[alsa-devel] [RFC/PATCH v7 01/12] media: Media device node support

[Laurent Pinchart]

Hi Greg,

Thank you for the review.

On Thursday 23 December 2010 04:32:53 Greg KH wrote:
> On Mon, Dec 20, 2010 at 12:36:24PM +0100, Laurent Pinchart wrote:
> > The media_devnode structure provides support for registering and
> > unregistering character devices using a dynamic major number. Reference
> > counting is handled internally, making device drivers easier to write
> > without having to solve the open/disconnect race condition issue over
> > and over again.
> 
> What race condition are you referring to?

In a nutshell, the race between device disconnection, which results in the 
device instance being delete (if not in use of course), and open() calls from 
userspace. The problem has been solved in V4L a couple of releases ago after 
suffering from this race for a too long time. As V4L devices (and now media 
devices) need to create both a struct device and a struct cdev instance, 
careful locking is needed.

> > +config MEDIA_CONTROLLER
> > +	bool "Media Controller API (EXPERIMENTAL)"
> > +	depends on EXPERIMENTAL
> > +	---help---
> > +	  Enable the media controller API used to query media devices internal
> > +	  topology and configure it dynamically.
> > +
> > +	  This API is mostly used by camera interfaces in embedded platforms.
> 
> That's nice, but why should I enable this?  Or will drivers enable it
> automatically?

Drivers depending on the media controller API will enable this, yes. The 
option will probably removed later when the API won't be deemed as 
experimental anymore.

> > +#define MEDIA_NUM_DEVICES	256
> 
> Why this limit?

Because I'm using a bitmap to store the used minor numbers, and I thus need a 
limit. I could get rid of it of it by using a linked list, but that will not 
be efficient (you could argue that the list will hold a few entries only most 
of the time, but in that case a limit of 256 minors wouldn't be a problem 
:-)).

> > +#define MEDIA_NAME		"media"
> 
> Are you sure this is a good name for a camera?

It's not just camera. Media devices are... well, media devices. Basically 
anything that can handle audio and/or video streams. The media controller API 
can be used by plain audio devices.

> > +static dev_t media_dev_t;
> 
> Only one major number?  Is it always dynamic?

Yes, one major and (for now) 256 minors. Is there a problem with it being 
dynamic ?

> > +
> > +/*
> > + *	Active devices
> > + */
> > +static DEFINE_MUTEX(media_devnode_lock);
> > +static DECLARE_BITMAP(media_devnode_nums, MEDIA_NUM_DEVICES);
> > +
> > +/* Called when the last user of the media device exits. */
> > +static void media_devnode_release(struct device *cd)
> > +{
> > +	struct media_devnode *mdev = to_media_devnode(cd);
> > +
> > +	mutex_lock(&media_devnode_lock);
> > +
> > +	/* Delete the cdev on this minor as well */
> > +	cdev_del(&mdev->cdev);
> > +
> > +	/* Mark device node number as free */
> > +	clear_bit(mdev->minor, media_devnode_nums);
> > +
> > +	mutex_unlock(&media_devnode_lock);
> > +
> > +	/* Release media_devnode and perform other cleanups as needed. */
> > +	if (mdev->release)
> > +		mdev->release(mdev);
> > +}
> 
> You forgot to free the device structure here as well, right?

That will be done by the release callback. The media_devnode structure is 
embedded in the media_device structure, which will be embedded in driver-
specific structures.

> > +static ssize_t media_read(struct file *filp, char __user *buf,
> > +		size_t sz, loff_t *off)
> > +{
> > +	struct media_devnode *mdev = media_devnode_data(filp);
> > +
> > +	if (!mdev->fops->read)
> > +		return -EINVAL;
> > +	if (!media_devnode_is_registered(mdev))
> > +		return -EIO;
> 
> How could this happen?

This can happen when a USB device is disconnected for instance.

> And are you sure -EIO is correct?

-ENXIO is probably better (I always confuse that with -ENODEV).

> > +	return mdev->fops->read(filp, buf, sz, off);
> > +}
> > +
> > +static ssize_t media_write(struct file *filp, const char __user *buf,
> > +		size_t sz, loff_t *off)
> > +{
> > +	struct media_devnode *mdev = media_devnode_data(filp);
> > +
> > +	if (!mdev->fops->write)
> > +		return -EINVAL;
> > +	if (!media_devnode_is_registered(mdev))
> > +		return -EIO;
> 
> Same as above, and same comment in other places (poll, ioctl.)

OK.

> > +/* Override for the open function */
> > +static int media_open(struct inode *inode, struct file *filp)
> > +{
> > +	struct media_devnode *mdev;
> > +	int ret;
> > +
> > +	/* Check if the media device is available. This needs to be done with
> > +	 * the media_devnode_lock held to prevent an open/unregister race:
> > +	 * without the lock, the device could be unregistered and freed between
> > +	 * the media_devnode_is_registered() and get_device() calls, leading to
> > +	 * a crash.
> > +	 */
> > +	mutex_lock(&media_devnode_lock);
> > +	mdev = container_of(inode->i_cdev, struct media_devnode, cdev);
> 
> By virtue of having the reference to the module held by the vfs, this
> shouldn't ever go away, even if the lock is not held.

inode->i_cdev is set to NULL by cdev_default_release() which can be called 
from media_devnode_unregister(). I could move to container_of outside the 
lock, but in that case I would have to check for mdev == NULL || 
!mdev_devnode_is_registered(mdev) (or move the NULL check inside 
mdev_devnode_is_registered). Is that what you would like ?

> > +	/* return ENXIO if the media device has been removed
> > +	   already or if it is not registered anymore. */
> > +	if (!media_devnode_is_registered(mdev)) {
> > +		mutex_unlock(&media_devnode_lock);
> > +		return -ENXIO;
> > +	}
> 
> So you can unregister a device at any time, even if the device is open,
> or about to be opened?

That's correct. That way drivers don't need to care about unregister/open 
races, media_devnode will handle it for them.

> Then that's fine, but you can put the lock after the container_of(), right?

If I add a NULL check (as explained above), yes.

> > +	/* and increase the device refcount */
> > +	get_device(&mdev->dev);
> 
> How is that holding anything into memory?

That will prevent the device instance from being freed until the device is 
closed, thereby holding both the device instance and the cdev instance in 
memory.

> Don't you want to keep the module that the fops pointer in the device in
> memory, not necessarily the device itself?

The cdev owner pointer is set to the fops owner. Unless I'm mistaken it will 
keep the module in memory. I need to keep the device in memory (or rather the 
media_devnode structure that embeds it) to handle file operations on a device 
that gets unregistered after it has been opened.

> > +	mutex_unlock(&media_devnode_lock);
> > +
> > +	filp->private_data = mdev;
> > +
> > +	if (mdev->fops->open) {
> > +		ret = mdev->fops->open(filp);
> > +		if (ret) {
> > +			put_device(&mdev->dev);
> > +			return ret;
> > +		}
> > +	}
> > +
> > +	return 0;
> > +}
> 
> No reference counting for the fops?  Why not?

Because cdev already increments the owner refcount on open().

> Anyway, it looks like what you really want is an "easier" way to handle
> a cdev and a struct device that will export the proper information to
> userspace, right?
> 
> Why not do this generically, fixing up the cdev interface (which really
> needs it) and not tie it to media devices at all, making it possible for
> _everyone_ to use this type of infrastructure?
> 
> That seems like the better thing to do here.

Sounds like a good idea. You're a better cdev expert than me, so could you 
give me a few pointers ? Do you want me to create a new object that will hold 
a struct cdev and a struct device together, or to embed the device structure 
into the existing cdev structure ?

-- 
Regards,

Laurent Pinchart