xerox_mfp: skip ENDPOINT CLEAR HALT command by default

Some xerox_mfp scanners, such as Samsung SCX-4521F, hangs when
their OUT ENDPOINT receives CLEAR HALT command, which is performed
in usb open and close functions of the backend.
The issue seem to be dependant on USB controller and assumingly
happens due to some sort of incompatibility of USB implementation
in the scanner.

Skip sending this command by default and add
SANE_XEROX_USB_HALT_WORKAROUND env variable to execute
CLEAR HALT commands.

backend/xerox_mfp-usb.c

@@ -21,6 +21,21 @@
 
 extern int sanei_debug_xerox_mfp;
 
+static int
+xerox_need_clear_halt()
+{
+    char *env;
+    int workaround = 0;
+
+    env = getenv("SANE_XEROX_USB_HALT_WORKAROUND");
+    if (env) {
+        workaround = atoi(env);
+        DBG(5, "xerox_need_clear_halt: workaround: %d\n", workaround);
+        return workaround;
+    }
+    return 0;
+}
+
 int
 usb_dev_request(struct device *dev,
                 SANE_Byte *cmd, size_t cmdlen,
@@ -70,7 +85,9 @@ usb_dev_open(struct device *dev)
         dev->dn = -1;
         return status;
     }
-    sanei_usb_clear_halt(dev->dn);
+    if (xerox_need_clear_halt()) {
+        sanei_usb_clear_halt(dev->dn);
+    }
     return SANE_STATUS_GOOD;
 }
 
@@ -92,7 +109,9 @@ usb_dev_close(struct device *dev)
             ret_cancel(dev, 0);
     }
 
-    sanei_usb_clear_halt(dev->dn);	/* unstall for next users */
+    if (xerox_need_clear_halt()) {
+        sanei_usb_clear_halt(dev->dn);	/* unstall for next users */
+    }
     sanei_usb_close(dev->dn);
     dev->dn = -1;
 }

doc/sane-usb.man

@@ -178,6 +178,15 @@ setting the environment variable
 to 1. This may work around issues which happen with particular kernel
 versions. Example:
 .I export SANE_USB_WORKAROUND=1.
+.TP
+.B SANE_XEROX_USB_HALT_WORKAROUND
+If your old (pre-2010) Xerox / Samsung / HP scanner is detected
+only once and subsequent usage requires replugging the cable, try
+setting the environment variable
+.B SANE_XEROX_USB_HALT_WORKAROUND
+to 1. This may work around issues which happen with particular USB
+controllers. Example:
+.I export SANE_XEROX_USB_HALT_WORKAROUND=1.
 
 .SH "SEE ALSO"
 .BR sane (7),