[sane-devel] sp15c vs. avision backends

[jazz_johnson at verizon.net]

On Thursday 10 January 2008, m. allan noah wrote:
>
> but only if they coordinate who's documents are on the scanner at that
> moment.

I suppose an argument can be made for both approaches to locking, but 
users sharing a network scanner must coordinate their usage by some means. 
Somebody has to put their documents on the scanner then start the scan. The 
SP15C doesn't have a "Start Scan" button, so the user must return to their 
desk to start the scan from whatever program they're using for scanning. 
During the scan (set_window, trigger_scan, sane_read) the scanner must be 
locked (reserved) of course. When other users see that the scanner has 
documents on it, they'll just have to wait their turn (no need for a queue 
manager).

> some backends now implement a locking feature to prevent 
> exactly this kind of access. it is made possible by the fact that
> sp15c.c closes the file handle at the end of attach, and does not
> re-open it til sane_start. this comment precedes the call to close: /*
> Why? */. Concurrent access is why, i suppose :) With backends that
> know how to read scanner sensors and buttons, this means a lot of
> opening and closing...


Well, I prefer the sp15c's concurrency. It means a user can use xsane for 
color/image adjustment, and gscans2pdf for its integration with tesseract, 
without having to open/close each program in turn -- they can keep their 
custom gamma tables and preview window sizes in xsane but temporarily  switch 
to another frontend without having to exit xsane.

---

Jeremy