hwmon: (bt1-pvt) Cache current update timeout

Instead of converting the update timeout data to the milliseconds each
time on the read procedure let's preserve the currently set timeout in the
dedicated driver private data cache. The cached value will be then used in
the timeout read method and in the alarm-less data conversion to prevent
the caller task hanging up in case if the PVT sensor is suddenly powered
down.

Fixes: 87976ce2825d ("hwmon: Add Baikal-T1 PVT sensor driver")
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Link: https://lore.kernel.org/r/20200920110924.19741-3-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

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>