[sane-devel] [RFC] Locking devices via sanei_access lib...

Oliver Rauch Oliver.Rauch@Rauch-Domain.DE
07 Feb 2005 16:50:27 +0100


Am Mon, 2005-02-07 um 14.10 schrieb Gerhard Jaeger:
> Hi,
> 
> On Monday 07 February 2005 12:41, Johannes Meixner wrote:
> > 
> > Hello,
> > 
> > On Feb 7 12:11 Gerhard Jaeger wrote (shortened):
> > > The idea is to provide a simple locking mechanism for the backends,
> > > to have exclusive access to one scanner during an operation:
> > ... 
> > > What do you guys think of this lib - is it useful? 
> > 
> > I even think exclusive access should be used by default
> > because I think normally it doesn't make sense when there
> > is more than one process which talks to the same scanner
> > (i.e. "the same scanner" but not "the same backend" because
> > one backend can drive several scanners).
> 
> I agree! Each device a backend talks to needs some exclusive
> locking!

We already had a discussion about this some time ago (2-3 years).
My opinion is a bit different.

On my work we have one scanner and about 30 people who have access
to the scanner. And I know that some people keep open the
scanning frontend all the time. With exclusive locking the device
would almost be unusable.

The umax (scsi) backend locks the scanner when sane_start is called
and releases the scanner when sane_cancel is called. This way there
is no problem to keep the frontend opened all the time.

I think it makes sense that opening the scanning frontend does not
lock the device. But in this case we need at least a button locking.

Oliver

> 
> > 
> > Once it happened while I did a nice (you may say stupid) stress test
> > 
> > for i in $( seq 100 )
> > do sane-find-scanner & scanimage -L &
> > done
> > 
> > that one of my USB scanners made funny noise: It seems it tried
> > to move its scanning unit beyond its physical limits.
> > I have no idea how this could have happened but I guess the scanner
> > was simply totally confused by the multiple concurrent accesses.
> 
> It should not happen, but even such stupid tests show, that the locking 
> needs to be done in another way, as it's currently done, if it's done.
> Rather the same "confusion" will happen when you have something
> like Rene Rebes' button daemon which does nothing else than checking the
> button status of a scanner - depending on the scanner, the backend needs
> to check some or at least one register and when another frontend currently
> is scanning - the scanner will not work properly afterwards (highly depends
> on that device)
> 
> > 
> > By the way:
> > The reason for the above stress-test was that this way I could
> > stop the kernel (no single error message in the logs  - just a
> > sudden stop) - meanwhile I do no longer use the crap SCSI
> > host adapter ;-)
> 
> This test should become another stress-test for a new backend ;)
> 
> 
> Ciao,
> Gerhard
>