How to attach file operations to sysfs attribute in platform driver?

Question

I wrote a platform driver for a peripheral we developed and would like to expose some configuration options to the sysfs. I have managed to create the appropriate files using attribute structs (see below) and sysfs_create_file in the probe function, but I can't figure out how to attach the show/store functions to the structs in a platform driver.

Most resources I found online used a device_attribute struct or something similar to create their files, is that also appropriate here? Is there another way to do this for a platform driver?

My attribute struct looks like this:

struct attribute subkey_attr = {
    .name = "subkeys",
    .mode = S_IWUGO | S_IRUGO,
};

And I register the file using this call:

riddler_kobject = &pdev->dev.kobj;
ret_val = sysfs_create_file(riddler_kobject, &subkey_attr);

Answer 1

It boils down to next:

• reuse existing kobject from struct device (from your struct platform_device) for sysfs_create_group() (instead of creating your own kobject)
• use DEVICE_ATTR() to declare struct device_attribute instead of regular __ATTR(), which creates struct kobj_attribute.

Here is how I created sysfs attributes for my platform driver.

1. Create structure you'll be using as private data in show() / store() operations for your sysfs attribute (file). For example:

   struct mydrv {
       struct device *dev;
       long myparam;
   };
   

2. Allocate this structure in your driver's probe():

   static int mydrv_probe(struct platform_device *pdev)
   {
       struct mydrv *mydrv;
   
       mydrv = devm_kzalloc(&pdev->dev, sizeof(*mydrv), GFP_KERNEL);
       mydrv->dev = &pdev->dev;
       platform_set_drvdata(pdev, mydrv);
   
       ...
   }
   

3. Create show() / store() functions:

   static ssize_t mydrv_myparam_show(struct device *dev,
           struct device_attribute *attr, char *buf)
   {
       struct mydrv *mydrv = dev_get_drvdata(dev);
       int len;
   
       len = sprintf(buf, "%d\n", mydrv->myparam);
       if (len <= 0)
           dev_err(dev, "mydrv: Invalid sprintf len: %d\n", len);
   
       return len;
   }
   
   static ssize_t mydrv_myparam_store(struct device *dev,
           struct device_attribute *attr, const char *buf, size_t count)
   {
       struct mydrv *mydrv = dev_get_drvdata(dev);
   
       kstrtol(buf, 10, &mydrv->myparam);
       return count;
   }
   

4. Create device attribute for those functions (right after those functions):

   static DEVICE_ATTR(myparam, S_IRUGO | S_IWUSR, mydrv_myparam_show,
                      mydrv_myparam_store);
   

5. Declare attributes table (listing in fact sysfs files for you driver):

   static struct attribute *mydrv_attrs[] = {
       &dev_attr_myparam.attr,
       NULL
   };
   

6. Declare attribute group (specifying in fact sysfs directory for your driver):

   static struct attribute_group mydrv_group = {
       .name = "mydrv",
       .attrs = mydrv_attrs,
   };
   
   static struct attribute_group *mydrv_groups[] = {
       &mydrv_group,
       NULL
   }
   

   which can be actually replaced with one line:

   ATTRIBUTE_GROUPS(mydrv);
   

7. Create sysfs directory and files in your driver's probe() function:

   static int mydrv_probe(struct platform_device *pdev)
   {
       int ret;
   
       ...
   
       ret = sysfs_create_group(&pdev->dev.kobj, &mydrv_group);
       if (ret) {
           dev_err(&pdev->dev, "sysfs creation failed\n");
           return ret;
       }
   
       ...
   }
   

8. Remove your sysfs files in your driver's remove() function:

   static int mydrv_remove(struct platform_device *pdev)
   {
       sysfs_remove_group(&pdev->dev.kobj, &mydrv_group);
       ...
   }
   

Race condition note

As @FranzForstmayr correctly pointed out, there may be race condition when adding sysfs files with sysfs_create_group() in mydrv_probe(). That's because user-space can be already notified that those files exist before mydrv_probe() called (where those files are actually being created by sysfs_create_group() function). This issue covered in details in "How to Create a sysfs File Correctly" article by Greg Kroah-Hartman.

So in our case of platform_device, instead of calling sysfs_create_group() (and its counterpart sysfs_remove_group()), you can use default attribute group. To do so, you need to assign corresponding .groups field of your struct device to your attribute groups variable:

static int mydrv_probe(struct platform_device *pdev)
{
    ...

    pdev->dev.groups = mydrv_groups;

    ...
}

DISCLAIMER: I didn't test this code, though it should work, because of this code.

See [1,2,3] links for more insights on mentioned race condition.

For more examples, run next command in kernel source directory:

$ git grep -l --all-match -e platform_device -e attribute -e '\.groups =' -- drivers/

Also you can search by "default attribute" in commit messages:

$ git log --no-merges --oneline --grep="default attribute" -- drivers/

Some commits I found this way: [4,5,6,7].

References

[1] My attributes are way too racy, what should I do?

[2] PATCH: sysfs: add devm_sysfs_create_group() and friends

[3] [GIT PATCH] Driver core patches for 3.11-rc2

[4] commit 1

[5] commit 2

[6] commit 3

[7] commit 4