Re: pre/post_reset methods for skel

On Wed, 9 May 2007, Oliver Neukum wrote:

> Am Dienstag, 8. Mai 2007 23:59 schrieb Alan Stern:
> > On Tue, 8 May 2007, Oliver Neukum wrote:
> > 
> > > Am Dienstag, 8. Mai 2007 20:59 schrieb Alan Stern:
> > > > > 2. I would prefer to have exclusion between open and reset, too.
> > > > 
> > > > Why?  I can understand wanting exclusion between read/write and reset.  
> > > > But there's no obvious reason to make open and reset exclusive.
> > > 
> > > I don't think we'd want mutual exclusion in these cases. In fact read/write
> > > should fail in case a device is being reset or has been reseted.
> > 
> > You probably can't prevent ongoing reads and writes from failing.  But you
> > can delay new reads/writes until the reset is finished, giving them a
> > fighting chance of succeeding.
> 
> I don't think we can let them succeed in the common case. Many devices,
> eg. modems, printers, ..., have settings which a driver cannot restore, as it
> does not understand the io.
> In all these cases you have to tell user space that the device has been
> reset, which means that io has to return an error code.

This isn't clear.  Let's say the device has been reset.  Does that mean
you want _all_ reads and writes to fail until the next open?  Or do you
just want the next I/O operation to fail unless there is an open first?

Either way implies that you want open to clear the stored error flag.  But 
what if there is a real I/O error -- not a reset?  Then you wouldn't want 
an open to clear the error flag, right?  Or would you?

The semantics behind this stored error flag thing are far too cloudy for 
my taste.

> When possible I want to avoid this scenario by delay.

You could always make skel_open() acquire dev->io_mutex.  That would force 
it to wait until an ongoing reset was complete.

Alan Stern


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