On 14.07.2017 11:00, 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.
Besides the energy measurement for all four rails, the chip also
measures the voltage, current and power. And there is one pin for
alert events, which are configurable.
PMBus is not supported.
Another feature to be mentioned is the relatively long time
between two subsequent CPU polls over I2C (due to the large
accumulator registers of the chip).
thanks,
Chris
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