On Wed, Mar 17, 2021 at 09:52:32AM -0600, Gregory Anders wrote:
Hi all,
I'm writing a char device driver and am having trouble getting the file to appear under /dev.
This isn't my first rodeo: in fact, I've written a few other drivers in the past and they have all worked as expected. This driver is based on the source code of those other drivers, so I'm fairly confident I'm doing everything correctly. So I'm stumped and looking for help.
Here is what I have in my init function:
#define DRIVER_NAME "foo"
static int __init mod_init(void) { dev_t devno; my_class = class_create(THIS_MODULE, DRIVER_NAME); ret = alloc_chrdev_region(&devno, 0, MAX_DEVICES, DRIVER_NAME);
my_major = MAJOR(devno);
... }
(Note that for brevity I'm omitting a lot of boilerplate/error handling, etc. But you can assume it's all there).
My driver is also a network driver; in fact, it's *primarily* a network driver and it provides a char device file that is used to configure the hardware. I am creating the char device whenever the network device is first opened (e.g. 'netdev_open' below is the 'ndo_open' field of the 'struct netdev_ops'):
struct private_data { struct cdev cdev; ... }
static int netdev_open(struct net_device *dev) { struct private_data *priv = netdev_priv(dev); dev_t devno; int minor; struct device *d;
minor = ida_alloc_max(&ida, MAX_DEVICES, GFP_KERNEL);
devno = MKDEV(my_major, minor); cdev_init(&priv->cdev, &my_fops); cdev_add(&priv->cdev, devno, 1);
/* This should create a device node with the same name as the * network interface, e.g. foo0 */ d = device_create(my_class, NULL, devno, priv, dev->name); if (IS_ERR(d)) { ... }
... }
Again, I'm omitting the error checking for the sake of brevity, but it is there in the actual code. This function runs successfully and the network device is successfully opened. The 'device_create' function does not return an error, but there is nothing beneath /dev as I would expect.
I'm really stumped here because everything I've been able to find online says that 'device_create' ought to create that device file. I can see my class under /sys/class/ and that directory contains a directory with the name of the device:
$ ls -1 /sys/class/my_class/ foo0
so it looks like the char device *is* being created, there's just no corresponding entry under /dev.
Is your char device listed in /sys/dev/char/ ? If not, then you have something wrong with your call to device_create(). If so, then you need to look at whatever tool is creating your device nodes in /dev. Usually the kernel handles this with devtmpfs, but I do not know what your system uses for this. Do you have a pointer to the real source code anywhere to be able to see it better to see if you are doing something wrong with the call to device_create()? Also, why a full major? Why not just use the misc_dev api instead that does all of the above "housekeeping" for you automagically? thanks, greg k-h