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;
}
Related
I have a board, SoC running Linux 5+, with a electrical relay. The relay is triggered by a GPIO. I am looking for a good way to define a relay in a device tree file.
I define LEDs as
led {
compatible = "gpio-leds";
debug {
label = "debug";
gpios = ...
default-state = "off";
};
};
This results in
# ls /sys/class/leds/
debug
I would like to have the relay be something similar such as
# ls /sys/class/{relays,outputs,gpios}/
relay1
What is a good way to achieve this?
Since relay behaves as simply as GPIO output (or more precisely GPO), what you need is just name the corresponding line. It can be done by assigning gpio-line-names property of the GPIO controller in the ACPI or Device Tree. With a use of libgpiod tools (such as gpiofind, gpioinfo), that access GPIO controller via character device node, you may find your line and do the operations on it. Note, GPIO sysfs interface is deprecated and it will be removed from the kernel on a horizon of ~5 years or so.
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.
I'm working on porting a driver I've written for the 2.6.x kernel series into 3.x (i.e. RH EL 6 -> RH EL 7). My driver solution actually comes in two modules: a modified form of ahci.c (from the kernel tree) and my own upper-layer character driver for access AHCI registers and even executing commands against the drive.
In porting to CentOS 7, I've run into an interesting problem. Changes to the drivers I'm building on remove the access to the scsi_host attributes in SYSFS. My question then is, can I append attributes onto devices already existing in SYSFS? Every example I've come across shows making the attributes at the point of device creation, e.g.:
static ssize_t port_show(struct kobject *kobj, struct kobj_attribute *attr,
char *buff);
static struct kobj_attribute pxclb_attr = __ATTR(pxclb, 0664, port_show, NULL);
static struct attribute *attrs[] = {
&pxclb_attr.attr,
NULL,
};
static struct attribute_group port_group = {
.attrs = attrs,
};
/* much other code */
sysfs_create_group(&kobj, &port_group);
This example comes from my character driver. All the searches I've done with Google, and referencing my Linux Foundation Drivers class book, all show attribute creation done at the time of device creation. Can I add to them at any time? It would seem that one could call sysfs_create_group() at any time. Is this true?
You can add attribute in sysfs at anytime you like.
The reason that almost all the drivers add attribute in probe is that userspace has strict expectations on when attributes get created. When a new device is registered in the kernel, a uevent is generated to notify userspace (like udev) that a new device is available. If attributes are added after the device is registered, then userspace won't get notified and userspace will not know about the new attributes.
I am writing a kernel module intended to functionally test a device driver kernel module for an ARM+FPGA SOC system. My approach involves finding which interrupts the device driver is using by querying the device tree. In the device driver itself, I register a platform driver using platform_driver_register and in the .probe function I am passed a platform_device* pointer that contains the device pointer. With this I can call of_match_device and irq_of_parse_and_map, retrieving the irq numbers.
I don't want to register a second platform driver just to query the device tree this way in the test module. Is there some other way I can query the device tree (perhaps more directly, by name perhaps?)
This is what I've found so far, and it seems to work. of_find_compatible_node does what I want. Once I have the device_node*, I can call irq_of_parse_and_map (since of_irq_get_byname doesn't seem to compile for me). I can use it something like the following:
#include <linux/of.h>
#include <linux/of_irq.h>
....
int get_dut_irq(char* dev_compatible_name)
{
struct device_node* dev_node;
int irq = -1;
dev_node = of_find_compatible_node(NULL, NULL, dev_compatible_name);
if (!dev_node)
return -1;
irq = irq_of_parse_and_map(dev_node, 0);
of_node_put(dev_node);
return irq;
}
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