Re: class_device/udev support for tapes?

[Arnd Bergmann <arnd@arndb.de>]

On Tuesday 02 December 2003 19:00, Greg KH wrote:
> 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...

Do you mean using the same device class for both the tape and sg chardevs?
Maybe I have misunderstood the current relationship between 'class'
and 'class_interface'. I thought that both sg and tape need to be 
classes, with a scsi tape belonging to both classes and having multiple
tape interfaces. However, scsi currently has a scsi_device class and
registers an sg interface for that.

> > 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

Well, the first three are different representations of the same physical
scsi tape, while the ntibm0 tape is a different non-scsi tape and
also has other chardev representations that I skipped in the example.
They have different drivers (bus types, major numbers, available
chardevs...) but use the same ioctl interface and should therefore be
in the same class (or have the same interface?).

> > 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?

Yes. AFAIK, they are almost identical but have minor interface differences.
For s390 and ATAPI, there are only two variants: the default rewinding
devices (/dev/rtibm0, /dev/rtibm1, /dev/ht0, ...) rewind the medium upon
close, the non-rewinding devices (/dev/ntibm0, /dev/ntibm1, /dev/nt0, ...)
don't. Other drivers (scsi, ftape, ...) have up to eight device nodes
per physical device, but I don't know the exact differences. It's a
silly design, but too late to change.
To make things worse, some drivers (well, at least s390 tape) have a block
device interface in addition to the character devices.

	Arnd <><



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