You can find numerous examples of descriptor-based GPIO usage in the kernel source.
Since the documentation specifies the GPIO descriptor as a property named
<function>-gpios, a grep of the directory arch/arm/boot/dts for the string "\-gpios" reports many possible examples.
In particular there's
./bcm2835-rpi-b.dts: hpd-gpios = <&gpio 46 GPIO_ACTIVE_HIGH>;
This hpd-gpios property belongs to the hdmi base-node defined in bcm283x.dtsi, and is used by the gpu/drm/vc4/vc4_hdmi.c driver.
/* General HDMI hardware state. */
struct vc4_hdmi {
...
int hpd_gpio;
...
};
static int vc4_hdmi_bind(struct device *dev, struct device *master, void *data)
{
...
/* Only use the GPIO HPD pin if present in the DT, otherwise
* we'll use the HDMI core's register.
*/
if (of_find_property(dev->of_node, "hpd-gpios", &value)) {
...
hdmi->hpd_gpio = of_get_named_gpio_flags(dev->of_node,
"hpd-gpios", 0,
&hpd_gpio_flags);
if (hdmi->hpd_gpio < 0) {
ret = hdmi->hpd_gpio;
goto err_unprepare_hsm;
}
...
}
If the hpd-gpios property is defined/found and successfully retrieved from the board's DeviceTree, then the driver's structure member hpd_gpio holds the GPIO pin number.
Since this driver does not call devm_gpio_request(), the framework apparently allocates the GPIO pin for the driver.
The driver can then access the GPIO pin.
static enum drm_connector_status
vc4_hdmi_connector_detect(struct drm_connector *connector, bool force)
{
...
if (vc4->hdmi->hpd_gpio) {
if (gpio_get_value_cansleep(vc4->hdmi->hpd_gpio) ^
vc4->hdmi->hpd_active_low)
return connector_status_connected;
else
return connector_status_disconnected;
}
I followed some tutorials that explained how to write Linux kernel modules and I am a bit confused. Even after reading the official "documentation", I have poor understanding of the concepts.
After creating a character device (register_chrdev), I see it is common to use a combination of the following functions:
class_create
class_device_create
device_create
I was not able to understand, what is a class, device and, class device and driver?
Which one of these actually responsible to create an entry under /proc/?
Rather than going into what's a class, or what's a device (I'm no expert in Linux kernel), I will address the question as follows.
After creating the character device, you want to be able to access it from the user space. To do this, you need to add a device node under /dev. You can do this in two ways.
Use mknod to manually add a device node (old)
mknod /dev/<name> c <major> <minor>
OR
Use udev
This is where the class_create and device_create or class_device_create (old) come in.
To notify udev from your kernel module, you first create a virtual device class using
struct class * class_create(owner, name)
Now, the name will appear in /sys/class/<name>.
Then, create a device and register it with sysfs.
struct device *device_create(struct class *class, struct device *parent,
dev_t devt, void *drvdata, const char *fmt, ...)
Now, device name will appear in /sys/devices/virtual/<class name>/<device name> and /dev/<device name>
It's not clear what you are asking about the /proc entry.
After your module is loaded, it will appear in /proc/modules (do a cat /proc/modules to see it). And, after you allocate the device numbers, say with
int register_chrdev_region(dev_t first, unsigned int count, char *name)
, the name will appear in /proc/devices (do a cat /proc/devices to see it).
And, please check the kernel sources for these functions as well, as they provide a good description of what they do in their comments.
The good old LDD3 does not provide these mechanisms, but it's a very good source.
The register_chrdev() function in kernel registers a character device:
int register_chrdev(unsigned int major, const char*name,
struct file_operations*ops));
If major is 0 the kernel dynamically allocates a major number and the register function returns it.
Now, let's assume a module foo.ko wants to use /dev/foo with a dynamic major number. How does the userspace learn what major number to pass to mknod to create /dev/foo ?
As soon as a character device gets registered with a dynamic major number, the corresponding information appears in /proc/devices and thus can be retrieved by a user-space application/script in order to create an appropriate node.
For a better example you may refer to Linux Device Drivers book (3rd edition), for instance, a script to read /proc/devices is shown on this page.
I'm studying Linux Device Driver programming 3rd edition and I have some questions about the open method, here's the "scull_open" method used in that book:
int scull_open(struct inode *inode, struct file *filp){
struct scull_dev *dev; /* device information */
dev = container_of(inode->i_cdev, struct scull_dev, cdev);
filp->private_data = dev; /* for other methods */
/* now trim to 0 the length of the device if open was write-only */
if ( (filp->f_flags & O_ACCMODE) == O_WRONLY) {
if (down_interruptible(&dev->sem))
return -ERESTARTSYS;
scull_trim(dev); /* ignore errors */
up(&dev->sem);
}
return 0; /* success */
}
And my questions are:
Shouldn't this function returns a file descriptor to the device just opened?
Isn't the "*filp" local to this function, then why we copy the contents of dev to it?
How we could use later in read and write methods?
could someone writes to my a typical "non-fatty" implementation of open method?
ssize_t scull_read(struct file *filp, char __user *buf, size_t count, loff_t *f_pos){
struct scull_dev *dev = filp->private_data;
...}
Userspace open function is what you are thinking of, that is a system call which returns a file descriptor int. Plenty of good references for that, such as APUE 3.3.
Device driver "open method" is a function within file_operations structure. It is different than userspace "file open". With the device driver installed, when user code does open of the device (e.g. accessing /dev/scull0), this "open method" would then get called.
Shouldn't this function returns a file descriptor to the device just opened ?
In linux device driver, open() returns either 0 or negative error code. File descriptor is internally managed by kernel.
Isn't the "*filp" local to this function, then why we copy the contents of dev to it ?
filp represents opened file and a pointer to it is passed to driver by kernel. Sometimes this filp is used sometimes it is not needed by driver. Copy contents of dev is required so that when some other function let us say read() is called driver can retrieve some device specific data.
How we could use later in read and write methods ?
One of the most common way to use filp in read/write() is to acquire lock. When device is opened driver will create a lock. Now when a read/write occurs same lock will be used to prevent data buffer from corruption in case multiple process are accessing same device.
could someone writes to my a typical "non-fatty" implementation of open method ?
As you are just studying please enjoy exploring more. An implementation can be found here