Date: Thu, 11 Dec 2008 13:59:24 +0100
From: Cornelia Huck <cornelia.huck@de.ibm.com>
To: Mark McLoughlin <markmc@redhat.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>, linux-kernel@vger.kernel.org,
       virtualization@lists.linux-foundation.org,
       Anthony Liguori <aliguori@us.ibm.com>,
       Kay Sievers <kay.sievers@vrfy.org>, Greg KH <gregkh@suse.de>,
       Mark McLoughlin <markmc@redhat.com>
Subject: Re: [PATCH 2/6] virtio: add register_virtio_root_device()
Message-ID: <20081211135924.4394cc56@gondolin>
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On Wed, 10 Dec 2008 17:45:35 +0000,
Mark McLoughlin <markmc@redhat.com> wrote:

> Add a function to allocate a root device object to group the
> devices from a given virtio implementation.
> 
> Also add a 'module' sysfs symlink to allow so that userspace
> can generically determine which virtio implementation a
> device is associated with. This will be used by Fedora
> mkinitrd to generically determine e.g. that virtio_pci is
> needed to mount a given root filesystem.

Nothing about this is really virtio-specific (just as
s390_root_dev_register() is not really s390-specific), and a 'module'
symlink doesn't really hurt in a generic implementation, even if it is
unneeded. I'm voting to put this in some generic, always built-in code
(or have the users select it) so we could also use it from s390.

> 
> Signed-off-by: Mark McLoughlin <markmc@redhat.com>
> ---
>  drivers/virtio/virtio.c |   71 +++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/virtio.h  |   10 ++++++
>  2 files changed, 81 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> index 018c070..61e6597 100644
> --- a/drivers/virtio/virtio.c
> +++ b/drivers/virtio/virtio.c
> @@ -1,6 +1,7 @@
>  #include <linux/virtio.h>
>  #include <linux/spinlock.h>
>  #include <linux/virtio_config.h>
> +#include <linux/err.h>
> 
>  /* Unique numbering for virtio devices. */
>  static unsigned int dev_index;
> @@ -200,6 +201,76 @@ void unregister_virtio_device(struct virtio_device *dev)
>  }
>  EXPORT_SYMBOL_GPL(unregister_virtio_device);
> 
> +/* A root device for virtio devices from a given backend.  This makes them
> + * appear as /sys/devices/{name}/0,1,2 not /sys/devices/0,1,2. It also allows
> + * us to have a /sys/devices/{name}/module symlink to the backend module. */
> +struct virtio_root_device
> +{
> +	struct device dev;
> +	struct module *owner;
> +};
> +
> +static struct virtio_root_device *to_virtio_root(struct device *dev)
> +{
> +        return container_of(dev, struct virtio_root_device, dev);
> +}
> +
> +static void release_virtio_root_device(struct device *dev)
> +{
> +	struct virtio_root_device *root = to_virtio_root(dev);
> +	if (root->owner)
> +		sysfs_remove_link(&root->dev.kobj, "module");
> +	kfree(root);
> +}

Can this code be a module? If yes, move the release callback to a
build-in as there are races with release-functions in modules.

> +
> +struct device *__register_virtio_root_device(const char *name,
> +					     struct module *owner)
> +{
> +	struct virtio_root_device *root;
> +	int err = -ENOMEM;
> +
> +	root = kzalloc(sizeof(struct virtio_root_device), GFP_KERNEL);
> +	if (!root)
> +		goto out;
> +
> +	err = dev_set_name(&root->dev, name);
> +	if (err)
> +		goto free_root;
> +
> +	err = device_register(&root->dev);
> +	if (err)
> +		goto free_root;
> +
> +	root->dev.parent  = NULL;
> +	root->dev.release = release_virtio_root_device;

You must set ->release before calling device_register(), and setting
the parent is unneeded.

> +
> +	if (owner) {
> +		struct module_kobject *mk = &owner->mkobj;
> +
> +		err = sysfs_create_link(&root->dev.kobj, &mk->kobj, "module");
> +                if (err) {
> +			device_unregister(&root->dev);
> +			return ERR_PTR(err);
> +		}
> +
> +		root->owner = owner;
> +	}
> +
> +	return &root->dev;
> +
> +free_root:
> +	kfree(root);

You need to call device_put() if you called device_register().

> +out:
> +	return ERR_PTR(err);
> +}
> +EXPORT_SYMBOL_GPL(__register_virtio_root_device);
> +
> +void unregister_virtio_root_device(struct device *root)
> +{
> +	device_unregister(root);
> +}
> +EXPORT_SYMBOL_GPL(unregister_virtio_root_device);
> +
>  static int virtio_init(void)
>  {
>  	if (bus_register(&virtio_bus) != 0)
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index 06005fa..66e6c67 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -93,6 +93,16 @@ struct virtio_device
>  int register_virtio_device(struct virtio_device *dev);
>  void unregister_virtio_device(struct virtio_device *dev);
> 
> +/* A root device is a dummy device used to group virtio devices from each
> + * implementation. */
> +struct device *__register_virtio_root_device(const char *name,
> +					     struct module *owner);
> +static inline struct device *register_virtio_root_device(const char *name)
> +{
> +	return __register_virtio_root_device(name, THIS_MODULE);
> +}
> +void unregister_virtio_root_device(struct device *root);
> +
>  /**
>   * virtio_driver - operations for a virtio I/O driver
>   * @driver: underlying device driver (populate name and owner).
> -- 
> 1.5.4.3
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