cdev_init and struct file_operations

cdev_init and struct file_operations

[John de la Garza]

In the book Linux Device Drivers a struct cdev is setup like this:

static void scull_setup_cdev(struct scull_dev *dev, int index)
{
    int err, devno = MKDEV(scull_major, scull_minor + index);
    
    cdev_init(&dev->cdev, &scull_fops);
    dev->cdev.owner = THIS_MODULE;
    dev->cdev.ops = &scull_fops;
...


the code for cdev_init does this:
void cdev_init(struct cdev *cdev, const struct file_operations *fops)
{
        memset(cdev, 0, sizeof *cdev);
        INIT_LIST_HEAD(&cdev->list);
        kobject_init(&cdev->kobj, &ktype_cdev_default);
        cdev->ops = fops;
}

Why does the code from ldd3 set dev->cdev.ops = &scull_fops again?

Doesn't cdev_init already do that?

cdev_init and struct file_operations

[Valdis.Kletnieks at vt.edu]

On Tue, 26 Aug 2014 20:48:41 -0400, John de la Garza said:
> In the book Linux Device Drivers a struct cdev is setup like this:
>
> static void scull_setup_cdev(struct scull_dev *dev, int index)
> {
>     int err, devno = MKDEV(scull_major, scull_minor + index);
>
>     cdev_init(&dev->cdev, &scull_fops);
>     dev->cdev.owner = THIS_MODULE;
>     dev->cdev.ops = &scull_fops;
> ...
>
>
> the code for cdev_init does this:
> void cdev_init(struct cdev *cdev, const struct file_operations *fops)
> {
>         memset(cdev, 0, sizeof *cdev);
>         INIT_LIST_HEAD(&cdev->list);
>         kobject_init(&cdev->kobj, &ktype_cdev_default);
>         cdev->ops = fops;
> }
>
> Why does the code from ldd3 set dev->cdev.ops = &scull_fops again?

Probably because when ldd3 was written several years ago, cdev_init()
didn't set it, so it wasn't "again"....

> Doesn't cdev_init already do that?

It does *now*.

Remember that ldd3 was written about 6 million lines of code ago....
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cdev_init and struct file_operations

[John de la Garza]

On Tue, Aug 26, 2014 at 09:18:57PM -0400, Valdis.Kletnieks at vt.edu wrote:
> On Tue, 26 Aug 2014 20:48:41 -0400, John de la Garza said:
> > Why does the code from ldd3 set dev->cdev.ops = &scull_fops again?
> 
> Probably because when ldd3 was written several years ago, cdev_init()
> didn't set it, so it wasn't "again"....
> 
> > Doesn't cdev_init already do that?
> 
> It does *now*.
> 
> Remember that ldd3 was written about 6 million lines of code ago....

It did then, too (unless I'm missing something)

linux$  git branch
* (detached from v2.6.12)
  master

from v2.6.12

void cdev_init(struct cdev *cdev, struct file_operations *fops)
{
        memset(cdev, 0, sizeof *cdev);
        INIT_LIST_HEAD(&cdev->list);
        cdev->kobj.ktype = &ktype_cdev_default;
        kobject_init(&cdev->kobj);
        cdev->ops = fops;
}