Many backends are single threaded currently, so this would be a pretty invasive change. Frankly, polling over the network once per second is not that much traffic, and certainly easier to implement.
allan On Fri, Dec 3, 2021 at 8:50 AM Paul Wolneykien <mano...@altlinux.org> wrote: > В Thu, 2 Dec 2021 17:29:13 -0800 > Ralph Little <skelb...@gmail.com> пишет: > > > However, regular polling can be > > detrimental as a general feature. One commentator suggested that over > > a network, this could become an unnecessary bandwidth hog. In SANE we > > would want to add polling features in a generic fashion and we should > > wary of undesirable side-effects. Flooding a network just because you > > are sitting in an idle instance of xsane is certainly such an > > example. It is something that we should discuss carefully before > > enhancing the standard. > > Yes, polling over the network is a very bad idea. That means, we need > something like subscribe+notify interface in SANE. What about the > following API upgrade? > > A new capability for dynamic options (not only the buttons, but all > things that could be changed by the scanner inself): > > + #define SANE_CAP_DYNAMIC > > Then a new action for sane_control_option() indicating we want to > subscribe to scanner-initiated updates of the given value: > > + SANE_ACTION_SUBSCRIBE > > Then a function type for a notification handler pointer: > > + typedef void (*sane_option_change_callback)(SANE_Int option, void > *value); > > Having these minor API changes, it's become possible to make a call to > sane_control_option() like the following: > > sane_control_option(sane_handle, option_number, SANE_ACTION_SUBSCRIBE, > my_handler, NULL); > > And if the option option_number is marked as SANE_CAP_DYNAMIC in the > option descriptor, the backend should setup the given notification > handler and invoke it each time the option is updated. > For options not explicitly marked as dynamic it should return > SANE_STATUS_UNSUPPORTED. So, no changes are required for the present > backends. > > How the backend could monitor the option changes? It's up to the > backend. But with the present codebase/hardware polling would be used > most of the time, I guess. That will be, however, a local-only polling > over USB or SCISI interfaces, not a polling over the network as the > backend code runs on the server connected to the scanner in the > client-server scenario. Of course we also need to implement the proper > RPC in order to support callbacks over the network, but that doesn't > affect the backends and doesn't require more API changes. > > As to a resident polling process inside the backend, we already have > sanei_thread_*() functions for that. > > -- "well, I stand up next to a mountain- and I chop it down with the edge of my hand"