usb-common.c: Add API-neutral nut_usb_get_string()

This function doesn't use usb_get_string_simple() from libusb0 or
libusb_get_string_descriptor_ascii() from libusb1 but replaces them to
be able to add a workaround for devices with broken langid descriptors.

If the langid descriptor is invalid, the en_US language is assumed.
(See networkupstools#1925)

It also adds retries to the string descriptor fetching, which may help
in some cases. (See networkupstools#414)
A small delay is introduced between the retries.

Signed-off-by: Tormod Volden <[email protected]>

drivers/usb-common.c

@@ -410,3 +410,85 @@ void warn_if_bad_usb_port_filename(const char *fn) {
 		__func__, fn);
 	return;
 }
+
+/* Retries were introduced for "Tripp Lite" devices, see
+ * https://github.com/networkupstools/nut/issues/414
+ */
+#define MAX_STRING_DESC_TRIES 3
+
+/* API neutral, handles retries */
+static int nut_usb_get_string_descriptor(
+	usb_dev_handle *udev,
+	int StringIdx,
+	int langid,
+	char *buf,
+	size_t buflen)
+{
+	int ret = -1;
+	int tries = MAX_STRING_DESC_TRIES;
+
+	while (tries--) {
+		ret = usb_get_string(udev, (usb_ctrl_strindex)StringIdx, langid, (usb_ctrl_charbuf)buf, buflen);
+		if (ret >= 0) {
+			break;
+		} else if (tries) {
+			upsdebugx(1, "%s: string descriptor %d request failed, retrying...", __func__, StringIdx);
+			usleep(50000); /* 50 ms, might help in some cases */
+		}
+	}
+	return ret;
+}
+
+/* API neutral, assumes en_US if langid descriptor is broken */
+int nut_usb_get_string(
+	usb_dev_handle *udev,
+	int StringIdx,
+	char *buf,
+	size_t buflen)
+{
+	int ret;
+	char buffer[255];
+	int langid;
+	int len;
+	int i;
+
+	if (!udev || StringIdx < 1 || StringIdx > 255) {
+		return -1;
+	}
+
+	/* request langid descriptor */
+	ret = nut_usb_get_string_descriptor(udev, 0, 0, buffer, 4);
+	if (ret < 0)
+		return ret;
+
+	if (ret == 4 && buffer[0] >= 4 && buffer[1] == USB_DT_STRING) {
+		langid = buffer[2] | (buffer[3] << 8);
+	} else {
+		upsdebugx(1, "%s: Broken language identifier, assuming en_US", __func__);
+		langid = 0x0409;
+	}
+
+	/* retrieve string in preferred language */
+	ret = nut_usb_get_string_descriptor(udev, StringIdx, langid, buffer, sizeof(buffer));
+	if (ret < 0) {
+#ifdef WIN32
+		/* only for libusb0 ? */
+		errno = -ret;
+#endif
+		return ret;
+	}
+
+	/* translate simple UTF-16LE to 8-bit */
+	len = ret < (int)buflen ? ret : (int)buflen;
+	len = len / 2 - 1; // 16-bit characters, without header
+	len = len < (int)buflen - 1 ? len : (int)buflen - 1; // reserve for null terminator
+	for (i = 0; i < len; i++) {
+		if (buffer[2 + i * 2 + 1] == 0)
+			buf[i] = buffer[2 + i * 2];
+		else
+			buf[i] = '?'; // not decoded
+	}
+	buf[i] = '\0';
+
+	return len;
+}

drivers/usb-common.h

@@ -560,4 +560,9 @@ void nut_usb_addvars(void);
  * here, to use in several USB-capable drivers. */
 void warn_if_bad_usb_port_filename(const char *fn);
 
+/* Retrieves string descriptor from device, decoded to 8-bit string,
+ * applies retries on failures, and assumes en_US in case the
+ * langid descriptor is invalid. */
+int nut_usb_get_string(usb_dev_handle *udev, int StringIdx, char *buf, size_t buflen);
+
 #endif /* NUT_USB_COMMON_H */