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Date:   Sun, 5 Apr 2020 09:27:38 -0700
From:   Guenter Roeck <linux@roeck-us.net>
To:     Kun Yi <kunyi@google.com>
Cc:     jdelvare@suse.com, robh+dt@kernel.org, mark.rutland@arm.com,
        openbmc@lists.ozlabs.org, joel@jms.id.au,
        linux-hwmon@vger.kernel.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH linux hwmon-next v3 1/3] hwmon: (sbtsi) Add basic support
 for SB-TSI sensors
Message-ID: <20200405162738.GA161211@roeck-us.net>
References: <20200405030118.191950-1-kunyi@google.com>
 <20200405030118.191950-2-kunyi@google.com>
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On Sat, Apr 04, 2020 at 08:01:16PM -0700, Kun Yi wrote:
> SB Temperature Sensor Interface (SB-TSI) is an SMBus compatible
> interface that reports AMD SoC's Ttcl (normalized temperature),
> and resembles a typical 8-pin remote temperature sensor's I2C interface
> to BMC.
> 
> This commit adds basic support using this interface to read CPU
> temperature, and read/write high/low CPU temp thresholds.
> 
> To instantiate this driver on an AMD CPU with SB-TSI
> support, the i2c bus number would be the bus connected from the board
> management controller (BMC) to the CPU. The i2c address is specified in
> Section 6.3.1 of the spec [1]: The SB-TSI address is normally 98h for socket 0
> and 90h for socket 1, but it could vary based on hardware address select pins.
> 
> [1]: https://www.amd.com/system/files/TechDocs/56255_OSRR.pdf
> 
> Test status: tested reading temp1_input, and reading/writing
> temp1_max/min.
> 
> Signed-off-by: Kun Yi <kunyi@google.com>
> ---
>  drivers/hwmon/Kconfig      |  10 ++
>  drivers/hwmon/Makefile     |   1 +
>  drivers/hwmon/sbtsi_temp.c | 259 +++++++++++++++++++++++++++++++++++++
>  3 files changed, 270 insertions(+)
>  create mode 100644 drivers/hwmon/sbtsi_temp.c
> 
> diff --git a/drivers/hwmon/Kconfig b/drivers/hwmon/Kconfig
> index 05a30832c6ba..9585dcd01d1b 100644
> --- a/drivers/hwmon/Kconfig
> +++ b/drivers/hwmon/Kconfig
> @@ -1412,6 +1412,16 @@ config SENSORS_RASPBERRYPI_HWMON
>  	  This driver can also be built as a module. If so, the module
>  	  will be called raspberrypi-hwmon.
>  
> +config SENSORS_SBTSI
> +	tristate "Emulated SB-TSI temperature sensor"
> +	depends on I2C
> +	help
> +	  If you say yes here you get support for emulated temperature
> +	  sensors on AMD SoCs with SB-TSI interface connected to a BMC device.
> +
> +	  This driver can also be built as a module. If so, the module will
> +	  be called sbtsi_temp.
> +
>  config SENSORS_SHT15
>  	tristate "Sensiron humidity and temperature sensors. SHT15 and compat."
>  	depends on GPIOLIB || COMPILE_TEST
> diff --git a/drivers/hwmon/Makefile b/drivers/hwmon/Makefile
> index b0b9c8e57176..cd109f003ce4 100644
> --- a/drivers/hwmon/Makefile
> +++ b/drivers/hwmon/Makefile
> @@ -152,6 +152,7 @@ obj-$(CONFIG_SENSORS_POWR1220)  += powr1220.o
>  obj-$(CONFIG_SENSORS_PWM_FAN)	+= pwm-fan.o
>  obj-$(CONFIG_SENSORS_RASPBERRYPI_HWMON)	+= raspberrypi-hwmon.o
>  obj-$(CONFIG_SENSORS_S3C)	+= s3c-hwmon.o
> +obj-$(CONFIG_SENSORS_SBTSI)	+= sbtsi_temp.o
>  obj-$(CONFIG_SENSORS_SCH56XX_COMMON)+= sch56xx-common.o
>  obj-$(CONFIG_SENSORS_SCH5627)	+= sch5627.o
>  obj-$(CONFIG_SENSORS_SCH5636)	+= sch5636.o
> diff --git a/drivers/hwmon/sbtsi_temp.c b/drivers/hwmon/sbtsi_temp.c
> new file mode 100644
> index 000000000000..e3ad6a9f7ec1
> --- /dev/null
> +++ b/drivers/hwmon/sbtsi_temp.c
> @@ -0,0 +1,259 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * sbtsi_temp.c - hwmon driver for a SBI Temperature Sensor Interface (SB-TSI)
> + *                compliant AMD SoC temperature device.
> + *
> + * Copyright (c) 2020, Google Inc.
> + * Copyright (c) 2020, Kun Yi <kunyi@google.com>
> + */
> +
> +#include <linux/err.h>
> +#include <linux/i2c.h>
> +#include <linux/init.h>
> +#include <linux/hwmon.h>
> +#include <linux/module.h>
> +#include <linux/mutex.h>
> +#include <linux/of_device.h>
> +#include <linux/of.h>
> +
> +/*
> + * SB-TSI registers only support SMBus byte data access. "_INT" registers are
> + * the integer part of a temperature value or limit, and "_DEC" registers are
> + * corresponding decimal parts.
> + */
> +#define SBTSI_REG_TEMP_INT		0x01 /* RO */
> +#define SBTSI_REG_STATUS		0x02 /* RO */
> +#define SBTSI_REG_CONFIG		0x03 /* RO */
> +#define SBTSI_REG_TEMP_HIGH_INT		0x07 /* RW */
> +#define SBTSI_REG_TEMP_LOW_INT		0x08 /* RW */
> +#define SBTSI_REG_TEMP_DEC		0x10 /* RW */
> +#define SBTSI_REG_TEMP_HIGH_DEC		0x13 /* RW */
> +#define SBTSI_REG_TEMP_LOW_DEC		0x14 /* RW */
> +#define SBTSI_REG_REV			0xFF /* RO */

The revision register is not actually used. 

> +
> +#define SBTSI_CONFIG_READ_ORDER_SHIFT	5
> +
> +#define SBTSI_TEMP_MIN	0
> +#define SBTSI_TEMP_MAX	255875
> +#define SBTSI_REV_MAX_VALID_ID	4

Not actually used, and I am not sure if it would make sense to check it.
If at all, it would only make sense if you also check SBTSIxFE (Manufacture
ID). Unfortunately, the actual SB-TSI specification seems to be non-public,
so I can't check if the driver as-is supports versions 0..3 (assuming those
exist).

> +
> +/* Each client has this additional data */
> +struct sbtsi_data {
> +	struct i2c_client *client;
> +	struct mutex lock;
> +};
> +
> +/*
> + * From SB-TSI spec: CPU temperature readings and limit registers encode the
> + * temperature in increments of 0.125 from 0 to 255.875. The "high byte"
> + * register encodes the base-2 of the integer portion, and the upper 3 bits of
> + * the "low byte" encode in base-2 the decimal portion.
> + *
> + * e.g. INT=0x19, DEC=0x20 represents 25.125 degrees Celsius
> + *
> + * Therefore temperature in millidegree Celsius =
> + *   (INT + DEC / 256) * 1000 = (INT * 8 + DEC / 32) * 125
> + */
> +static inline int sbtsi_reg_to_mc(s32 integer, s32 decimal)
> +{
> +	return ((integer << 3) + (decimal >> 5)) * 125;
> +}
> +
> +/*
> + * Inversely, given temperature in millidegree Celsius
> + *   INT = (TEMP / 125) / 8
> + *   DEC = ((TEMP / 125) % 8) * 32
> + * Caller have to make sure temp doesn't exceed 255875, the max valid value.
> + */
> +static inline void sbtsi_mc_to_reg(s32 temp, u8 *integer, u8 *decimal)
> +{
> +	temp /= 125;
> +	*integer = temp >> 3;
> +	*decimal = (temp & 0x7) << 5;
> +}
> +
> +static int sbtsi_read(struct device *dev, enum hwmon_sensor_types type,
> +		      u32 attr, int channel, long *val)
> +{
> +	struct sbtsi_data *data = dev_get_drvdata(dev);
> +	s32 temp_int, temp_dec;
> +	int err, reg_int, reg_dec;
> +	u8 read_order;
> +
> +	if (type != hwmon_temp)
> +		return -EINVAL;
> +
> +	read_order = 0;
> +	switch (attr) {
> +	case hwmon_temp_input:
> +		/*
> +		 * ReadOrder bit specifies the reading order of integer and
> +		 * decimal part of CPU temp for atomic reads. If bit == 0,
> +		 * reading integer part triggers latching of the decimal part,
> +		 * so integer part should be read first. If bit == 1, read
> +		 * order should be reversed.
> +		 */
> +		err = i2c_smbus_read_byte_data(data->client, SBTSI_REG_CONFIG);
> +		if (err < 0)
> +			return err;
> +
As I understand it, the idea is to set this configuration bit once and then
just use it. Any chance to do that ? This would save an i2c read operation
each time the temperature is read, and the if/else complexity below.

> +		read_order = (u8)err & BIT(SBTSI_CONFIG_READ_ORDER_SHIFT);

Nit: typecast is unnecessary.

> +		reg_int = SBTSI_REG_TEMP_INT;
> +		reg_dec = SBTSI_REG_TEMP_DEC;
> +		break;
> +	case hwmon_temp_max:
> +		reg_int = SBTSI_REG_TEMP_HIGH_INT;
> +		reg_dec = SBTSI_REG_TEMP_HIGH_DEC;
> +		break;
> +	case hwmon_temp_min:
> +		reg_int = SBTSI_REG_TEMP_LOW_INT;
> +		reg_dec = SBTSI_REG_TEMP_LOW_DEC;
> +		break;
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	if (read_order == 0) {
> +		temp_int = i2c_smbus_read_byte_data(data->client, reg_int);
> +		temp_dec = i2c_smbus_read_byte_data(data->client, reg_dec);
> +	} else {
> +		temp_dec = i2c_smbus_read_byte_data(data->client, reg_dec);
> +		temp_int = i2c_smbus_read_byte_data(data->client, reg_int);
> +	}

Just a thought: if you use regmap and tell it that the limit registers
are non-volatile, this wouldn't actually read from the chip more than once.

Also, since the read involves reading two registers, and the first read
locks the value for the second, you'll need mutex protection when reading
the current temperature (not for limits, though).

> +
> +	if (temp_int < 0)
> +		return temp_int;
> +	if (temp_dec < 0)
> +		return temp_dec;
> +
> +	*val = sbtsi_reg_to_mc(temp_int, temp_dec);
> +
> +	return 0;
> +}
> +
> +static int sbtsi_write(struct device *dev, enum hwmon_sensor_types type,
> +		       u32 attr, int channel, long val)
> +{
> +	struct sbtsi_data *data = dev_get_drvdata(dev);
> +	int reg_int, reg_dec, err;
> +	u8 temp_int, temp_dec;
> +
> +	if (type != hwmon_temp)
> +		return -EINVAL;
> +
> +	switch (attr) {
> +	case hwmon_temp_max:
> +		reg_int = SBTSI_REG_TEMP_HIGH_INT;
> +		reg_dec = SBTSI_REG_TEMP_HIGH_DEC;
> +		break;
> +	case hwmon_temp_min:
> +		reg_int = SBTSI_REG_TEMP_LOW_INT;
> +		reg_dec = SBTSI_REG_TEMP_LOW_DEC;
> +		break;
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	val = clamp_val(val, SBTSI_TEMP_MIN, SBTSI_TEMP_MAX);
> +	mutex_lock(&data->lock);
> +	sbtsi_mc_to_reg(val, &temp_int, &temp_dec);
> +	err = i2c_smbus_write_byte_data(data->client, reg_int, temp_int);
> +	if (err)
> +		goto exit;
> +
> +	err = i2c_smbus_write_byte_data(data->client, reg_dec, temp_dec);
> +exit:
> +	mutex_unlock(&data->lock);
> +	return err;
> +}
> +
> +static umode_t sbtsi_is_visible(const void *data,
> +				enum hwmon_sensor_types type,
> +				u32 attr, int channel)
> +{
> +	switch (type) {
> +	case hwmon_temp:
> +		switch (attr) {
> +		case hwmon_temp_input:
> +			return 0444;
> +		case hwmon_temp_min:
> +			return 0644;
> +		case hwmon_temp_max:
> +			return 0644;
> +		}
> +		break;
> +	default:
> +		break;
> +	}
> +	return 0;
> +}
> +
> +static const struct hwmon_channel_info *sbtsi_info[] = {
> +	HWMON_CHANNEL_INFO(chip,
> +			   HWMON_C_REGISTER_TZ),
> +	HWMON_CHANNEL_INFO(temp,
> +			   HWMON_T_INPUT | HWMON_T_MIN | HWMON_T_MAX),

For your consideration: SB-TSI supports reporting high/low alerts.
With this, it would be possible to implement respective alarm attributes.
In conjunction with https://patchwork.kernel.org/patch/11277347/mbox/,
it should also be possible to add interrupt and thus userspace notification
for those attributes.

SBTSI also supports setting the update rate (SBTSIx04) and setting
the temperature offset (SBTSIx11, SBTSIx12), which could also be
implemented as standard attributes.

I won't require that for the initial version, just something to keep
in mind.

> +	NULL
> +};
> +
> +static const struct hwmon_ops sbtsi_hwmon_ops = {
> +	.is_visible = sbtsi_is_visible,
> +	.read = sbtsi_read,
> +	.write = sbtsi_write,
> +};
> +
> +static const struct hwmon_chip_info sbtsi_chip_info = {
> +	.ops = &sbtsi_hwmon_ops,
> +	.info = sbtsi_info,
> +};
> +
> +static int sbtsi_probe(struct i2c_client *client,
> +		       const struct i2c_device_id *id)
> +{
> +	struct device *dev = &client->dev;
> +	struct device *hwmon_dev;
> +	struct sbtsi_data *data;
> +
> +	data = devm_kzalloc(dev, sizeof(struct sbtsi_data), GFP_KERNEL);
> +	if (!data)
> +		return -ENOMEM;
> +
> +	data->client = client;
> +	mutex_init(&data->lock);
> +
> +	hwmon_dev =
> +		devm_hwmon_device_register_with_info(dev, client->name, data,
> +						     &sbtsi_chip_info, NULL);
> +
> +	return PTR_ERR_OR_ZERO(hwmon_dev);
> +}
> +
> +static const struct i2c_device_id sbtsi_id[] = {
> +	{"sbtsi", 0},
> +	{}
> +};
> +MODULE_DEVICE_TABLE(i2c, sbtsi_id);
> +
> +static const struct of_device_id __maybe_unused sbtsi_of_match[] = {
> +	{
> +		.compatible = "amd,sbtsi",
> +	},
> +	{ },
> +};
> +MODULE_DEVICE_TABLE(of, sbtsi_of_match);
> +
> +static struct i2c_driver sbtsi_driver = {
> +	.class = I2C_CLASS_HWMON,
> +	.driver = {
> +		.name = "sbtsi",
> +		.of_match_table = of_match_ptr(sbtsi_of_match),
> +	},
> +	.probe = sbtsi_probe,
> +	.id_table = sbtsi_id,
> +};
> +
> +module_i2c_driver(sbtsi_driver);
> +
> +MODULE_AUTHOR("Kun Yi <kunyi@google.com>");
> +MODULE_DESCRIPTION("Hwmon driver for AMD SB-TSI emulated sensor");
> +MODULE_LICENSE("GPL");