Re: [PATCH 2/3] vfio/mdev: inline needed class_compat functionality

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Dec 03, 2024 at 09:11:47PM +0100, Heiner Kallweit wrote:
> vfio/mdev is the last user of class_compat, and it doesn't use it for
> the intended purpose. See kdoc of class_compat_register():
> Compatibility class are meant as a temporary user-space compatibility
> workaround when converting a family of class devices to a bus devices.

True, so waht is mdev doing here?

> In addition it uses only a part of the class_compat functionality.
> So inline the needed functionality, and afterwards all class_compat
> code can be removed.
> 
> No functional change intended. Compile-tested only.
> 
> Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
> ---
>  drivers/vfio/mdev/mdev_core.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
> index ed4737de4..a22c49804 100644
> --- a/drivers/vfio/mdev/mdev_core.c
> +++ b/drivers/vfio/mdev/mdev_core.c
> @@ -18,7 +18,7 @@
>  #define DRIVER_AUTHOR		"NVIDIA Corporation"
>  #define DRIVER_DESC		"Mediated device Core Driver"
>  
> -static struct class_compat *mdev_bus_compat_class;
> +static struct kobject *mdev_bus_kobj;



>  
>  static LIST_HEAD(mdev_list);
>  static DEFINE_MUTEX(mdev_list_lock);
> @@ -76,7 +76,7 @@ int mdev_register_parent(struct mdev_parent *parent, struct device *dev,
>  	if (ret)
>  		return ret;
>  
> -	ret = class_compat_create_link(mdev_bus_compat_class, dev, NULL);
> +	ret = sysfs_create_link(mdev_bus_kobj, &dev->kobj, dev_name(dev));

This feels really wrong, why create a link to a random kobject?  Who is
using this kobject link?

>  	if (ret)
>  		dev_warn(dev, "Failed to create compatibility class link\n");
>  
> @@ -98,7 +98,7 @@ void mdev_unregister_parent(struct mdev_parent *parent)
>  	dev_info(parent->dev, "MDEV: Unregistering\n");
>  
>  	down_write(&parent->unreg_sem);
> -	class_compat_remove_link(mdev_bus_compat_class, parent->dev, NULL);
> +	sysfs_remove_link(mdev_bus_kobj, dev_name(parent->dev));
>  	device_for_each_child(parent->dev, NULL, mdev_device_remove_cb);
>  	parent_remove_sysfs_files(parent);
>  	up_write(&parent->unreg_sem);
> @@ -251,8 +251,8 @@ static int __init mdev_init(void)
>  	if (ret)
>  		return ret;
>  
> -	mdev_bus_compat_class = class_compat_register("mdev_bus");
> -	if (!mdev_bus_compat_class) {
> +	mdev_bus_kobj = class_pseudo_register("mdev_bus");

But this isn't a class, so let's not fake it please.  Let's fix this
properly, odds are all of this code can just be removed entirely, right?

thanks,

greg k-h