Re: [PATCH v2 12/14] fs/configfs: Add a callback to determine attribute visibility

[Dan Williams <dan.j.williams@intel.com>]

Tom Lendacky wrote:
> On 3/13/24 16:37, Joel Becker wrote:
> > On Fri, Mar 08, 2024 at 12:35:27PM -0600, Tom Lendacky wrote:
> >> In order to support dynamic decisions as to whether an attribute should be
> >> created, add a callback that returns a bool to indicate whether the
> >> attribute should be display. If no callback is registered, the attribute
> >> is displayed by default.
> > 
> > I'm curious what the strong value is in this extra callback.  As opposed
> > to not generating the attribute in the absence of a TPM (why create a
> > config_item at all?), merely having an empty response from the attribute,
> > or having `->show()` return -ENODEV or similar.
> 
> The value is to reduce the complexity of registering with the TSM support 
> across multiple vendors. There is a base set of attributes that are common 
> across vendors and some that are specific to vendors. Creating this 
> structure in the TSM support can get unwieldy. This would make it simple 
> to determine if support is provided since the attribute will either be 
> present or not.
> 
> This would also make the support similar to sysfs in the ability to 
> dynamically hide or show attributes.
> 
> > 
> >>
> >> Cc: Joel Becker <jlbec@evilplan.org>
> >> Cc: Christoph Hellwig <hch@lst.de>
> >> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> >> ---
> >>   fs/configfs/file.c       |  13 +++++
> >>   include/linux/configfs.h | 114 +++++++++++++++++++++++++++------------
> >>   2 files changed, 93 insertions(+), 34 deletions(-)
> >>
> >> diff --git a/fs/configfs/file.c b/fs/configfs/file.c
> >> index 0ad32150611e..c758bcc11235 100644
> >> --- a/fs/configfs/file.c
> >> +++ b/fs/configfs/file.c
> >> @@ -451,6 +451,12 @@ int configfs_create_file(struct config_item * item, const struct configfs_attrib
> >>   	umode_t mode = (attr->ca_mode & S_IALLUGO) | S_IFREG;
> >>   	int error = 0;
> >>   
> >> +	if (attr->ca_is_visible) {
> >> +		mode = attr->ca_is_visible(item, attr);
> >> +		if (!mode)
> >> +			return 0;
> > 
> > What value do we get from carrying the mode through here?  The API
> > proposed is "visible or not", which is a boolean.  Overloading that with
> > "also set the mode" is confusing, and it also can lead to the divergent
> > codepath problem you mentioned in your response, where
> > `->ca_is_visible()` fails to return the mode correctly.  If this was simpl
> > a boolean hook, the code could read like so:
> 
> A boolean would work. There was a request to make this similar to the 
> sysfs attribute visibility. I certainly can make this a simple bool 
> function if that is preferable.

The inspiration was sysfs is_visible() and an idea to reproduce the
"static declaration + dynamic visibility" model that sysfs allows.
However, in the near term boolean visibility is sufficient since the
attributes are either on/off by vendor not read-write/read-only by
vendor implementation.