fel: Try to detect VM environments and print a warning for those

fel.c

@@ -1069,6 +1069,17 @@ int main(int argc, char **argv)
 		if (*argv[i] == '-')
 			pr_fatal("Invalid option %s\n", argv[i]);
 
+	/* Check for virtual machine */
+	const char *emulation = fel_check_vm();
+	if (emulation) {
+		/* print a warning message, but otherwise try to continue normally */
+		printf("Virtual machine detected! (%s)\n\n", emulation);
+		printf("The FEL protocol (handler in the BROM) is not very robust. Trying to use it\n"
+		       "via emulated USB is known to be problematic on some virtual machines. If you\n"
+		       "insist on doing this, expect spurious errors due to timing issues etc. These\n"
+		       "do _not_ mean that sunxi-fel is at fault, and it can't do anything about it.\n\n");
+	}
+
 	/* Process options that don't require a FEL device handle */
 	if (device_list)
 		felusb_list_devices(); /* and exit program afterwards */

fel_lib.c

@@ -820,3 +820,47 @@ feldev_list_entry *list_fel_devices(size_t *count)
 	if (count) *count = devices;
 	return list;
 }
+
+/*
+ * Check for virtual machine (USB emulation). The FEL protocol is rather
+ * sensitive to errors and timing issues, so such environments are known
+ * to NOT work well with sunxi-fel.
+ *
+ * Returns either NULL (if no virtual machine could be detected), or a string
+ * pointer that identifies the environment. (e.g. "VirtualBox", "VMware", ...)
+ */
+const char *fel_check_vm(void) {
+	const char *result = NULL;
+
+	ssize_t rc;
+	libusb_context *ctx;
+	libusb_device **usb;
+	struct libusb_device_descriptor desc;
+
+	libusb_init(&ctx);
+	rc = libusb_get_device_list(ctx, &usb);
+	if (rc < 0)
+		usb_error(rc, "libusb_get_device_list()", 1);
+
+	// iterate over device descriptors, checking their vendor IDs
+	while (--rc >= 0) {
+		libusb_get_device_descriptor(usb[rc], &desc);
+		switch (desc.idVendor) {
+		case 0x0E0F:
+			result = "VMware";
+			break;
+		case 0x15AD:
+			result = "VMware";
+			break;
+		case 0x80EE:
+			result = "VirtualBox";
+			break;
+		}
+		if (result)
+			break; // exit while loop
+	}
+	libusb_free_device_list(usb, true);
+	libusb_exit(ctx);
+
+	return result;
+}

fel_lib.h

@@ -81,4 +81,7 @@ void fel_clrsetbits_le32(feldev_handle *dev,
 bool fel_get_sid_root_key(feldev_handle *dev, uint32_t *result,
 			  bool force_workaround);
 
+/* check for virtual machine (USB emulation) */
+const char *fel_check_vm(void);
+
 #endif /* _SUNXI_TOOLS_FEL_LIB_H */