Device number linux module issue - linux-kernel

Question

I'm developing character devices for a Linux kernel distro Poky 1.7 built with Yocto and installed on a Freescale imx6sx target board. They're simple modules, that are just used to read/write IO registers of the SoC.
Everything has worked well till I've load the modules after login, by simple calling in the target console
modprobe mydevice
Problems have begun since I've automatically load this modules at boot time. First time system startup after flashing my board, my custom modules are loaded and I can open them, do read/write operations...therefore they're working.
Restarting the system (switching it off and on or rebooting), modules are still loaded, but I'm unable to use them.
After some trials, what I've seen is that the problem is dynamic allocation of the device number they use, in particular function alloc_chrdev_region( ) called in the __init( ) function of the module.
In fact, really first time the system is switched on after has been flashed, the device file's device number in /dev is up-to-date with the /sys/class/myclass/mydevice/dev device number, while from the first reboot they doesn't match anymore. Briefly:
First boot:
/dev/mydevice device number [Major, Minor] = 247,0
/sys/class/myclass/mydevice/dev device number [Major, Minor] = 247,0
Second and later boots:
/dev/mydevice device number [Major, Minor] = 247,0
/sys/class/myclass/mydevice/dev device number [Major, Minor] = 248,0
So, it seems like the device file is kept in memory instead of been updated at every boot.
Here is main code for __init( ) and __exit( ) functions:
static struct class *g_deviceClass;
static struct cdev g_characterDevice;
static dev_t g_deviceNumber;
static int __init npe_power_drv_init(void)
{
int32_t result;
if ( alloc_chrdev_region(&g_deviceNumber, 0, 1, DEVICE_NAME) < 0 )
{
return -EAGAIN;
}
cdev_init(&g_characterDevice, &npe_power_drv_ops);
g_deviceClass = class_create(THIS_MODULE, CLASS_NAME);
if( device_create(g_deviceClass, NULL, g_deviceNumber, NULL, DEVICE_NAME) == NULL )
{
class_destroy(g_deviceClass);
unregister_chrdev_region(g_deviceNumber, 1);
return -EAGAIN;
}
result = cdev_add(&g_characterDevice, g_deviceNumber, 1);
if( result < 0 )
{
device_destroy(g_deviceClass, g_deviceNumber);
class_destroy(g_deviceClass);
unregister_chrdev_region(g_deviceNumber, 1);
return -EAGAIN;
}
return 0;
}
static void __exit npe_power_drv_exit(void)
{
device_destroy(g_deviceClass, g_deviceNumber);
class_destroy(g_deviceClass);
cdev_del(&g_characterDevice);
unregister_chrdev_region(g_deviceNumber, 1);
}
module_init(npe_power_drv_init);
module_exit(npe_power_drv_exit);
Using static device number allocation, register_chrdev_region( ) everything works fine. Sure I can use this latter way to implement my code, but I would like to know why I've got this behaviour.
Many thanks for any help
Andrea