avision.c: Add "skip-adf" option

The code which tries aggressively to reset/initialize HP 82xx/83xx
Automatic Document Feeders doesn't work well if there's no ADF attached,
because the scanner reports a non-zero ADF model type in this case.
The retry code makes several attempts to initialize the ADF, and then
bails out with an "Operation not supported" error.

Add a new "option skip-adf" which will cause the backend to believe
the "ADF not present" status if that's what the scanner reports.

Fixes #313751

backend/avision.c

@@ -1277,6 +1277,9 @@ static SANE_Bool force_calibration = SANE_FALSE;
 static SANE_Bool force_a4 = SANE_FALSE;
 static SANE_Bool force_a3 = SANE_FALSE;
 
+/* trust ADF-presence flag, even if ADF model is nonzero */
+static SANE_Bool skip_adf = SANE_FALSE;
+
 /* hardware resolutions to interpolate from */
 static const int  hw_res_list_c5[] =
   {
@@ -3218,11 +3221,13 @@ get_accessories_info (Avision_Scanner* s)
     {
       dev->inquiry_duplex = 1;
       dev->inquiry_duplex_interlaced = 0;
-    } else if (result[0] == 0 && result[2] != 0) {
+    } else if (result[0] == 0 && result[2] != 0 && !skip_adf) {
       /* Sometimes the scanner will report that there is no ADF attached, yet
        * an ADF model number will still be reported.  This happens on the
        * HP8200 series and possibly others.  In this case we need to reset the
-       * the adf and try reading it again.
+       * the adf and try reading it again.  Skip this if the configuration says
+       * to do so, so that we don't fail out the scanner as being broken and
+       * unsupported if there isn't actually an ADF present.
        */
       DBG (3, "get_accessories_info: Found ADF model number but the ADF-present flag is not set. Trying to recover...\n");
       status = adf_reset (s);
@@ -7627,6 +7632,11 @@ sane_reload_devices (void)
 		     linenumber);
 		force_a3 = SANE_TRUE;
 	      }
+	      else if (strcmp (word, "skip-adf") == 0) {
+		DBG (3, "sane_reload_devices: config file line %d: enabling skip-adf\n",
+		     linenumber);
+		skip_adf = SANE_TRUE;
+	      }
 	      else if (strcmp (word, "static-red-calib") == 0) {
 		DBG (3, "sane_reload_devices: config file line %d: static red calibration\n",
 		     linenumber);

doc/sane-avision.man

@@ -36,6 +36,7 @@ a hash mark (#) are ignored. A sample configuration file is shown below:
 \ 
  option force\-a4
  option force\-a3
+ option skip\-adf
  option disable\-gamma\-table
  option disable\-calibration
 \ 
@@ -61,6 +62,15 @@ known to return bogus data are marked in the backend
 so if you need this option please report this to the
 backend maintainer. USE WITH CARE!
 .TP
+skip\-adf:
+Forces the backend to ignore an inconsistent ADF
+status returned by the scanner (ADF not present, but
+ADF model number non-zero).  Without this option, the
+backend will make several attempts to reset the ADF
+and retry the query in this situation, and will fail
+with a "not supported" error if the ADF still doesn't
+respond.
+.TP
 disable\-gamma\-table:
 Disables the usage of the scanner's gamma-table. You
 might try this if your scans hang or only produces