ztex: if we had only errors in one round we do not count the errors

to detect a totally non working fpga we only do that if the last
round had some valid nonces.

if we would count the errors the automatic megahertz adaption
would drop and never recover.

driver-ztex.c

@@ -223,6 +223,8 @@ static int64_t ztex_scanhash(struct thr_info *thr, struct work *work,
 
 	overflow = false;
 	int count = 0;
+	int validNonces = 0;
+	double errorCount = 0;
 
 	applog(LOG_DEBUG, "%s: entering poll loop", ztex->repr);
 	while (!(overflow || thr->work_restart)) {
@@ -274,10 +276,13 @@ static int64_t ztex_scanhash(struct thr_info *thr, struct work *work,
 
 				// do not count errors in the first 500ms after sendHashData (2x250 wait time)
 				if (count > 2) {
-					ztex->errorCount[ztex->freqM] += 1.0 / ztex->numNonces;
 					thr->cgpu->hw_errors++;
+					errorCount += (1.0 / ztex->numNonces);
 				}
 			}
+			else
+				validNonces++;
+
 
 			for (j=0; j<=ztex->extraSolutions; j++) {
 				nonce = hdata[i].goldenNonce[j];
@@ -313,6 +318,18 @@ static int64_t ztex_scanhash(struct thr_info *thr, struct work *work,
 		}
 	}
 
+	// only add the errorCount if we had at least some valid nonces or
+	// had no valid nonces in the last round
+	if (ztex->nonceCheckValid > 0 && validNonces == 0) {
+		applog(LOG_ERR, "%s: resetting %.1f errors", ztex->repr, errorCount);
+	}
+	else {
+		ztex->errorCount[ztex->freqM] += errorCount;
+	}
+
+	// remember the number of valid nonces for the check in the next round
+	ztex->nonceCheckValid = validNonces;
+
 	ztex->errorRate[ztex->freqM] = ztex->errorCount[ztex->freqM] /	ztex->errorWeight[ztex->freqM] * (ztex->errorWeight[ztex->freqM] < 100? ztex->errorWeight[ztex->freqM] * 0.01: 1.0);
 	if (ztex->errorRate[ztex->freqM] > ztex->maxErrorRate[ztex->freqM])
 		ztex->maxErrorRate[ztex->freqM] = ztex->errorRate[ztex->freqM];

libztex.c

@@ -701,6 +701,9 @@ int libztex_prepare_device(struct libusb_device *dev, struct libztex_device** zt
 		newdev->maxErrorRate[cnt] = 0;
 	}
 
+	// fake that the last round found something valid
+	newdev->nonceCheckValid = 1;
+
 	newdev->usbbus = libusb_get_bus_number(dev);
 	newdev->usbaddress = libusb_get_device_address(dev);
 	sprintf(newdev->repr, "ZTEX %s-1", newdev->snString);

libztex.h

@@ -73,6 +73,8 @@ struct libztex_device {
 	double errorRate[256];
 	double maxErrorRate[256];
 
+	int16_t nonceCheckValid;
+
 	int16_t numberOfFpgas;
 	int selectedFpga;
 	bool parallelConfigSupport;