Driver code in kernel module doesn't execute? - linux-kernel

Question

Why this kernel module doesn't do anything when i load it?
#include <linux/init.h>
#include <linux/module.h>
#include <linux/platform_device.h>
#define DEVICE_NAME "hello-1.00.a"
#define DRIVER_NAME "hello"
MODULE_LICENSE("Dual BSD/GPL");
static int hello_init(struct platform_device *pdev){
printk(KERN_ALERT "Hello, world\n");
return 0;
}
static int hello_exit(struct platform_device *pdev){
printk(KERN_ALERT "Goodbye, cruel world\n");
return 0;
}
static const struct of_device_id myled_of_match[] =
{
{.compatible = DEVICE_NAME},
{},
};
MODULE_DEVICE_TABLE(of, myled_of_match);
static struct platform_driver hello_driver =
{
.driver = {
.name = DRIVER_NAME,
.owner = THIS_MODULE,
.of_match_table = myled_of_match
},
.probe = hello_init,
.remove = hello_exit
};
module_platform_driver(hello_driver);
It musts print Hello, world\n, if i do lsmod the module appear to be loaded:
lsmod
hello_world 1538 0 - Live 0xbf000000 (O)
but nothing is printed neither in the console nor in dmesg.
If i use module_init and module_exit all works, but i need the pointer platform_device *pdev to the device, what can i do?
EDIT:
the original module looks like this:
#include <linux/init.h>
#include <linux/module.h>
static int hello_init(void){
printk(KERN_ALERT "Hello, world\n");
return 0;
}
static void hello_exit(void){
printk(KERN_ALERT "Goodbye, cruel world\n");
}
module_init(hello_init);
module_exit(hello_exit);
In my device tree blob is present this entry:
hello {
compatible = "dglnt,hello-1.00.a";
reg = <0x41220000 0x10000>;
};

Answer 1

If i use module_init and module_exit all works
That short "original" code only consists of the module framework. The init routine is guaranteed to be called when the module is loaded, and the exit routine is called prior to unloading. That "original" code is not a driver.
The longer kernel module is a driver and getting loaded, but since it has the default init and exit code that does nothing (as generated by the expansion of the module_platform_driver() macro), there are no messages. The driver code in the loadable module is not guaranteed to be called when the kernel uses a Device Tree.
Why this kernel module doesn't do anything when i load it?
The probe function of the driver (which would output messages) is probably not getting called because there is nothing in your Device Tree that indicates that this device driver is needed.
The snippet of the board's Device Tree has
compatible = "dglnt,hello-1.00.a";
but the driver declares that it should specified as
#define DEVICE_NAME "hello-1.00.a"
...
{.compatible = DEVICE_NAME},
These strings should match so that the driver can bind with this referenced device in the Device Tree node.
Also the device node should be declared as
status = "okay";
to override any default status that could disable the device.
A properly configured node in the Device Tree should get the driver's probe function to be executed as expected.