rtc: s35390a: fix reading out alarm

There are several issues fixed in this patch:

 - When alarm isn't enabled, set .enabled to zero instead of returning
   -EINVAL.
 - Ignore how IRQ1 is configured when determining if IRQ2 is on.
 - The three alarm registers have an enable flag which must be
   evaluated.
 - The chip always triggers when the seconds register gets 0.

Note that the rtc framework however doesn't handle the result correctly
because it doesn't check wday being initialized and so interprets an
alarm being set for 10:00 AM in three days as 10:00 AM tomorrow (or
today if that's not over yet).
Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-s35390a.c

@@ -242,6 +242,8 @@ static int s35390a_set_alarm(struct i2c_client *client, struct rtc_wkalrm *alm)
 
 	if (alm->time.tm_wday != -1)
 		buf[S35390A_ALRM_BYTE_WDAY] = bin2bcd(alm->time.tm_wday) | 0x80;
+	else
+		buf[S35390A_ALRM_BYTE_WDAY] = 0;
 
 	buf[S35390A_ALRM_BYTE_HOURS] = s35390a_hr2reg(s35390a,
 			alm->time.tm_hour) | 0x80;
@@ -269,23 +271,43 @@ static int s35390a_read_alarm(struct i2c_client *client, struct rtc_wkalrm *alm)
 	if (err < 0)
 		return err;
 
-	if (bitrev8(sts) != S35390A_INT2_MODE_ALARM)
-		return -EINVAL;
+	if ((bitrev8(sts) & S35390A_INT2_MODE_MASK) != S35390A_INT2_MODE_ALARM) {
+		/*
+		 * When the alarm isn't enabled, the register to configure
+		 * the alarm time isn't accessible.
+		 */
+		alm->enabled = 0;
+		return 0;
+	} else {
+		alm->enabled = 1;
+	}
 
 	err = s35390a_get_reg(s35390a, S35390A_CMD_INT2_REG1, buf, sizeof(buf));
 	if (err < 0)
 		return err;
 
 	/* This chip returns the bits of each byte in reverse order */
-	for (i = 0; i < 3; ++i) {
+	for (i = 0; i < 3; ++i)
 		buf[i] = bitrev8(buf[i]);
-		buf[i] &= ~0x80;
-	}
 
-	alm->time.tm_wday = bcd2bin(buf[S35390A_ALRM_BYTE_WDAY]);
-	alm->time.tm_hour = s35390a_reg2hr(s35390a,
-						buf[S35390A_ALRM_BYTE_HOURS]);
-	alm->time.tm_min = bcd2bin(buf[S35390A_ALRM_BYTE_MINS]);
+	/*
+	 * B0 of the three matching registers is an enable flag. Iff it is set
+	 * the configured value is used for matching.
+	 */
+	if (buf[S35390A_ALRM_BYTE_WDAY] & 0x80)
+		alm->time.tm_wday =
+			bcd2bin(buf[S35390A_ALRM_BYTE_WDAY] & ~0x80);
+
+	if (buf[S35390A_ALRM_BYTE_HOURS] & 0x80)
+		alm->time.tm_hour =
+			s35390a_reg2hr(s35390a,
+				       buf[S35390A_ALRM_BYTE_HOURS] & ~0x80);
+
+	if (buf[S35390A_ALRM_BYTE_MINS] & 0x80)
+		alm->time.tm_min = bcd2bin(buf[S35390A_ALRM_BYTE_MINS] & ~0x80);
+
+	/* alarm triggers always at s=0 */
+	alm->time.tm_sec = 0;
 
 	dev_dbg(&client->dev, "%s: alm is mins=%d, hours=%d, wday=%d\n",
 			__func__, alm->time.tm_min, alm->time.tm_hour,