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 >
