RE: sysfs attribute example

["Stephen Morgan" <stephenm@glw.com>]

> -----Original Message-----
> From: Greg KH [mailto:greg@kroah.com]
> Sent: Monday, June 13, 2005 6:54 PM
> To: Stephen Morgan
> Cc: Kay Sievers; linux-hotplug-devel
> Subject: Re: sysfs attribute example
>
>
> On Mon, Jun 13, 2005 at 06:01:26PM -0500, Stephen Morgan wrote:
> > > -----Original Message-----
> > > From: Kay Sievers [mailto:kay.sievers@vrfy.org]
> > > Sent: Friday, June 10, 2005 11:10 AM
> > > To: Stephen Morgan
> > > Cc: linux-hotplug-devel
> > > Subject: Re: sysfs attribute example
> > >
> > >
> > > On Fri, Jun 10, 2005 at 12:05:07PM -0500, Stephen Morgan wrote:
> > > > Hello all,
> > > >
> > > > Can someone point me to an example of exporting a sysfs
> attribute from a
> > > > driver?  (Or tell me if I shouldn't.)  I'm trying to get a
> > > value from my USB
> > > > device driver's probe() into an attribute so my udev .rules
> > > file can pick it
> > > > up.
> > > >
> > > > I've tried to follow what's in LDD 3rd ed. (page 374)
> > > "...simply fill in an
> > > > attribute and pass it to sysfs_create_file()"
> > > >
> > > > From my probe() function:
> > > >
> > > >   mydev->my_attribute.name = "my_attribute_name";
> > > >   mydev->my_attribute.owner = THIS_MODULE;
> > > >   mydev->my_attribute.mode = S_IRUGO;
> > > >
> > > >   sysfs_create_file(&(interface->class_dev->kobj),
> > > &(mydev->my_attribute));
> > > >
> > > > When a device is probed, the file "my_attribute_name" shows up
> > > in sysfs, but
> > > > I get a "kernel NULL pointer dereference at virtual address
> 00000004"
> > > > (address changes with the USB port I plug into).
> > > >
> > > > I'm also not seeing how to attach a value to the attribute.
> > > So, clearly,
> > > > I'm missing something crucial here :-)
> > >
> > > Sure, you do. :)
> > >
> > > Look for: DEVICE_ATTR() in the kernel code.
> > >
> > > Good luck,
> > > Kay
> > >
> >
> > Hi Kay,
> >
> > Thanks for your help.
> >
> > This has gotten me much closer.  However, the show() function
> seems to be
> > receiving a pointer to the usb_interface's class_device instead of its
> > device.  Any ideas why?
>
> Because that's where you registered the attribute?  :)

Okay, but if you register with the device (as opposed to the class_device),
you don't get the SYSFS attribute created.

>
> Care to show us your patch?

Here are the pertinent lines (the interim solution used here is to cast the
device pointer to a class_device pointer):

**********************************************
  static ssize_t mydev_number_show(struct device *dev, char *buf)
  {
    int retval = 0;
    struct my_dev *mydev = NULL;
    int index;
    struct class_device * cdev = (struct class_device *) dev;

    printk(" dev pointer: %p\n", cdev->dev);
    printk(" kobject name: %s\n", cdev->dev->kobj.k_name);
    printk(" bus_id: %s\n", cdev->dev->bus_id);

    mydev = cdev->dev->driver_data;
    index = mydev->index;
    printk(" my device index: %d\n", index);

    return sprintf(buf, "%d\n",index);
  }

  static DEVICE_ATTR(mydev_number, S_IRUGO, mydev_number_show, NULL);

And in my probe() function:

  usb_register_dev(interface, &my_class);
  usb_set_intfdata(interface, mydev);
  sysfs_create_file(&(interface->class_dev->kobj),
&(dev_attr_mydev_number.attr));

************************************************

Thanks,

Stephen



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