[sane-devel] Interactive use of scanner buttons

[Kelly Price]

Well, that and if the access to the scanner is locked at the saned
level, polling isn't going to tie it up and saned can always return a
"We're busy scanning" signal.

On Fri, Dec 3, 2021 at 9:03 AM m. allan noah <kitno455 at gmail.com> wrote:
>
> 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 <manowar at altlinux.org> wrote:
>>
>> В Thu, 2 Dec 2021 17:29:13 -0800
>> Ralph Little <skelband at 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"



-- 
Kelly "STrRedWolf" Price
http://redwolf.ws