[PATCH] USB: Don't ask for string descriptor lengths

Okay, here's a revised patch (as332b).  This tries first to ask for 255
bytes, and if that fails then it asks for the length and the full
descriptor.  Hopefully nobody will object to applying this version...

You know, it occurs to me that the have_langid field in usb_device could
easily be eliminated.  Just set string_langid to -1 during initialization
and test for whether or not it is >= 0.  I'll do that some other time.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>

drivers/usb/core/message.c

@@ -605,12 +605,128 @@ int usb_get_descriptor(struct usb_device *dev, unsigned char type, unsigned char
  * Returns the number of bytes received on success, or else the status code
  * returned by the underlying usb_control_msg() call.
  */
-int usb_get_string(struct usb_device *dev, unsigned short langid, unsigned char index, void *buf, int size)
+int usb_get_string(struct usb_device *dev, unsigned short langid,
+		unsigned char index, void *buf, int size)
 {
-	return usb_control_msg(dev, usb_rcvctrlpipe(dev, 0),
+	int i;
+	int result;
+
+	for (i = 0; i < 3; ++i) {
+		/* retry on length 0 or stall; some devices are flakey */
+		result = usb_control_msg(dev, usb_rcvctrlpipe(dev, 0),
 			USB_REQ_GET_DESCRIPTOR, USB_DIR_IN,
 			(USB_DT_STRING << 8) + index, langid, buf, size,
 			HZ * USB_CTRL_GET_TIMEOUT);
+		if (!(result == 0 || result == -EPIPE))
+			break;
+	}
+	return result;
+}
+
+static int usb_string_sub(struct usb_device *dev, unsigned int langid,
+		unsigned int index, unsigned char *buf)
+{
+	int rc;
+
+	/* Try to read the string descriptor by asking for the maximum
+	 * possible number of bytes */
+	rc = usb_get_string(dev, langid, index, buf, 255);
+
+	/* If that failed try to read the descriptor length, then
+	 * ask for just that many bytes */
+	if (rc < 0) {
+		rc = usb_get_string(dev, langid, index, buf, 2);
+		if (rc == 2)
+			rc = usb_get_string(dev, langid, index, buf, buf[0]);
+	}
+
+	if (rc >= 0) {
+		/* There might be extra junk at the end of the descriptor */
+		if (buf[0] < rc)
+			rc = buf[0];
+		if (rc < 2)
+			rc = -EINVAL;
+	}
+	return rc;
+}
+
+/**
+ * usb_string - returns ISO 8859-1 version of a string descriptor
+ * @dev: the device whose string descriptor is being retrieved
+ * @index: the number of the descriptor
+ * @buf: where to put the string
+ * @size: how big is "buf"?
+ * Context: !in_interrupt ()
+ * 
+ * This converts the UTF-16LE encoded strings returned by devices, from
+ * usb_get_string_descriptor(), to null-terminated ISO-8859-1 encoded ones
+ * that are more usable in most kernel contexts.  Note that all characters
+ * in the chosen descriptor that can't be encoded using ISO-8859-1
+ * are converted to the question mark ("?") character, and this function
+ * chooses strings in the first language supported by the device.
+ *
+ * The ASCII (or, redundantly, "US-ASCII") character set is the seven-bit
+ * subset of ISO 8859-1. ISO-8859-1 is the eight-bit subset of Unicode,
+ * and is appropriate for use many uses of English and several other
+ * Western European languages.  (But it doesn't include the "Euro" symbol.)
+ *
+ * This call is synchronous, and may not be used in an interrupt context.
+ *
+ * Returns length of the string (>= 0) or usb_control_msg status (< 0).
+ */
+int usb_string(struct usb_device *dev, int index, char *buf, size_t size)
+{
+	unsigned char *tbuf;
+	int err;
+	unsigned int u, idx;
+
+	if (size <= 0 || !buf || !index)
+		return -EINVAL;
+	buf[0] = 0;
+	tbuf = kmalloc(256, GFP_KERNEL);
+	if (!tbuf)
+		return -ENOMEM;
+
+	/* get langid for strings if it's not yet known */
+	if (!dev->have_langid) {
+		err = usb_string_sub(dev, 0, 0, tbuf);
+		if (err < 0) {
+			dev_err (&dev->dev,
+				"string descriptor 0 read error: %d\n",
+				err);
+			goto errout;
+		} else if (err < 4) {
+			dev_err (&dev->dev, "string descriptor 0 too short\n");
+			err = -EINVAL;
+			goto errout;
+		} else {
+			dev->have_langid = -1;
+			dev->string_langid = tbuf[2] | (tbuf[3]<< 8);
+				/* always use the first langid listed */
+			dev_dbg (&dev->dev, "default language 0x%04x\n",
+				dev->string_langid);
+		}
+	}
+	
+	err = usb_string_sub(dev, dev->string_langid, index, tbuf);
+	if (err < 0)
+		goto errout;
+
+	size--;		/* leave room for trailing NULL char in output buffer */
+	for (idx = 0, u = 2; u < err; u += 2) {
+		if (idx >= size)
+			break;
+		if (tbuf[u+1])			/* high byte */
+			buf[idx++] = '?';  /* non ISO-8859-1 character */
+		else
+			buf[idx++] = tbuf[u];
+	}
+	buf[idx] = 0;
+	err = idx;
+
+ errout:
+	kfree(tbuf);
+	return err;
 }
 
 /**
@@ -1257,102 +1373,6 @@ int usb_set_configuration(struct usb_device *dev, int configuration)
 	return ret;
 }
 
-/**
- * usb_string - returns ISO 8859-1 version of a string descriptor
- * @dev: the device whose string descriptor is being retrieved
- * @index: the number of the descriptor
- * @buf: where to put the string
- * @size: how big is "buf"?
- * Context: !in_interrupt ()
- * 
- * This converts the UTF-16LE encoded strings returned by devices, from
- * usb_get_string_descriptor(), to null-terminated ISO-8859-1 encoded ones
- * that are more usable in most kernel contexts.  Note that all characters
- * in the chosen descriptor that can't be encoded using ISO-8859-1
- * are converted to the question mark ("?") character, and this function
- * chooses strings in the first language supported by the device.
- *
- * The ASCII (or, redundantly, "US-ASCII") character set is the seven-bit
- * subset of ISO 8859-1. ISO-8859-1 is the eight-bit subset of Unicode,
- * and is appropriate for use many uses of English and several other
- * Western European languages.  (But it doesn't include the "Euro" symbol.)
- *
- * This call is synchronous, and may not be used in an interrupt context.
- *
- * Returns length of the string (>= 0) or usb_control_msg status (< 0).
- */
-int usb_string(struct usb_device *dev, int index, char *buf, size_t size)
-{
-	unsigned char *tbuf;
-	int err, len;
-	unsigned int u, idx;
-
-	if (size <= 0 || !buf || !index)
-		return -EINVAL;
-	buf[0] = 0;
-	tbuf = kmalloc(256, GFP_KERNEL);
-	if (!tbuf)
-		return -ENOMEM;
-
-	/* get langid for strings if it's not yet known */
-	if (!dev->have_langid) {
-		err = usb_get_descriptor(dev, USB_DT_STRING, 0, tbuf, 4);
-		if (err < 0) {
-			dev_err (&dev->dev,
-				"string descriptor 0 read error: %d\n",
-				err);
-			goto errout;
-		} else if (err < 4 || tbuf[0] < 4) {
-			dev_err (&dev->dev, "string descriptor 0 too short\n");
-			err = -EINVAL;
-			goto errout;
-		} else {
-			dev->have_langid = -1;
-			dev->string_langid = tbuf[2] | (tbuf[3]<< 8);
-				/* always use the first langid listed */
-			dev_dbg (&dev->dev, "default language 0x%04x\n",
-				dev->string_langid);
-		}
-	}
-
-	/*
-	 * ask for the length of the string 
-	 */
-
-	err = usb_get_string(dev, dev->string_langid, index, tbuf, 2);
-	if (err == -EPIPE || err == 0) {
-		dev_dbg(&dev->dev, "RETRY string %d read/%d\n", index, 2);
-		err = usb_get_string(dev, dev->string_langid, index, tbuf, 2);
-	}
-	if(err<2)
-		goto errout;
-	len=tbuf[0];	
-	
-	err = usb_get_string(dev, dev->string_langid, index, tbuf, len);
-	if (err == -EPIPE || err == 0) {
-		dev_dbg(&dev->dev, "RETRY string %d read/%d\n", index, len);
-		err = usb_get_string(dev, dev->string_langid, index, tbuf, len);
-	}
-	if (err < 0)
-		goto errout;
-
-	size--;		/* leave room for trailing NULL char in output buffer */
-	for (idx = 0, u = 2; u < err; u += 2) {
-		if (idx >= size)
-			break;
-		if (tbuf[u+1])			/* high byte */
-			buf[idx++] = '?';  /* non ISO-8859-1 character */
-		else
-			buf[idx++] = tbuf[u];
-	}
-	buf[idx] = 0;
-	err = idx;
-
- errout:
-	kfree(tbuf);
-	return err;
-}
-
 // synchronous request completion model
 EXPORT_SYMBOL(usb_control_msg);
 EXPORT_SYMBOL(usb_bulk_msg);