Re: [PATCHv8 07/15] cec: add HDMI CEC framework

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

On Tue, Aug 18, 2015 at 11:00:20AM +0100, Russell King - ARM Linux wrote:
> On Tue, Aug 18, 2015 at 10:26:32AM +0200, Hans Verkuil wrote:
> > +	/* Part 2: Initialize and register the character device */
> > +	cdev_init(&cecdev->cdev, &cec_devnode_fops);
> > +	cecdev->cdev.owner = owner;
> > +
> > +	ret = cdev_add(&cecdev->cdev, MKDEV(MAJOR(cec_dev_t), cecdev->minor),
> > +									1);
> > +	if (ret < 0) {
> > +		pr_err("%s: cdev_add failed\n", __func__);
> > +		goto error;
> > +	}
> > +
> > +	/* Part 3: Register the cec device */
> > +	cecdev->dev.bus = &cec_bus_type;
> > +	cecdev->dev.devt = MKDEV(MAJOR(cec_dev_t), cecdev->minor);
> > +	cecdev->dev.release = cec_devnode_release;
> > +	if (cecdev->parent)
> > +		cecdev->dev.parent = cecdev->parent;
> > +	dev_set_name(&cecdev->dev, "cec%d", cecdev->minor);
> > +	ret = device_register(&cecdev->dev);
> 
> It's worth pointing out that you can greatly simplify the lifetime
> handling (you don't need to get and put cecdev->dev) if you make
> the cdev a child of the cecdev->dev.
> 
> If you grep for kobj.parent in drivers/ you'll see many drivers are
> doing this.
> 
> 	cecdev->cdev.kobj.parent = &cecdev->dev.kobj;
> 
> but you will need to call device_initialize() on cecdev->dev first,
> and use device_add() here.

This is basically a requirement if one embeds both device and a cdev
into the same structure. Trying to do get/put in the driver is racy,
you need to let framework know (by setting cdve's parent to the device
structure).

Thanks.

-- 
Dmitry