Re: New Driver for electrical energy measurement

[Christian Gromm <christian.gromm@microchip.com>]

On Wed, 12 Jul 2017 14:51:01 +0200
Greg KH <gregkh@linuxfoundation.org> wrote:

> On Wed, Jul 12, 2017 at 02:18:54PM +0200, Christian Gromm wrote:
> > 
> > Hi,
> > 
> > Microchip is planning to introduce a driver for a new companion
> > chip series capable of electical energy measurement. However, we
> > are not sure which location in the source tree is the most
> > suitable. First thought was to put it into /drivers/powercap. But
> > now we are in discussions whether it would make even more sense to
> > introduce a new driver for electrical energy measuring or
> > monitoring.
> 
> Why not just have it be an IIO device, I think they have energy
> sensors already (adding the iio mailing list to find out...)

We thought about hwmon, but not IIO.

> 
> > 
> > Following is a rough overview of the part we want the driver for.
> > Any input on this will be appreciated.
> > 
> > thanks,
> > Chris
> > 
> > === Introduction
> > 
> > Following the recent focus on low-power systems, Microchip has
> > designed a companion chip series capable of measuring the
> > electrical energy flow of an electrical system.  The series in
> > question, EM Chip  is capable of measuring the energy flow (in or
> > out) of an electrical system by monitoring one or more power
> > rails.  The 1st chip of the series, EM Chip is able to measure the
> > net energy flow over 4 different power rails.  The chip itself is a
> > small QFN16 (4mm x 4mm x 0.5mm) or WLCSP16 (2.25mm x 2.2mm) package.
> > 
> > === Theory of Operation
> > 
> > In order to measure the amount of energy entering/exiting a system,
> > a small shunt resistor is interleaved on a power rail. By measuring
> > the small voltage drop across the shunt resistor (a known value),
> > the electrical current is measured.
> > 
> > 	I = U/R
> > 
> > Depending on whether the system is consuming or producing energy,
> > the measured current value can be either a positive or negative
> > number.
> > 
> > To get the instantaneous power figure we will have to measure the
> > power rail’s voltage and multiply it with the measured value for
> > the current
> > 
> > 	P = V * I = V * U_shunt/R
> > 
> > Knowing the instantaneous power and by making a high enough sample
> > rate (for measuring the current and the power rail voltage), we can
> > assume the measured value for power is equal to the average power
> > figure for the amount of time between 2 sampling moments.
> > 
> > Now that we also know the average power for a given time interval
> > (dt_x = time between 2 sampling moments; sampling speed is known),
> > we can measure the energy amount entering or exiting the system
> > between 2 sampling moments
> > 
> > 	E_partial_x = P * dt_x
> > 
> > Having the energy information available, we can continue to add the
> > subsequent energy values for as long as the system is active. The
> > amount of energy is the sum of all the partial energy values
> > measured for each time interval E = Sum (E_partial_x), where x can
> > take values from 0 till infinite.
> > 
> > 
> > === Chip Operation
> > 
> > The EM Chip chip uses the same principles of operation as presented
> > in the “Theory of Operation” and it does so for a number of 4
> > channels. 4 independent power rails can be energy monitored in the
> > same time.  The chip is controlled over I2C/SMBus by an I2C/SMBus
> > master.
> > 
> > In order to reduce the amount of information traffic between the EM
> > Chip and the entity asking for its information, the chip features
> > large accumulator energy registers for the accumulated energy
> > values. It can accumulate up to 2^24 power values. Depending on the
> > selected chip’s sampling speed, such a number of samples are
> > produced in about 4.5 hours (fastest sampling speed) up to 582.5
> > hours or more than 24 days (lowest sampling speed).
> > 
> > EM Chip measures the power as a 28-bits number. The 28-bits number
> > is the multiplication result of the 16-bits number used to measure
> > the power rail voltage and the 12-bits number used for measuring
> > the voltage drop across the power rail shunt resistor. 
> > 
> > If bidirectional energy flow is required, the power is measured as a
> > 27-bits number and 1 bit for the sign. When no bidirectional flow is
> > needed, the power value is measured as an unsigned 28-bits number.
> > 
> > Due to the relatively large size of the accumulated energy
> > registers inside the chip (48 bits), in the worst case scenario
> > (power values are full scale numbers), a number of 2^20 full-scale
> > power values can be measured before energy register’s overflow.
> > Using the fastest sampling speed, the accumulated energy registers
> > overflows only after a bit over 17 minutes, while at the lowest
> > sampling speed, it would overflow in over 36 hours.
> > 
> > Thus, the chip requires infrequent reads of the accumulated energy
> > registers. Even in the worst case scenario, the time between 2
> > consecutive energy accumulator reads can be over 17 minutes !
> > 
> > In order to keep a longer history of energy measurements, an
> > I2C/SMBus master (e.g. SoC) would read the accumulated energy
> > register values and then use larger “soft” accumulated energy
> > registers to extend the maximum overflow period.
> > 
> > The EM Chip chip can monitor rails up to 32V. It can monitor the
> > energy amounts used by various sub-components of a system (e.g.
> > CPU, GPU, memory, hard-drives, USB ports, backlight, wireless
> > adapters, cameras, displays, …)
> > 
> > EM Chip is able to start operating immediately after power-up with
> > no CPU intervention at all. Such a feature is useful, because it
> > can show the amount of energy consumed by a system before the
> > latter finished booting its operating system.
> > 
> > The chip’s own current draw is around 20uA (in low-power mode,
> > lowest sampling speed - 8 samples/sec) to 675uA (for the highest
> > sampling speed - 1024 samples/sec)
> > 
> > When operated in low-power mode, it can be used to monitor the
> > stand-by energy draw of the system. As an example, such a mode is
> > useful when a system is suspended to RAM or to measure the energy
> > usage from the power on till the booting process is finished.
> > 
> > === Linux Driver
> > 
> > While the chip is due to be publicly released in Q3 2017, a
> > selected number of PC OEM manufacturers will include one or more EM
> > Chip chips on their systems.  We would like to include a driver for
> > EM Chip chip and its follow-up products, such that Linux Kernel
> > will be able to provide the energy information as soon as computing
> > systems using this series of chips will become available.
> > 
> > By providing accurate energy measurements, the computing systems
> > along with their operating systems will be able to run more
> > efficiently.
> 
> 
> Do you have any kernel code already to show how this will get hooked
> up to the device?  Do you need device tree bindings for the sensors,
> or are they on a discoverable bus?

No, I don't. I wanted to bring this up on the mailing list first,
before I get started.
Sensors are attached via I2C.

thanks,
Chris



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