[sane-devel] Canon Mx 700 - Scanbuttons

[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
> > SANE_ACTION_SET_VALUE.
> >
> 
> 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.

Thanks for your valuable feedback
Louis