On Wed, 12 Jul 2017 14:51:01 +0200 Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> 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 - _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
