w1: ds2438: support for writing to offset register

Added a sysfs entry to support writing to the offset register on page1.
This register is used to calibrate the chip canceling offset errors in the
current ADC. This means that, over time, reading the IAD register will not
return the correct current measurement, it will have an offset. Writing to
the offset register if the two's complement of the current register while
passing zero current to the load will calibrate the measurements. This
change was tested on real hardware and it was able to calibrate the chip
correctly.

Signed-off-by: Luiz Sampaio <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/ABI/stable/sysfs-driver-w1_ds2438

···
 Description:	read the contents of the page1 of the DS2438
 		see Documentation/w1/slaves/w1_ds2438.rst for detailed information
 Users:		any user space application which wants to communicate with DS2438
+
+What:		/sys/bus/w1/devices/.../offset
+Date:		April 2021
+Contact:	Luiz Sampaio <[e-mail]>
+Description:	write the contents to the offset register of the DS2438
+		see Documentation/w1/slaves/w1_ds2438.rst for detailed information
+Users:		any user space application which wants to communicate with DS2438

Documentation/w1/slaves/w1_ds2438.rst

···
 wind speed/direction measuring, humidity sensing, etc.
 
 Current support is provided through the following sysfs files (all files
-except "iad" are readonly):
+except "iad" and "offset" are readonly):
 
 "iad"
 -----
···
 Internally when this file is read, the additional CRC byte is also obtained
 from the slave device. If it is correct, the 8 bytes page data are passed
 to userspace, otherwise an I/O error is returned.
+
+"offset"
+-------
+This file controls the 2-byte Offset Register of the chip.
+Writing a 2-byte value will change the Offset Register, which changes the
+current measurement done by the chip. Changing this register to the two's complement
+of the current register while forcing zero current through the load will calibrate
+the chip, canceling offset errors in the current ADC.
+
 
 "temperature"
 -------------

drivers/w1/slaves/w1_ds2438.c

···
 	return -1;
 }
 
+static int w1_ds2438_change_offset_register(struct w1_slave *sl, u8 *value)
+{
+	unsigned int retries = W1_DS2438_RETRIES;
+	u8 w1_buf[9];
+	u8 w1_page1_buf[DS2438_PAGE_SIZE + 1 /*for CRC*/];
+
+	if (w1_ds2438_get_page(sl, 1, w1_page1_buf) == 0) {
+		memcpy(&w1_buf[2], w1_page1_buf, DS2438_PAGE_SIZE - 1); /* last register reserved */
+		w1_buf[7] = value[0]; /* change only offset register */
+		w1_buf[8] = value[1];
+		while (retries--) {
+			if (w1_reset_select_slave(sl))
+				continue;
+			w1_buf[0] = W1_DS2438_WRITE_SCRATCH;
+			w1_buf[1] = 0x01; /* write to page 1 */
+			w1_write_block(sl->master, w1_buf, 9);
+
+			if (w1_reset_select_slave(sl))
+				continue;
+			w1_buf[0] = W1_DS2438_COPY_SCRATCH;
+			w1_buf[1] = 0x01;
+			w1_write_block(sl->master, w1_buf, 2);
+			return 0;
+		}
+	}
+	return -1;
+}
+
 static int w1_ds2438_get_voltage(struct w1_slave *sl,
 				 int adc_input, uint16_t *voltage)
 {
···
 	return ret;
 }
 
+static ssize_t offset_write(struct file *filp, struct kobject *kobj,
+			    struct bin_attribute *bin_attr, char *buf,
+			    loff_t off, size_t count)
+{
+	struct w1_slave *sl = kobj_to_w1_slave(kobj);
+	int ret;
+
+	mutex_lock(&sl->master->bus_mutex);
+
+	if (w1_ds2438_change_offset_register(sl, buf) == 0)
+		ret = count;
+	else
+		ret = -EIO;
+
+	mutex_unlock(&sl->master->bus_mutex);
+
+	return ret;
+}
+
 static ssize_t temperature_read(struct file *filp, struct kobject *kobj,
 				struct bin_attribute *bin_attr, char *buf,
 				loff_t off, size_t count)
···
 static BIN_ATTR_RW(iad, 0);
 static BIN_ATTR_RO(page0, DS2438_PAGE_SIZE);
 static BIN_ATTR_RO(page1, DS2438_PAGE_SIZE);
+static BIN_ATTR_WO(offset, 2);
 static BIN_ATTR_RO(temperature, 0/* real length varies */);
 static BIN_ATTR_RO(vad, 0/* real length varies */);
 static BIN_ATTR_RO(vdd, 0/* real length varies */);
···
 	&bin_attr_iad,
 	&bin_attr_page0,
 	&bin_attr_page1,
+	&bin_attr_offset,
 	&bin_attr_temperature,
 	&bin_attr_vad,
 	&bin_attr_vdd,