ASoC: tas2770: expose die temp to hwmon

TAS2770 includes an ADC which reports the chip's die temperature.
As per the datasheet, the temperature in degrees Celsius is derived
by taking the raw value stored in the ADC's registers, dividing by 16,
then subtracting 93.

Create and register a hwmon device to expose the chip's die temperature
to the hwmon interface.

The ADC is shut down during software shutdown mode, and its registers
are initialised to 0 on reset. This means that the die temperature will
read -93 *C until the chip has been fully powered up at least once (e.g.
the PCM its attached to is opened). Once the chip is put into software
shutdown again, the ADC will also shut down. The last value sampled
before this happens will persist in the ADC's registers.

Co-developed-by: Martin Povišer <[e-mail]>
Signed-off-by: Martin Povišer <[e-mail]>
Signed-off-by: James Calligeros <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

sound/soc/codecs/tas2770.c

···
 #include <linux/err.h>
 #include <linux/init.h>
 #include <linux/delay.h>
+#include <linux/hwmon.h>
 #include <linux/pm.h>
 #include <linux/i2c.h>
 #include <linux/gpio/consumer.h>
···
 	},
 };
 
+static int tas2770_read_die_temp(struct tas2770_priv *tas2770, long *result)
+{
+	int ret = 0;
+	int reading, msb, lsb;
+
+	ret = regmap_read(tas2770->regmap, TAS2770_TEMP_MSB, &msb);
+	if (ret)
+		return ret;
+
+	ret = regmap_read(tas2770->regmap, TAS2770_TEMP_LSB, &lsb);
+	if (ret)
+		return ret;
+
+	reading = (msb << 4) | (lsb >> 4);
+
+	/*
+	 * As per datasheet: divide register by 16 and subtract 93 to get
+	 * degrees Celsius. hwmon requires millidegrees. Let's avoid rounding
+	 * errors by subtracting 93 * 16 then multiplying by 1000 / 16.
+	 *
+	 * NOTE: The ADC registers are initialised to 0 on reset. This means
+	 * that the temperature will read -93 *C until the chip is brought out
+	 * of software shutdown (e.g. the PCM it's attached to is opened). The
+	 * ADC is also shut down in software shutdown/low-power mode, so the
+	 * value read back from its registers will be the last value sampled
+	 * before entering software shutdown.
+	 */
+	*result = (reading - (93 * 16)) * (1000 / 16);
+	return 0;
+}
+
+static umode_t tas2770_hwmon_is_visible(const void *data,
+					enum hwmon_sensor_types type, u32 attr,
+					int channel)
+{
+	if (type != hwmon_temp)
+		return 0;
+
+	switch (attr) {
+	case hwmon_temp_input:
+		return 0444;
+	default:
+		break;
+	}
+
+	return 0;
+}
+
+static int tas2770_hwmon_read(struct device *dev,
+			      enum hwmon_sensor_types type,
+			      u32 attr, int channel, long *val)
+{
+	struct tas2770_priv *tas2770 = dev_get_drvdata(dev);
+	int ret;
+
+	switch (attr) {
+	case hwmon_temp_input:
+		ret = tas2770_read_die_temp(tas2770, val);
+		break;
+	default:
+		ret = -EOPNOTSUPP;
+		break;
+	}
+
+	return ret;
+}
+
+static const struct hwmon_channel_info *const tas2770_hwmon_info[] = {
+	HWMON_CHANNEL_INFO(temp, HWMON_T_INPUT),
+	NULL
+};
+
+static const struct hwmon_ops tas2770_hwmon_ops = {
+	.is_visible	= tas2770_hwmon_is_visible,
+	.read		= tas2770_hwmon_read,
+};
+
+static const struct hwmon_chip_info tas2770_hwmon_chip_info = {
+	.ops	= &tas2770_hwmon_ops,
+	.info	= tas2770_hwmon_info,
+};
+
 static const struct regmap_config tas2770_i2c_regmap;
 
 static int tas2770_codec_probe(struct snd_soc_component *component)
···
 		if (PTR_ERR(tas2770->reset_gpio) == -EPROBE_DEFER) {
 			tas2770->reset_gpio = NULL;
 			return -EPROBE_DEFER;
+		}
+	}
+
+	if (IS_REACHABLE(CONFIG_HWMON)) {
+		struct device *hwmon;
+
+		hwmon = devm_hwmon_device_register_with_info(&client->dev, "tas2770",
+							tas2770,
+							&tas2770_hwmon_chip_info,
+							NULL);
+		if (IS_ERR(hwmon)) {
+			return dev_err_probe(&client->dev, PTR_ERR(hwmon),
+					     "Failed to register temp sensor\n");
 		}
 	}