Writing a Sysfs module

Question

I'm attempting to learn how to write a sysfs module, and am confused at the basic initialization. In this document looking at the kobject.h file, there are several different functions related to creating a sysfs entry.

From the looks of it, the function "kobject_init_and_add" seems like the right thing to use, which takes the following:

 90 int kobject_init_and_add(struct kobject *kobj,
 91                          struct kobj_type *ktype, struct kobject *parent,
 92                          const char *fmt, ...);

struct kobject and struct kobj_type are straightforward enough, but I don't understand what the *parent kobject and *fmt cstring are supposed to be.

Further, after initializing these objects, I would need to remove them at the exit_module function, but there are two options that seem possible: kobject_del and kobject_puts. What are the differences between these?

Part of my confusion comes from the fact that while googling for the answer, I see tutorials which says to use functions like kobject_register instead, but that function doesn't actually exist.

vinod maverick · Accepted Answer · 2017-01-31 12:11:31Z

2

Yes there are lot of example on mainline kernel which you can refers for your implementatin. For Your doubts I am adding the some example code"

Module Probe/init function

static struct kobject   *module_kobject;
module_kobject=kobject_create_and_add("module_status",NULL);
sysfs_create_group(module_kobject,&module_attr);

Module Remove/exit function

sysfs_remove_group(module_kobject,&module_attr);
kobject_put(module_kobject);

If you want to expose more than one attribute on the user space; than you need to define the group as well

static struct attribute_group module_attr={
.attrs = module_attribute,
};

There is some more implementation and function you may need like:

static ssize_t module_show_status(struct kobject *kobj,struct kobj_attribute *attr,char *buf);
static ssize_t module_store__status(struct kobject *kobj,struct kobj_attribute *attr,const char *buf,size_t len);

I think you can start your sysfs module implementation based on the above code and Feel free for any help.

edited Jan 31, 2017 at 12:11

answered Jan 21, 2017 at 17:28

vinod maverick

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4 Comments

Zephyr Over a year ago

So for attributes like the &module_attr in the sysfs_create_group example, where is that &module_attr being defined? Because that seems to be where I am having trouble. I had read you could use the __ATTR macro but that is causing errors for me.

vinod maverick Over a year ago

You can define like the below structure static struct attribute_group module_attr={ .attrs = module_attribute, };

Zephyr Over a year ago

Understood, my question is, does the 'module_attribute' you use there need to be defined in advance, or just declared?

vinod maverick Over a year ago

You need to define the attribute of module_attribute, I am just posting the way you need to implement in your code: static ssize_t module_show_minimum_value(struct kobject *kobj,struct kobj_attribute *attr,char *buf){ // Add your code }; static struct kobj_attribute module_attribute= __ATTR(min_value,0777,module_show_minimum_value ,NULL); static struct attribute_group module_attr={ .attrs = module_attribute, };

Rami Rosen · Accepted Answer · 2017-01-20 21:41:44Z

0

There are many kernel modules that create sysfs entries. For example, http://lxr.free-electrons.com/source/net/bridge/br_sysfs_br.c This module uses kobject_create_and_add(), which gets as a parameter a kobject instance, created by sysfs_create_group(). I believe that looking into such module, and trying to code step by step, following the patterns in that module, can help. Also look in http://lxr.free-electrons.com/source/Documentation/kobject.txt

Rami Rosen

answered Jan 20, 2017 at 21:41

Rami Rosen

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1 Comment

Zephyr Over a year ago

Thank you, I will try this. Are you aware of any such example I could look at for instantiating an attribute? The sysfs you link above has an instance where several attributes are bundled together into an attribute group, and then assigned, but I don't quite understand how to create those attributes to begin with.

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