xerox_mfp: do not append slack lines

xerox_mfp devices return a page height in millimetres.  We previously
converted that to pixels, guaranteed the image's height in pixels,
and added some slack lines if the scanner returned too few.

Return an image height of -1 instead, and get rid of the slack lines.

backend/xerox_mfp.c

@@ -355,7 +355,7 @@ static SANE_Status dev_stop(struct device *dev)
     dev->reserved = 0;
     dev_cmd(dev, CMD_RELEASE_UNIT);
     DBG(3, "total image %d*%d size %d (win %d*%d), %d*%d %d data: %d, out %d bytes\n",
-        dev->para.pixels_per_line, dev->para.lines,
+        dev->para.pixels_per_line, dev->total_lines,
         dev->total_img_size,
         dev->win_width, dev->win_len,
         dev->pixels_per_line, dev->ulines, dev->blocks,
@@ -652,7 +652,7 @@ static void set_parameters(struct device *dev)
         px_to_len = 1213.9 / dev->val[OPT_RESOLUTION].w;
 #endif
     }
-    dev->para.lines = dev->win_len / px_to_len;
+    dev->total_lines = dev->win_len / px_to_len;
     if (dev->composition == MODE_LINEART ||
         dev->composition == MODE_HALFTONE) {
         dev->para.format = SANE_FRAME_GRAY;
@@ -1200,19 +1200,6 @@ static int dev_acquire(struct device *dev)
     return 1;
 }
 
-static int fill_slack(struct device *dev, SANE_Byte *buf, int maxlen)
-{
-    const int slack = dev->total_img_size - dev->total_out_size;
-    const int havelen = MIN(slack, maxlen);
-    int j;
-
-    if (havelen <= 0)
-        return 0;
-    for (j = 0; j < havelen; j++)
-        buf[j] = 255;
-    return havelen;
-}
-
 static int copy_plain_trim(struct device *dev, SANE_Byte *buf, int maxlen, int *olenp)
 {
     int j;
@@ -1225,7 +1212,7 @@ static int copy_plain_trim(struct device *dev, SANE_Byte *buf, int maxlen, int *
         if (y >= dev->vertical)
             break; /* slack */
         if (x < dev->para.bytes_per_line &&
-            (y + dev->y_off) < dev->para.lines) {
+            (y + dev->y_off) < dev->total_lines) {
             *buf++ = dev->data[(dev->dataoff + j) & DATAMASK];
             (*olenp)++;
         }
@@ -1262,7 +1249,7 @@ static int copy_mix_bands_trim(struct device *dev, SANE_Byte *buf, int maxlen, i
         const int y_rly = y + y_off + dev->y_off; /* global y */
 
         if (x < dev->para.pixels_per_line &&
-            y_rly < dev->para.lines) {
+            y_rly < dev->total_lines) {
             *buf++ = dev->data[(dev->dataoff + band + x + y * linesize) & DATAMASK];
             (*olenp)++;
         }
@@ -1312,8 +1299,7 @@ sane_read(SANE_Handle h, SANE_Byte *buf, SANE_Int maxlen, SANE_Int *lenp)
 
             /* but we may need to fill slack */
             if (buf && lenp && slack > 0) {
-                *lenp = fill_slack(dev, buf, maxlen);
-                dev->total_out_size += *lenp;
+                dev->total_out_size += MIN(slack, maxlen);
                 DBG(9, "<> slack: %d, filled: %d, maxlen %d\n",
                     slack, *lenp, maxlen);
                 return SANE_STATUS_GOOD;
@@ -1399,10 +1385,10 @@ sane_read(SANE_Handle h, SANE_Byte *buf, SANE_Int maxlen, SANE_Int *lenp)
             DBG(9, "<> olen: %d, clrlen: %d, blocklen: %d/%d, maxlen %d (%d %d %d)\n",
                 olen, clrlen, dev->blocklen, dev->datalen, maxlen,
                 dev->dataindex / dev->bytes_per_line + dev->y_off,
-                dev->y_off, dev->para.lines);
+                dev->y_off, dev->total_lines);
 
             /* slack beyond last line */
-            if (dev->dataindex / dev->bytes_per_line + dev->y_off >= dev->para.lines) {
+            if (dev->dataindex / dev->bytes_per_line + dev->y_off >= dev->total_lines) {
                 dev->datalen = 0;
                 dev->dataoff = 0;
             }
@@ -1488,7 +1474,7 @@ sane_start(SANE_Handle h)
         dev->para.bytes_per_line = dev->para.pixels_per_line;
     }
 
-    dev->total_img_size = dev->para.bytes_per_line * dev->para.lines;
+    dev->total_img_size = dev->para.bytes_per_line * dev->total_lines;
 
     if (isSupportedDevice(dev) &&
         dev->composition == MODE_RGB24) {

backend/xerox_mfp.h

@@ -119,6 +119,7 @@ struct device {
     int total_img_size;		/* predicted image size */
     int total_out_size;		/* total we sent to user */
     int total_data_size;		/* total of what scanner sent us */
+    int total_lines;		/* number of lines expected */
 
     /* transport to use */
     transport *io;