[sane-devel] Canon Mx 700 - Scanbuttons

Wilhelm wilhelm.meier at fh-kl.de
Sun Oct 28 05:15:54 UTC 2012

Am 27.10.2012 19:34, schrieb Louis Lagendijk:
> On Sat, 2012-10-27 at 12:25 -0400, m. allan noah wrote:
>>> I finally found some time to dig into the backend to see what it does
>>> and how to use it (too many other things to do and been ill for a
>>> while). I now understand how things work.
>>> The button-1 and button-2 options are set when sane_control_option() is
>>> called for option button_update with action SANE_ACTION_SET_VALUE.
>>> SANE_ACTION_SET_VALUE button_update is used to poll the scanner and set
>>> button-1 and button-2. But scanbd does not support
>> The sane standard does not really cover the proper implementation of
>> buttons, but I would say that the mechanism you described is weird,
>> and should be changed. Reading the value of the button should do
>> whatever is required to get the value from the scanner.
> I agree, it took me a while to get my brain around it. Sending a setting
> command for one option to initiate a read cache action is funny indeed.
> But do we not run the risk that changes will break other applications
> that rely on the current, weird behavior?
>> It should not
>> be required to set another option to update the value of the button.
>> In the case where the value for all buttons is requested in one
>> command to the scanner, the backend should cache the values as
>> required. The fujitsu backend uses this scheme, perhaps the code could
>> be adapted.
> Well, the caching is done when button_update is set. It should indeed be
> sufficient to read the buttons and cache the result at that point in
> time.
> I will adapt the code accordingly, but leave the button_update stuff in
> so we do not break anything for other users.

Does that mean, that reading the button value will reflect the button 
changes in the future *without* setting the button-update option?

If you change the stuff, do you mind also changing the option-type of 
button-1/-2 to SANE_TYPE_BUTTON (it is actually SANE_TYPE_INT)?

Is anyone aware of similar operation (setting one option for querying 
another) in other backends (in that case I would consider updating 
scanbd to suppport such a schema as well)?



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