[PATCH] w1: add ability to set (SRAM) and store (EEPROM) configuration for temp sensors like DS18B20

[x29a]

based on the PR (https://github.com/raspberrypi/linux/pull/1412) for the raspberry pi kernel, i would like to propose my changes upstream as well.

The changes entitle basic write operations for the 1Wire driver which are needed to change e.g. the precision of temperature sensors like the very popular DS18B20. Writing to SRAM and commiting the values to EEPROM is possible.

Since many sensors come "preconfigured" with a lower precision, people are stuck that precision when running on a kernel based device (unlike the Dallas 1Wire library for e.g. Arduino, which supports writing the configuration/scratchpad).

Signed-off-by: x29a <0.x29a.0@xxxxxxxxx>
---
 Documentation/w1/slaves/w1_therm |  10 +-
 drivers/w1/slaves/w1_therm.c     | 218 +++++++++++++++++++++++++++++++++++++--
 drivers/w1/w1.h                  |   2 +
 3 files changed, 222 insertions(+), 8 deletions(-)

diff --git a/Documentation/w1/slaves/w1_therm b/Documentation/w1/slaves/w1_therm
index 13411fe..d1f93af 100644
--- a/Documentation/w1/slaves/w1_therm
+++ b/Documentation/w1/slaves/w1_therm
@@ -33,7 +33,15 @@ temperature conversion at a time.  If none of the devices are parasite
 powered it would be possible to convert all the devices at the same
 time and then go back to read individual sensors.  That isn't
 currently supported.  The driver also doesn't support reduced
-precision (which would also reduce the conversion time).
+precision (which would also reduce the conversion time) when reading values.
+
+Writing a value between 9 and 12 to the sysfs w1_slave file will change the
+precision of the sensor for the next readings. This value is in (volatile)
+SRAM, so it is reset when the sensor gets power-cycled.
+
+To store the current precision configuration into EEPROM, the value 0
+has to be written to the sysfs w1_slave file. Since the EEPROM has a limited
+amount of writes (>50k), this command should be used wisely.
 
 The module parameter strong_pullup can be set to 0 to disable the
 strong pullup, 1 to enable autodetection or 2 to force strong pullup.
diff --git a/drivers/w1/slaves/w1_therm.c b/drivers/w1/slaves/w1_therm.c
index 2f029e8..581a300 100644
--- a/drivers/w1/slaves/w1_therm.c
+++ b/drivers/w1/slaves/w1_therm.c
@@ -92,10 +92,13 @@ static void w1_therm_remove_slave(struct w1_slave *sl)
 static ssize_t w1_slave_show(struct device *device,
 	struct device_attribute *attr, char *buf);
 
+static ssize_t w1_slave_store(struct device *device,
+	struct device_attribute *attr, const char *buf, size_t size);
+
 static ssize_t w1_seq_show(struct device *device,
 	struct device_attribute *attr, char *buf);
 
-static DEVICE_ATTR_RO(w1_slave);
+static DEVICE_ATTR_RW(w1_slave);
 static DEVICE_ATTR_RO(w1_seq);
 
 static struct attribute *w1_therm_attrs[] = {
@@ -154,8 +157,17 @@ struct w1_therm_family_converter
 	u16			reserved;
 	struct w1_family	*f;
 	int			(*convert)(u8 rom[9]);
+	int			(*precision)(struct device *device, int val);
+	int			(*eeprom)(struct device *device);
 };
 
+/* write configuration to eeprom */
+static inline int w1_therm_eeprom(struct device *device);
+
+/* Set precision for conversion */
+static inline int w1_DS18B20_precision(struct device *device, int val);
+static inline int w1_DS18S20_precision(struct device *device, int val);
+
 /* The return value is millidegrees Centigrade. */
 static inline int w1_DS18B20_convert_temp(u8 rom[9]);
 static inline int w1_DS18S20_convert_temp(u8 rom[9]);
@@ -163,26 +175,194 @@ static inline int w1_DS18S20_convert_temp(u8 rom[9]);
 static struct w1_therm_family_converter w1_therm_families[] = {
 	{
 		.f		= &w1_therm_family_DS18S20,
-		.convert 	= w1_DS18S20_convert_temp
+		.convert	= w1_DS18S20_convert_temp,
+		.precision	= w1_DS18S20_precision,
+		.eeprom		= w1_therm_eeprom
 	},
 	{
 		.f		= &w1_therm_family_DS1822,
-		.convert 	= w1_DS18B20_convert_temp
+		.convert	= w1_DS18B20_convert_temp,
+		.precision	= w1_DS18S20_precision,
+		.eeprom		= w1_therm_eeprom
 	},
 	{
 		.f		= &w1_therm_family_DS18B20,
-		.convert 	= w1_DS18B20_convert_temp
+		.convert	= w1_DS18B20_convert_temp,
+		.precision	= w1_DS18B20_precision,
+		.eeprom		= w1_therm_eeprom
 	},
 	{
 		.f		= &w1_therm_family_DS28EA00,
-		.convert	= w1_DS18B20_convert_temp
+		.convert	= w1_DS18B20_convert_temp,
+		.precision	= w1_DS18S20_precision,
+		.eeprom		= w1_therm_eeprom
 	},
 	{
 		.f		= &w1_therm_family_DS1825,
-		.convert	= w1_DS18B20_convert_temp
+		.convert	= w1_DS18B20_convert_temp,
+		.precision	= w1_DS18S20_precision,
+		.eeprom		= w1_therm_eeprom
 	}
 };
 
+static inline int w1_therm_eeprom(struct device *device)
+{
+	struct w1_slave *sl = dev_to_w1_slave(device);
+	struct w1_master *dev = sl->master;
+	u8 rom[9], external_power;
+	int ret, max_trying = 10;
+	u8 *family_data = sl->family_data;
+
+	ret = mutex_lock_interruptible(&dev->bus_mutex);
+	if (ret != 0)
+		goto post_unlock;
+
+	if (!sl->family_data) {
+		ret = -ENODEV;
+		goto pre_unlock;
+	}
+
+	/* prevent the slave from going away in sleep */
+	atomic_inc(THERM_REFCNT(family_data));
+	memset(rom, 0, sizeof(rom));
+
+	while (max_trying--) {
+		if (!w1_reset_select_slave(sl)) {
+			unsigned int tm = 10;
+			unsigned long sleep_rem;
+
+			/* check if in parasite mode */
+			w1_write_8(dev, W1_READ_PSUPPLY);
+			external_power = w1_read_8(dev);
+
+			if (w1_reset_select_slave(sl))
+				continue;
+
+			/* 10ms strong pullup/delay after the copy command */
+			if (w1_strong_pullup == 2 ||
+			    (!external_power && w1_strong_pullup))
+				w1_next_pullup(dev, tm);
+
+			w1_write_8(dev, W1_COPY_SCRATCHPAD);
+
+			if (external_power) {
+				mutex_unlock(&dev->bus_mutex);
+
+				sleep_rem = msleep_interruptible(tm);
+				if (sleep_rem != 0) {
+					ret = -EINTR;
+					goto post_unlock;
+				}
+
+				ret = mutex_lock_interruptible(&dev->bus_mutex);
+				if (ret != 0)
+					goto post_unlock;
+			} else if (!w1_strong_pullup) {
+				sleep_rem = msleep_interruptible(tm);
+				if (sleep_rem != 0) {
+					ret = -EINTR;
+					goto pre_unlock;
+				}
+			}
+
+			break;
+		}
+	}
+
+pre_unlock:
+	mutex_unlock(&dev->bus_mutex);
+
+post_unlock:
+	atomic_dec(THERM_REFCNT(family_data));
+	return ret;
+}
+
+/* DS18S20 does not feature configuration register */
+static inline int w1_DS18S20_precision(struct device *device, int val)
+{
+	return 0;
+}
+
+static inline int w1_DS18B20_precision(struct device *device, int val)
+{
+	struct w1_slave *sl = dev_to_w1_slave(device);
+	struct w1_master *dev = sl->master;
+	u8 rom[9], crc;
+	int ret, max_trying = 10;
+	u8 *family_data = sl->family_data;
+	uint8_t precision_bits;
+	uint8_t mask = 0x60;
+
+	if(val > 12 || val < 9) {
+		pr_warn("Unsupported precision\n");
+		return -1;
+	}
+
+	ret = mutex_lock_interruptible(&dev->bus_mutex);
+	if (ret != 0)
+		goto post_unlock;
+
+	if (!sl->family_data) {
+		ret = -ENODEV;
+		goto pre_unlock;
+	}
+
+	/* prevent the slave from going away in sleep */
+	atomic_inc(THERM_REFCNT(family_data));
+	memset(rom, 0, sizeof(rom));
+
+	/* translate precision to bitmask (see datasheet page 9) */
+	switch (val) {
+	case 9:
+		precision_bits = 0x00;
+		break;
+	case 10:
+		precision_bits = 0x20;
+		break;
+	case 11:
+		precision_bits = 0x40;
+		break;
+	case 12:
+	default:
+		precision_bits = 0x60;
+		break;
+	}
+
+	while (max_trying--) {
+		crc = 0;
+
+		if (!w1_reset_select_slave(sl)) {
+			int count = 0;
+
+			/* read values to only alter precision bits */
+			w1_write_8(dev, W1_READ_SCRATCHPAD);
+			if ((count = w1_read_block(dev, rom, 9)) != 9)
+				dev_warn(device, "w1_read_block() returned %u instead of 9.\n",	count);
+
+			crc = w1_calc_crc8(rom, 8);
+			if (rom[8] == crc) {
+				rom[4] = (rom[4] & ~mask) | (precision_bits & mask);
+
+				if (!w1_reset_select_slave(sl)) {
+					w1_write_8(dev, W1_WRITE_SCRATCHPAD);
+					w1_write_8(dev, rom[2]);
+					w1_write_8(dev, rom[3]);
+					w1_write_8(dev, rom[4]);
+
+					break;
+				}
+			}
+		}
+	}
+
+pre_unlock:
+	mutex_unlock(&dev->bus_mutex);
+
+post_unlock:
+	atomic_dec(THERM_REFCNT(family_data));
+	return ret;
+}
+
 static inline int w1_DS18B20_convert_temp(u8 rom[9])
 {
 	s16 t = le16_to_cpup((__le16 *)rom);
@@ -220,6 +400,30 @@ static inline int w1_convert_temp(u8 rom[9], u8 fid)
 	return 0;
 }
 
+static ssize_t w1_slave_store(struct device *device,
+			      struct device_attribute *attr, const char *buf,
+			      size_t size)
+{
+	int val, ret;
+	struct w1_slave *sl = dev_to_w1_slave(device);
+	int i;
+
+	ret = kstrtoint(buf, 0, &val);
+	if (ret)
+		return ret;
+
+	for (i = 0; i < ARRAY_SIZE(w1_therm_families); ++i) {
+		if (w1_therm_families[i].f->fid == sl->family->fid) {
+			/* zero value indicates to write current configuration to eeprom */
+			if (0 == val)
+				ret = w1_therm_families[i].eeprom(device);
+			else
+				ret = w1_therm_families[i].precision(device, val);
+			break;
+		}
+	}
+	return ret ? : size;
+}
 
 static ssize_t w1_slave_show(struct device *device,
 	struct device_attribute *attr, char *buf)
@@ -311,7 +515,7 @@ static ssize_t w1_slave_show(struct device *device,
 	for (i = 0; i < 9; ++i)
 		c -= snprintf(buf + PAGE_SIZE - c, c, "%02x ", rom[i]);
 	c -= snprintf(buf + PAGE_SIZE - c, c, ": crc=%02x %s\n",
-			   crc, (verdict) ? "YES" : "NO");
+		      crc, (verdict) ? "YES" : "NO");
 	if (verdict)
 		memcpy(family_data, rom, sizeof(rom));
 	else
diff --git a/drivers/w1/w1.h b/drivers/w1/w1.h
index 56a49ba..129895f 100644
--- a/drivers/w1/w1.h
+++ b/drivers/w1/w1.h
@@ -58,6 +58,8 @@ struct w1_reg_num
 #define W1_ALARM_SEARCH		0xEC
 #define W1_CONVERT_TEMP		0x44
 #define W1_SKIP_ROM		0xCC
+#define W1_COPY_SCRATCHPAD	0x48
+#define W1_WRITE_SCRATCHPAD	0x4E
 #define W1_READ_SCRATCHPAD	0xBE
 #define W1_READ_ROM		0x33
 #define W1_READ_PSUPPLY		0xB4
-- 
2.5.0