On 07/14/2017 11:00 AM, Guenter Roeck wrote: > On 07/14/2017 01:07 AM, Jonathan Cameron wrote: >> On Wed, 12 Jul 2017 15:19:40 +0200 >> Christian Gromm <christian.gromm@xxxxxxxxxxxxx> wrote: >> >>> 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. >> >> Sure, I don't blame you having myself gone through several subsystems with >> a driver in the past! >> >> As Greg mentioned there are existing IIO drivers doing similar power >> measurements. So yes, a driver for such a part would be welcome. >> >> I can see we may be 'straining' some of the interfaces a bit for those >> 48 bit values, but we can probably do something simple using the >> two 32 bit values available from a raw read in IIO. >> >> Going to get awkward the first time we get a 65+ bit device though. >> There goes my assumptions again :( >> >> Anyhow, nice sounding part so looking forward to seeing some code when >> you are ready. >> >> Note we do have additional energy drivers still in staging (dating back >> to the early days of IIO) but those are 3 phase units so not so relevant >> (and their interfaces need some work which is why they are still in >> staging). >> >> The adc/ina2xx_adc.c driver is probably a better place to start. >> >> Hmm. The only thing that makes me doubt IIO as the obvious place is >> that the focus is clearly on PC energy monitoring. Now you can bridge >> IIO to hwmon but I'd like a bit of input from Guenter on >> whether that is a sensible approach here. >> > > I don't think I have enough information for a sensible answer. What > else besides raw energy measurement do the chips support ? Do they have > limits / alarms ? Do they measure voltage and current as well ? > If not, iio would be the better place. If the chips support PMBus > (which would make sense, but it doesn't sound like it), hwmon would > obviously be better, since it already has the necessary infrastructure. There is also some precedence for these kinds of devices in the power-supply framework. E.g. the LTC2941 and similar work in a similar way to whats described above (https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/power/supply/ltc2941-battery-gauge.c http://cds.linear.com/docs/en/datasheet/2941fb.pdf). Isn't it lovely to have so much choice? _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
