hwmon: Add Baikal-T1 PVT sensor driver

Baikal-T1 SoC provides an embedded process, voltage and temperature
sensor to monitor an internal SoC environment (chip temperature, supply
voltage and process monitor) and on time detect critical situations,
which may cause the system instability and even damages. The IP-block
is based on the Analog Bits PVT sensor, but is equipped with a
dedicated control wrapper, which provides a MMIO registers-based access
to the sensor core functionality (APB3-bus based) and exposes an
additional functions like thresholds/data ready interrupts, its status
and masks, measurements timeout. All of these is used to create a hwmon
driver being added to the kernel by this commit.

The driver implements support for the hardware monitoring capabilities
of Baikal-T1 process, voltage and temperature sensors. PVT IP-core
consists of one temperature and four voltage sensors, each of which is
implemented as a dedicated hwmon channel config.

The driver can optionally provide the hwmon alarms for each sensor the
PVT controller supports. The alarms functionality is made compile-time
configurable due to the hardware interface implementation peculiarity,
which is connected with an ability to convert data from only one sensor
at a time. Additional limitation is that the controller performs the
thresholds checking synchronously with the data conversion procedure.
Due to these limitations in order to have the hwmon alarms
automatically detected the driver code must switch from one sensor to
another, read converted data and manually check the threshold status
bits. Depending on the measurements timeout settings this design may
cause additional burden on the system performance. By default if the
alarms kernel config is disabled the data conversion is performed by
the driver on demand when read operation is requested via corresponding
_input-file.

Co-developed-by: Maxim Kaurkin <maxim.kaurkin@baikalelectronics.ru>
Signed-off-by: Maxim Kaurkin <maxim.kaurkin@baikalelectronics.ru>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: devicetree@vger.kernel.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/bt1-pvt.rst

@@ -0,0 +1,117 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+
+Kernel driver bt1-pvt
+=====================
+
+Supported chips:
+
+  * Baikal-T1 PVT sensor (in SoC)
+
+    Prefix: 'bt1-pvt'
+
+    Addresses scanned: -
+
+    Datasheet: Provided by BAIKAL ELECTRONICS upon request and under NDA
+
+Authors:
+    Maxim Kaurkin <maxim.kaurkin@baikalelectronics.ru>
+    Serge Semin <Sergey.Semin@baikalelectronics.ru>
+
+Description
+-----------
+
+This driver implements support for the hardware monitoring capabilities of the
+embedded into Baikal-T1 process, voltage and temperature sensors. PVT IP-core
+consists of one temperature and four voltage sensors, which can be used to
+monitor the chip internal environment like heating, supply voltage and
+transistors performance. The driver can optionally provide the hwmon alarms
+for each sensor the PVT controller supports. The alarms functionality is made
+compile-time configurable due to the hardware interface implementation
+peculiarity, which is connected with an ability to convert data from only one
+sensor at a time. Additional limitation is that the controller performs the
+thresholds checking synchronously with the data conversion procedure. Due to
+these in order to have the hwmon alarms automatically detected the driver code
+must switch from one sensor to another, read converted data and manually check
+the threshold status bits. Depending on the measurements timeout settings
+(update_interval sysfs node value) this design may cause additional burden on
+the system performance. So in case if alarms are unnecessary in your system
+design it's recommended to have them disabled to prevent the PVT IRQs being
+periodically raised to get the data cache/alarms status up to date. By default
+in alarm-less configuration the data conversion is performed by the driver
+on demand when read operation is requested via corresponding _input-file.
+
+Temperature Monitoring
+----------------------
+
+Temperature is measured with 10-bit resolution and reported in millidegree
+Celsius. The driver performs all the scaling by itself therefore reports true
+temperatures that don't need any user-space adjustments. While the data
+translation formulae isn't linear, which gives us non-linear discreteness,
+it's close to one, but giving a bit better accuracy for higher temperatures.
+The temperature input is mapped as follows (the last column indicates the input
+ranges)::
+
+	temp1: CPU embedded diode	-48.38C - +147.438C
+
+In case if the alarms kernel config is enabled in the driver the temperature input
+has associated min and max limits which trigger an alarm when crossed.
+
+Voltage Monitoring
+------------------
+
+The voltage inputs are also sampled with 10-bit resolution and reported in
+millivolts. But in this case the data translation formulae is linear, which
+provides a constant measurements discreteness. The data scaling is also
+performed by the driver, so returning true millivolts. The voltage inputs are
+mapped as follows (the last column indicates the input ranges)::
+
+	in0: VDD		(processor core)		0.62V - 1.168V
+	in1: Low-Vt		(low voltage threshold)		0.62V - 1.168V
+	in2: High-Vt		(high voltage threshold)	0.62V - 1.168V
+	in3: Standard-Vt	(standard voltage threshold)	0.62V - 1.168V
+
+In case if the alarms config is enabled in the driver the voltage inputs
+have associated min and max limits which trigger an alarm when crossed.
+
+Sysfs Attributes
+----------------
+
+Following is a list of all sysfs attributes that the driver provides, their
+permissions and a short description:
+
+=============================== ======= =======================================
+Name				Perm	Description
+=============================== ======= =======================================
+update_interval			RW	Measurements update interval per
+					sensor.
+temp1_type			RO	Sensor type (always 1 as CPU embedded
+					diode).
+temp1_label			RO	CPU Core Temperature sensor.
+temp1_input			RO	Measured temperature in millidegree
+					Celsius.
+temp1_min			RW	Low limit for temp input.
+temp1_max			RW	High limit for temp input.
+temp1_min_alarm			RO	Temperature input alarm. Returns 1 if
+					temperature input went below min limit,
+					0 otherwise.
+temp1_max_alarm			RO	Temperature input alarm. Returns 1 if
+					temperature input went above max limit,
+					0 otherwise.
+temp1_offset			RW	Temperature offset in millidegree
+					Celsius which is added to the
+					temperature reading by the chip. It can
+					be used to manually adjust the
+					temperature measurements within 7.130
+					degrees Celsius.
+in[0-3]_label			RO	CPU Voltage sensor (either core or
+					low/high/standard thresholds).
+in[0-3]_input			RO	Measured voltage in millivolts.
+in[0-3]_min			RW	Low limit for voltage input.
+in[0-3]_max			RW	High limit for voltage input.
+in[0-3]_min_alarm		RO	Voltage input alarm. Returns 1 if
+					voltage input went below min limit,
+					0 otherwise.
+in[0-3]_max_alarm		RO	Voltage input alarm. Returns 1 if
+					voltage input went above max limit,
+					0 otherwise.
+=============================== ======= =======================================

Documentation/hwmon/index.rst

@@ -44,6 +44,7 @@ Hardware Monitoring Kernel Drivers
    asc7621
    aspeed-pwm-tacho
    bel-pfe
+   bt1-pvt
    coretemp
    da9052
    da9055

drivers/hwmon/Kconfig

@@ -414,6 +414,31 @@ config SENSORS_ATXP1
 	  This driver can also be built as a module. If so, the module
 	  will be called atxp1.
 
+config SENSORS_BT1_PVT
+	tristate "Baikal-T1 Process, Voltage, Temperature sensor driver"
+	depends on MIPS_BAIKAL_T1 || COMPILE_TEST
+	help
+	  If you say yes here you get support for Baikal-T1 PVT sensor
+	  embedded into the SoC.
+
+	  This driver can also be built as a module. If so, the module will be
+	  called bt1-pvt.
+
+config SENSORS_BT1_PVT_ALARMS
+	bool "Enable Baikal-T1 PVT sensor alarms"
+	depends on SENSORS_BT1_PVT
+	help
+	  Baikal-T1 PVT IP-block provides threshold registers for each
+	  supported sensor. But the corresponding interrupts might be
+	  generated by the thresholds comparator only in synchronization with
+	  a data conversion. Additionally there is only one sensor data can
+	  be converted at a time. All of these makes the interface impossible
+	  to be used for the hwmon alarms implementation without periodic
+	  switch between the PVT sensors. By default the data conversion is
+	  performed on demand from the user-space. If this config is enabled
+	  the data conversion will be periodically performed and the data will be
+	  saved in the internal driver cache.
+
 config SENSORS_DRIVETEMP
 	tristate "Hard disk drives with temperature sensors"
 	depends on SCSI && ATA

drivers/hwmon/Makefile

@@ -54,6 +54,7 @@ obj-$(CONFIG_SENSORS_ASC7621)	+= asc7621.o
 obj-$(CONFIG_SENSORS_ASPEED)	+= aspeed-pwm-tacho.o
 obj-$(CONFIG_SENSORS_ATXP1)	+= atxp1.o
 obj-$(CONFIG_SENSORS_AXI_FAN_CONTROL) += axi-fan-control.o
+obj-$(CONFIG_SENSORS_BT1_PVT)	+= bt1-pvt.o
 obj-$(CONFIG_SENSORS_CORETEMP)	+= coretemp.o
 obj-$(CONFIG_SENSORS_DA9052_ADC)+= da9052-hwmon.o
 obj-$(CONFIG_SENSORS_DA9055)+= da9055-hwmon.o