Re: class_device/udev support for tapes?

[Greg KH <greg@kroah.com>]

On Tue, Dec 02, 2003 at 06:35:18PM +0100, Arnd Bergmann wrote:
> Has anyone come up with a solution for using udev for tape devices?

The scsi people need to add support for this into the st driver.  They
also need to add it to their sg driver for scsi generic devices.  They
know about this, and hopefully are working on it...

> Unlike block devices, there is no master device with anything
> similar to partitions, but all the interfaces are equal. One way
> to represent them would be to create a class device without a 
> dev entry and have the chardev interfaces as child kobjects of that:
> 
> /sys/class/tape/st0/st0/dev
> /sys/class/tape/st0/st0a/dev
> /sys/class/tape/st0/nst0/dev
> /sys/class/tape/st0/device -> ../../...
> /sys/class/tape/rtibm0/ntibm0/dev

What is "st0" here?  Why not just:

/sys/class/scsi_tape/st0/dev
/sys/class/scsi_tape/st0a/dev
/sys/class/scsi_tape/nst0/dev
/sys/class/scsi_tape/ntibm0/dev

> This would of course be inconsistant with all the other classes.
> The second option I see is naming the 'dev' attribute differently:
> 
> /sys/class/tape/st0/dev-st
> /sys/class/tape/st0/dev-sta
> /sys/class/tape/st0/dev-nst

Hm, I probably don't understand tapes.  What's the difference between
st, sta, and nst?  Are they all different char devices for the same tape
device?  Doing different things?

> This is also somewhat inconsistant from udev's point of view, but not
> so much from the kernel.

True, but we can always work around that in udev, if it's necessary.

> The third option is to simply register one class device for each logical
> device:
> 
> /sys/class/tape/st0/dev
> /sys/class/tape/st0/device -> ../../...
> /sys/class/tape/st0a/dev
> /sys/class/tape/st0a/device -> ../../...
> /sys/class/tape/nst0/dev
> /sys/class/tape/nst0/device -> ../../...
> /sys/class/tape/rtibm0/dev
> 
> This is at least possible with the least number of changes to udev
> and the view of class devices, but not as intuitive for the user.
> 
> I am totally unsure which option should be used or if there is maybe
> a better way that I  failed to see.
> 
> The other question is how to best design the kernel interface for 
> registering a tape class device with a variable number of chardev nodes.

You might want to take this to the linux-scsi list, as the people there
should know how to do this.

thanks,

greg k-h


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