rtc: ds1307: properly handle oscillator failure flags

Stop enabling the oscillator and removing the oscillator failure flags in
probe. Instead, return -EINVAL in .read_time when the oscillaotr is not
start or when it failed at some point. The oscillator gets enabled on the
first .set_time after failure and the failure flags are cleared.

This also removes the possibility of an infinite loop at probe where a
failing RTC will make the goto read_rtc to be taken every time.

Tested on mcp79411.
Reported-by: Mastro Gippo <gipmad@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-ds1307.c

@@ -225,6 +225,45 @@ static int ds1307_get_time(struct device *dev, struct rtc_time *t)
 		return -EINVAL;
 	}
 
+	tmp = regs[DS1307_REG_SECS];
+	switch (ds1307->type) {
+	case ds_1307:
+	case m41t0:
+	case m41t00:
+	case m41t11:
+		if (tmp & DS1307_BIT_CH)
+			return -EINVAL;
+		break;
+	case ds_1308:
+	case ds_1338:
+		if (tmp & DS1307_BIT_CH)
+			return -EINVAL;
+
+		ret = regmap_read(ds1307->regmap, DS1307_REG_CONTROL, &tmp);
+		if (ret)
+			return ret;
+		if (tmp & DS1338_BIT_OSF)
+			return -EINVAL;
+		break;
+	case ds_1340:
+		if (tmp & DS1340_BIT_nEOSC)
+			return -EINVAL;
+
+		ret = regmap_read(ds1307->regmap, DS1340_REG_FLAG, &tmp);
+		if (ret)
+			return ret;
+		if (tmp & DS1340_BIT_OSF)
+			return -EINVAL;
+		break;
+	case mcp794xx:
+		if (!(tmp & MCP794XX_BIT_ST))
+			return -EINVAL;
+
+		break;
+	default:
+		break;
+	}
+
 	t->tm_sec = bcd2bin(regs[DS1307_REG_SECS] & 0x7f);
 	t->tm_min = bcd2bin(regs[DS1307_REG_MIN] & 0x7f);
 	tmp = regs[DS1307_REG_HOUR] & 0x3f;
@@ -289,7 +328,17 @@ static int ds1307_set_time(struct device *dev, struct rtc_time *t)
 	if (t->tm_year > 199 && chip->century_bit)
 		regs[chip->century_reg] |= chip->century_bit;
 
-	if (ds1307->type == mcp794xx) {
+	switch (ds1307->type) {
+	case ds_1308:
+	case ds_1338:
+		regmap_update_bits(ds1307->regmap, DS1307_REG_CONTROL,
+				   DS1338_BIT_OSF, 0);
+		break;
+	case ds_1340:
+		regmap_update_bits(ds1307->regmap, DS1340_REG_FLAG,
+				   DS1340_BIT_OSF, 0);
+		break;
+	case mcp794xx:
 		/*
 		 * these bits were cleared when preparing the date/time
 		 * values and need to be set again before writing the
@@ -297,6 +346,9 @@ static int ds1307_set_time(struct device *dev, struct rtc_time *t)
 		 */
 		regs[DS1307_REG_SECS] |= MCP794XX_BIT_ST;
 		regs[DS1307_REG_WDAY] |= MCP794XX_BIT_VBATEN;
+		break;
+	default:
+		break;
 	}
 
 	dev_dbg(dev, "%s: %7ph\n", "write", regs);
@@ -1705,7 +1757,6 @@ static int ds1307_probe(struct i2c_client *client,
 		break;
 	}
 
-read_rtc:
 	/* read RTC registers */
 	err = regmap_bulk_read(ds1307->regmap, chip->offset, regs,
 			       sizeof(regs));
@@ -1714,77 +1765,13 @@ static int ds1307_probe(struct i2c_client *client,
 		goto exit;
 	}
 
-	/*
-	 * minimal sanity checking; some chips (like DS1340) don't
-	 * specify the extra bits as must-be-zero, but there are
-	 * still a few values that are clearly out-of-range.
-	 */
-	tmp = regs[DS1307_REG_SECS];
-	switch (ds1307->type) {
-	case ds_1307:
-	case m41t0:
-	case m41t00:
-	case m41t11:
-		/* clock halted?  turn it on, so clock can tick. */
-		if (tmp & DS1307_BIT_CH) {
-			regmap_write(ds1307->regmap, DS1307_REG_SECS, 0);
-			dev_warn(ds1307->dev, "SET TIME!\n");
-			goto read_rtc;
-		}
-		break;
-	case ds_1308:
-	case ds_1338:
-		/* clock halted?  turn it on, so clock can tick. */
-		if (tmp & DS1307_BIT_CH)
-			regmap_write(ds1307->regmap, DS1307_REG_SECS, 0);
-
-		/* oscillator fault?  clear flag, and warn */
-		if (regs[DS1307_REG_CONTROL] & DS1338_BIT_OSF) {
-			regmap_write(ds1307->regmap, DS1307_REG_CONTROL,
-				     regs[DS1307_REG_CONTROL] &
-				     ~DS1338_BIT_OSF);
-			dev_warn(ds1307->dev, "SET TIME!\n");
-			goto read_rtc;
-		}
-		break;
-	case ds_1340:
-		/* clock halted?  turn it on, so clock can tick. */
-		if (tmp & DS1340_BIT_nEOSC)
-			regmap_write(ds1307->regmap, DS1307_REG_SECS, 0);
-
-		err = regmap_read(ds1307->regmap, DS1340_REG_FLAG, &tmp);
-		if (err) {
-			dev_dbg(ds1307->dev, "read error %d\n", err);
-			goto exit;
-		}
-
-		/* oscillator fault?  clear flag, and warn */
-		if (tmp & DS1340_BIT_OSF) {
-			regmap_write(ds1307->regmap, DS1340_REG_FLAG, 0);
-			dev_warn(ds1307->dev, "SET TIME!\n");
-		}
-		break;
-	case mcp794xx:
-		/* make sure that the backup battery is enabled */
-		if (!(regs[DS1307_REG_WDAY] & MCP794XX_BIT_VBATEN)) {
+	if (ds1307->type == mcp794xx &&
+	    !(regs[DS1307_REG_WDAY] & MCP794XX_BIT_VBATEN)) {
 		regmap_write(ds1307->regmap, DS1307_REG_WDAY,
 			     regs[DS1307_REG_WDAY] |
 			     MCP794XX_BIT_VBATEN);
 	}
 
-		/* clock halted?  turn it on, so clock can tick. */
-		if (!(tmp & MCP794XX_BIT_ST)) {
-			regmap_write(ds1307->regmap, DS1307_REG_SECS,
-				     MCP794XX_BIT_ST);
-			dev_warn(ds1307->dev, "SET TIME!\n");
-			goto read_rtc;
-		}
-
-		break;
-	default:
-		break;
-	}
-
 	tmp = regs[DS1307_REG_HOUR];
 	switch (ds1307->type) {
 	case ds_1340: