DM USB: refine the polling thread for libusb events

The thread function usb_dev_sys_thread is the source of libusb events,
which is also the basis of USB/xHCI emulation. During S3 process,
function libusb_handle_events_timeout may fail and return -1, which
could result of exit of the thread (BTW, this failure is resulted from
USB native reseting behavior during S3). Under this situation, the emulation
of USB/xHCI could never continue.

This patch fix this issue by continuing to call libusb_handle_events_timeout
even it reture -1. The return value will be finally ok after S3 process is
completed.

This patch will continue poll after failure and hence fix this issue.

Tracked-On: #1893
Signed-off-by: Xiaoguang Wu <xiaoguang.wu@intel.com>
Reviewed-by: Liang Yang <liang3.yang@intel.com>
Acked-by: Yu Wang <yu1.wang@intel.com>

devicemodel/hw/platform/usb_pmapper.c

@@ -10,6 +10,7 @@
 #include <stdio.h>
 #include <assert.h>
 #include <string.h>
+#include <unistd.h>
 #include "usb.h"
 #include "usbdi.h"
 #include "usb_pmapper.h"
@@ -1112,10 +1113,16 @@ static void *
 usb_dev_sys_thread(void *arg)
 {
 	struct timeval t = {1, 0};
+	int rc = 0;
 
-	while (g_ctx.thread_exit == 0 &&
-		libusb_handle_events_timeout(g_ctx.libusb_ctx, &t) >= 0)
-		; /* nothing */
+	while (g_ctx.thread_exit == 0) {
+		rc = libusb_handle_events_timeout(g_ctx.libusb_ctx, &t);
+		if (rc < 0)
+			/* TODO: maybe one second as interval is too long which
+			 * may result of slower USB enumeration process.
+			 */
+			sleep(1);
+	}
 
 	UPRINTF(LINF, "poll thread exit\n\r");
 	return NULL;