Subject: Re: [PATCH v2 02/10] sysfs: introduce callback attribute_group::bin_size

From: Bjorn Helgaas <[email-masked]>
Date: Tue, 5 Nov 2024 10:12:30 -0600

On Sun, Nov 03, 2024 at 05:03:31PM +0000, Thomas Weißschuh wrote:
> Several drivers need to dynamically calculate the size of an binary
> attribute. Currently this is done by assigning attr->size from the
> is_bin_visible() callback.

s/an binary/a binary/

> This has drawbacks:
> * It is not documented.
> * A single attribute can be instantiated multiple times, overwriting the
>   shared size field.
> * It prevents the structure to be moved to read-only memory.
> 
> Introduce a new dedicated callback to calculate the size of the
> attribute.
> 
> Signed-off-by: Thomas Weißschuh <[email-masked]>
> ---
>  fs/sysfs/group.c      | 2 ++
>  include/linux/sysfs.h | 8 ++++++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/fs/sysfs/group.c b/fs/sysfs/group.c
> index 45b2e92941da1f49dcc71af3781317c61480c956..8b01a7eda5fb3239e138372417d01967c7a3f122 100644
> --- a/fs/sysfs/group.c
> +++ b/fs/sysfs/group.c
> @@ -98,6 +98,8 @@ static int create_files(struct kernfs_node *parent, struct kobject *kobj,
>  				if (!mode)
>  					continue;
>  			}
> +			if (grp->bin_size)
> +				size = grp->bin_size(kobj, *bin_attr, i);
>  
>  			WARN(mode & ~(SYSFS_PREALLOC | 0664),
>  			     "Attribute %s: Invalid permissions 0%o\n",
> diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
> index c4e64dc112063f7cb89bf66059d0338716089e87..4746cccb95898b24df6f53de9421ea7649b5568f 100644
> --- a/include/linux/sysfs.h
> +++ b/include/linux/sysfs.h
> @@ -87,6 +87,11 @@ do {							\
>   *		SYSFS_GROUP_VISIBLE() when assigning this callback to
>   *		specify separate _group_visible() and _attr_visible()
>   *		handlers.
> + * @bin_size:
> + *		Optional: Function to return the size of a binary attribute
> + *		of the group. Will be called repeatedly for each binary
> + *		attribute in the group. Overwrites the size field embedded
> + *		inside the attribute itself.

"Overwrites" suggests that we write over the size field in the single
shared attribute.  But that's not what create_files() does.

create_files() instantiates sysfs files from the attribute template.
Previously each instance used the size from the shared attribute.
With this patch, if ->bin_size() exists, its return value is the size
of this particular instance, over*riding* the default size from the
shared attribute.

This description follows the language of other function pointers,
which was the right approach.  But I think the existing language would
be more helpful if it called out the difference between the attribute
itself (a potentially read-only singleton structure shared by all
kobjects with this attribute) and the instantiation of that attribute
for each kobject.

For example,

  @bin_size:
	      Optional: Function to return the size of this kobject's
	      instantiation of a binary attribute.  If present, it is
	      called for each bin_attribute in the group and overrides
	      the default size from the bin_attribute template.

This is nice work, thanks for doing it!

>   * @attrs:	Pointer to NULL terminated list of attributes.
>   * @bin_attrs:	Pointer to NULL terminated list of binary attributes.
>   *		Either attrs or bin_attrs or both must be provided.
> @@ -97,6 +102,9 @@ struct attribute_group {
>  					      struct attribute *, int);
>  	umode_t			(*is_bin_visible)(struct kobject *,
>  						  struct bin_attribute *, int);
> +	size_t			(*bin_size)(struct kobject *,
> +					    const struct bin_attribute *,
> +					    int);
>  	struct attribute	**attrs;
>  	struct bin_attribute	**bin_attrs;
>  };
> 
> -- 
> 2.47.0
>