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

Bjorn Helgaas
2024-11-05T16:12:31Z

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 <linux@weissschuh.net>
> ---
> 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
>