Attr Groups in sysfs.

[Greg KH]

On Mon, Apr 18, 2011 at 04:29:57PM +0200, Gasparini Andrea wrote:
> Hi,
> 
> On Mon, Apr 18, 2011 at 4:14 PM, Greg KH <greg at kroah.com> wrote:
> >> I'm missing only the myattr_group name... anyone has a clue of how can
> >> I access this?
> >
> > You can't.
> 
> ok.
> 
> >> And adding to my kobject this way:
> >>     retval = sysfs_create_group(mykobj, &my_attr_group_0);
> >
> > Ick, no, please don't do that, you want the group to be created before
> > the device shows up to userspace, otherwise you just raced with
> > userspace and bad things will happen.
> 
> Understand, you're completely right.
> I'll try to refactor in a different way to avoid race.
> 
> > Why are you using a struct kobject directly and not a struct device
> > instead?  Please don't use kobjects unless you are doing something very
> > strange, and even then, please reconsider.
> >
> > Care to explain what you are using sysfs for?
> 
> Sure, It's quite complicated, in fact ;)
> I'm doing an "extension" for a video device, in terms of a module that
> handle some addictional logic that do some sort of stats about the
> stream passed through it.
> For example, I've a certain number of areas inside a frame, given in
> form of (x,y,width,heigth) that behave to the same device, so I'd like
> to call them: mydev/region_{0,1,2...}/{x,y,width,height}
> 
> That's why I'd like to have the "region_0" name, so I can distinguish
> between the right attribute group.

That's fine, but again, don't use a kobject, use the normal struct
device you already have in your driver for this device.

But wait, don't nest attributes that deep, an attribute group should
only be one level deep on a device, otherwise you are thinking it is a
real device that lives there, and userspace tools might have a hard time
finding the data.

Or, create a new "device" called region_*, attach it to a bus for your
device, and then use the attribute group for that device.

Yeah, it's a bit complex, but you are trying to get sysfs and the driver
model to do things it was not designed for, so you will have to write
more code than you might have thought.

Have you looked at the media device framework that the v4l layer now
provides you?  That might have enough functionality for what you are
wanting to do here.

And of course, you need to document your sysfs files properly with
entries in Documentation/ABI/

Good luck,

greg k-h