xerox_mfp: When scanning from ADF, do not stop the device at page completion

- Stopping and releasing the device causes the ADF to purge any remaining pages,
thus only the first page gets properly scanned (observed with Samsung
SCX-4729FW).
- Instead, when sourcing from the ADF, at each page's completion it should
continue and issue "OBJECT POSITION" command to proceed to the next page.
- Once all pages have been processed, the subsequent "OBJECT POSITION" command
returns the proper "CHECK CONDITION" status  (ADF is empty) and triggers the
device stop and release.
- Additionally, the status of the ADF (loaded/empty) needs to be requeried
at the `sane_start()` for proper handling of the "Auto" mode. Otherwise,
the scanning session retains the ADF state from the start of the session,
even though ADF will become empty. Thus the "Auto" mode will not properly
switch to Flatbed as intended in the client application (observed in
Simple-Scan).

> NOTE: the initial "READ" command in `sane_start()` is extraneous and
> appears to always return STATUS_BUSY (indeed, the scanner is usually
> busy loading the page). The actual READ is correctly done in
> `dev_acquire()`. It may appear benign in a single page scan mode, however,
> it becomes malfunctioning in the mutipage ADF mode. The initial READ,
> when successful, will cause the skipping of the read block from processing,
> as the actual processing is tied to content read from `dev_acquire()`.
>
> Therefore, the initial READ command in the `sane_start()` is removed.

backend/xerox_mfp.c

@@ -703,6 +703,14 @@ static void set_parameters(struct device *dev)
     }
 }
 
+/* determine if document is to be sourced from ADF */
+static int sourcing_from_adf(struct device *dev)
+{
+    return (dev->doc_source == DOC_ADF ||
+            (dev->doc_source == DOC_AUTO && dev->doc_loaded));
+}
+
+
 /* resolve all options related to scan window */
 /* called after option changed and in set_window */
 static int fix_window(struct device *dev)
@@ -732,11 +740,10 @@ static int fix_window(struct device *dev)
     dev->doc_source = doc_source_to_code[string_match_index(doc_sources, dev->val[OPT_SOURCE].s)];
 
     /* max window len is dependent of document source */
-    if (dev->doc_source == DOC_FLATBED ||
-        (dev->doc_source == DOC_AUTO && !dev->doc_loaded))
-        dev->max_len = dev->max_len_fb;
-    else
+    if (sourcing_from_adf(dev))
         dev->max_len = dev->max_len_adf;
+    else
+        dev->max_len = dev->max_len_fb;
 
     /* parameters */
     dev->win_y_range.max = SANE_FIX((double)dev->max_len / PNT_PER_MM);
@@ -880,8 +887,9 @@ dev_inquiry(struct device *dev)
                       dev->res[0x3f];
     dev->line_order = dev->res[0x31];
     dev->compressionTypes = dev->res[0x32];
-    dev->doc_loaded = (dev->res[0x35] == 0x02) &&
-                      (dev->res[0x26] & 0x03);
+    dev->has_adf = ((dev->res[0x26] & 0x03) != 0);
+    dev->doc_loaded = (dev->res[0x35] == 0x02)
+                       && dev->has_adf;
 
     init_options(dev);
     reset_options(dev);
@@ -892,6 +900,25 @@ dev_inquiry(struct device *dev)
     return SANE_STATUS_GOOD;
 }
 
+
+static SANE_Status
+dev_inquiry_adf_status(struct device *dev)
+{
+    if (!dev_cmd(dev, CMD_INQUIRY))
+        return SANE_STATUS_IO_ERROR;
+
+    dev->has_adf = ((dev->res[0x26] & 0x03) != 0);
+    dev->doc_loaded = (dev->res[0x35] == 0x02)
+                       && dev->has_adf;
+
+    DBG(3, "%s: ADF present: %s, loaded: %s\n", __func__,
+        (dev->has_adf ? "true" : "false"),
+        (dev->doc_loaded ? "true" : "false"));
+
+    return SANE_STATUS_GOOD;
+}
+
+
 const SANE_Option_Descriptor *
 sane_get_option_descriptor(SANE_Handle h, SANE_Int opt)
 {
@@ -1362,7 +1389,10 @@ sane_read(SANE_Handle h, SANE_Byte *buf, SANE_Int maxlen, SANE_Int *lenp)
                 remove(encTmpFileName);
             }
             /* that's all */
-            dev_stop(dev);
+            /* finished receving the document; */
+            /* stop and release the unit, unless sourcing from ADF */
+            if (!sourcing_from_adf(dev))
+                dev_stop(dev);
             return SANE_STATUS_EOF;
         }
 
@@ -1466,7 +1496,6 @@ SANE_Status
 sane_start(SANE_Handle h)
 {
     struct device *dev = h;
-
     DBG(3, "%s: %p\n", __func__, h);
 
     dev->cancel = 0;
@@ -1477,22 +1506,24 @@ sane_start(SANE_Handle h)
     dev->blocks = 0;
 
     if (!dev->reserved) {
+        if (dev->has_adf
+            && (dev->doc_source == DOC_AUTO || dev->doc_source == DOC_ADF)) {
+            if (dev_inquiry_adf_status(dev) != SANE_STATUS_GOOD)
+                return dev_stop(dev);
+        }
+
         if (!dev_cmd_wait(dev, CMD_RESERVE_UNIT))
             return dev->state;
         dev->reserved++;
+
+        if (!dev_set_window(dev) ||
+            (dev->state && dev->state != SANE_STATUS_DEVICE_BUSY))
+            return dev_stop(dev);
     }
 
-    if (!dev_set_window(dev) ||
-        (dev->state && dev->state != SANE_STATUS_DEVICE_BUSY))
-        return dev_stop(dev);
-
     if (!dev_cmd_wait(dev, CMD_OBJECT_POSITION))
         return dev_stop(dev);
 
-    if (!dev_cmd(dev, CMD_READ) ||
-        (dev->state && dev->state != SANE_STATUS_DEVICE_BUSY))
-        return dev_stop(dev);
-
     dev->scanning = 1;
     dev->final_block = 0;
     dev->blocklen = 0;

backend/xerox_mfp.h

@@ -64,6 +64,7 @@ struct device {
     int state;			/* current state */
     int reserved;			/* CMD_RESERVE_UNIT */
     int reading;			/* READ_IMAGE is sent */
+    int has_adf;			/* ADF is present */
 
     SANE_Byte *data;		/* postprocessing cyclic buffer 64k */
     int datalen;			/* how data in buffer */