Simplifying devfs: minor == unit

[Ed Schouten]

* Kostik Belousov <kostikbel at gmail.com> wrote:
> > - I've seen most drivers only use the device cloner, because they need
> >   descriptor local storage. It turns out more drivers need this than I
> >   initially thought. kib@ has a patch for this, so I hope this gets
> >   committed one of these {days,weeks,months}.
> The patch was committed ~ a week ago.

Great. Looks like I wasn't paying attention back then.

> > - After we've got file descriptor local storage, I think we can live
> >   without the cloner. This means we could consider removing the minor
> >   number argument from make_dev(), removing the unique unit number
> >   restriction we currently have inside devfs, which causes many drivers
> >   to use number pools for no obvious reason.
> I think we cannot live without clones regardless of devfs_cdevpriv.
> The model assumed for the pty, snp and probably several other devices
> actually requires new cdev instead of the priv data.

The pty driver does not use the clone_* interface. It only uses the
eventhandler, which should indeed be left intact. The snp driver does
use the clone_* interface, but not in a way that can't be done using the
eventhandler, validating the device name and calling make_dev()
directly.

Please take a look at src/usr.sbin/watch/watch.c:open_snp(). We might as
well turn snp(4) into a single /dev/snp, where the kernel space driver
uses per-descriptor data to distinguish the instances. This provides
some advantages:

- No more silly open()-loops.

- A system administrator can change the permissions on /dev/snp, which
  automatically sets a system wide policy, instead on one of the device
  nodes.

- We don't fill up the system with a lot of unused nodes.

	for i in `seq 1000`
	do
		ls /dev/bpf$i > /dev/null
	done

> > I was thinking about discussing this patch with my mentor + committing
> > it somewhere in the nearby future. Any comments?
> 
> Making minor == unit number looks to be not a bad idea, please, look at
> the saga of the tty_pty.c revs. 1.153, 156, 1.157. Making the devices use
> si_drv0 directly probably is not so good since we remove the indirection
> layer that is already present and allows for some (minor) freedom in the
> devfs/kern_conf implementation.

But why isn't this done for si_drv1 and si_drv2 then? My idea is to turn
si_drv0 in an integer field that can be freely used. There is reason to
force a policy on this field.

-- 
 Ed Schouten <ed at 80386.nl>
 WWW: http://80386.nl/
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