linux-kernel - Re: RFC: Add USBDEVFS_TRY

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Date:	Wed, 24 Aug 2011 17:18:57 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Greg KH <greg@...ah.com>
cc:	Hans de Goede <hdegoede@...hat.com>,
	USB list <linux-usb@...r.kernel.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: RFC: Add USBDEVFS_TRY_DISCONNECT ioctl

On Wed, 24 Aug 2011, Greg KH wrote:

> On Wed, Aug 24, 2011 at 04:32:31PM -0400, Alan Stern wrote:
> > Okay, here's a sample patch.  Actually it's three patches, listed one 
> > after another, but people can apply it like a single patch.
> > 
> >      1.	Introduce the USBDEVFS_TRY_DISCONNECT ioctl and the check_busy
> > 	callback it uses.  Implement the callback in the usbfs driver; 
> > 	this gives a way for programs to unbind kernel drivers without
> > 	unbinding other userspace drivers.
> > 
> >      2.	Implement device-file reference tracking in the SCSI layer,
> > 	and the device_open and device_close callbacks it uses.
> 
> Does this handle if the filesystem is being created or fscked, as it's
> not mounted at that time.

Yes, because the device file is held open by mkfs or fsck.  You can 
test this easily enough, in a nondestructive way, by using this little 
shell script:

	echo -n 'Press RETURN to continue... '
	read </dev/tty

Stick that in a file, and run the file with input redirected to the 
appropriate /dev/sd? file.

> > @@ -1647,9 +1653,16 @@ static int proc_ioctl(struct dev_state *
> >  	else switch (ctl->ioctl_code) {
> >  
> >  	/* disconnect kernel driver from interface */
> > +	case USBDEVFS_TRY_DISCONNECT:
> >  	case USBDEVFS_DISCONNECT:
> >  		if (intf->dev.driver) {
> >  			driver = to_usb_driver(intf->dev.driver);
> > +			if (ctl->ioctl_code == USBDEVFS_TRY_DISCONNECT &&
> > +					driver->check_busy) {
> > +				retval = driver->check_busy(intf);
> > +				if (retval)
> > +					break;
> > +			}
> 
> I don't like the fact that if a driver doesn't contain check_busy() then
> it will automatically fall back to looking like it was a DISCONNECT
> call, which could give userspace a false sense of "everything was fine"
> when trying this out.
> 
> Why not fail if that callback is not present?

It could be made to work that way.  I had to choose, so I chose to make
TRY_DISCONNECT work like DISCONNECT when the callback was missing.
Doing it as you suggest might be better though, because then the user
program could decide what to do if the kernel driver doesn't support
TRY_DISCONNECT.

What would be a good error code for that case?  -EOPNOTSUPP?  Or the 
traditional -ENOTTY?

> I can't comment on the scsi layer, but what about devices that don't use
> scsi?  Like "raw" block drivers?

You mean things like Pete Zaitcev's ub driver?  They would need an
equivalent change.

Remember, this was just a trial patch to demonstrate the idea.  It
wasn't intended to be complete.  (Another thing I skipped was updating
the usb_driver structures in all the little submodules in the
drivers/usb/storage directory.)

Alan Stern

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