Re: [PATCH v2 1/2] dt-bindings: iio: adc: adding dt-bindings for PAC193X

[Marius.Cristea]

Hi

On Fri, 2023-10-27 at 15:26 +0100, Jonathan Cameron wrote:
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> 
> On Thu, 26 Oct 2023 17:08:07 +0100
> Conor Dooley <conor@xxxxxxxxxx> wrote:
> 
> > On Thu, Oct 26, 2023 at 03:23:46PM +0000,
> > Marius.Cristea@xxxxxxxxxxxxx wrote:
> > > Hi Conor,
> > > 
> > > On Wed, 2023-10-25 at 16:08 +0100, Conor Dooley wrote:
> > > > Hey Marius,
> > > > 
> > > > On Wed, Oct 25, 2023 at 04:44:03PM +0300,
> > > > marius.cristea@xxxxxxxxxxxxx wrote:
> > > > > From: Marius Cristea <marius.cristea@xxxxxxxxxxxxx>
> > > > > 
> > > > > This is the device tree schema for iio driver for
> > > > > Microchip PAC193X series of Power Monitors with Accumulator.
> > > > > 
> > > > > 
......
> > > > > +
> > > > > +  reg:
> > > > > +    maxItems: 1
> > > > > +
> > > > > +  "#address-cells":
> > > > > +    const: 1
> > > > > +
> > > > > +  "#size-cells":
> > > > > +    const: 0
> > > > > +
> > > > > +  interrupts:
> > > > > +    description: IRQ line of the ADC
> > > > > +    maxItems: 1
> > > > > +
> > > > > +  drive-open-drain:
> > > > > +    description: The IRQ signal is configured as open-drain.
> > > > > +    type: boolean
> > > > > +    maxItems: 1
> > > > > +
> > > > > +  microchip,slow-io:
> > > > > +    type: boolean
> > > > > +    description: |
> > > > > +      A GPIO used to trigger a change is sampling rate
> > > > > (lowering
> > > > > the chip power consumption).
> > > > > +      In default mode, if this pin is forced high, sampling
> > > > > rate
> > > > > is forced to eight
> > > > > +      samples/second. When it is forced low, the sampling
> > > > > rate is
> > > > > 1024 samples/second unless
> > > > > +      a different sample rate has been programmed.
> > > > 
> > > > This description doesn't really make sense to me - if a GPIO is
> > > > used
> > > > to
> > > > drive the pin low or high, why do we need a property? A DT
> > > > property
> > > > implies that this is a static configuration depending on the
> > > > board,
> > > > but
> > > > reading the description this seems to be something that can be
> > > > toggled
> > > > at runtime.
> > > > I do note though, that this GPIO is not documented in the
> > > > binding, so
> > > > I
> > > > suppose what really needs to happen here is document the gpio
> > > > so that
> > > > the driver can determine at runtime what state this pin is in?
> > > > 
> > > > Also, you say "In default mode", but don't mention what the
> > > > non-
> > > > default
> > > > mode is. What happens in the other mode?
> > 
> > > This is a "double function" pin. On the PAC193x there is the
> > > SLOW/ALERT
> > > pin. At runtime this pin could be configured as an input to the
> > > PAC and
> > > the functionality will be "SLOW" that means if it is forced high,
> > > the
> > > PAC will work in low power mode by changing the sample rate to 8
> > > SPS.
> > > If it's forced low the PAC will work at it's full sample rate.
> > 
> > Since this is a runtime thing, it doesn't make sense to have a
> > property
> > that is set at dts creation time that decides what mode the pin is
> > in.
> > 
> > > "SLOW" is the default function of the pin but it may be
> > > programmed to
> > > function as ALERT pin (Open Collector when functioning as ALERT,
> > > requires pull-up resistor to VDD I/O). This time the pin will be
> > > set as
> > > output from PAC (ALERT functionality) to trigger an interrupt to
> > > the
> > > system (this is covered by the interrupts and drive-open-drain).
> > 
> > Hmm, at the risk of getting out of my depth with what the GPIO
> > subsystem
> > is capable of doing, I would expect to see something like
> > 
> > sampling-rate-gpios:
> >   description:
> >     <what you have above>
> >   maxItems: 1
> > 
> > Which would allow the driver to either drive this pin via the gpio
> > subsystem, or to use the interrupt property to use it as an
> > interrupt
> > instead.
> > 
> > Perhaps Jonathan etc knows better for these sort of dual mode pins.
> 
> Beyond them being a pain? The fun is they may get wired to interrupt
> controllers that are also GPIOs or they may not (and the other way
> around
> with them wired to GPIO pins that aren't interrupt pins).
> 
> I don't understand the usecase for the SLOW control.
> Given it seems software can override the use for SLOW I'd be tempted
> to
> always do that.
> Thus making this pin useable only as an optional interrupt.
> 
> If someone hard wires it to high or low that is harmless if we aren't
> letting it control anything.
> 

Here I was trying to define/describe 3 possible situations:
- 1) the pin is not used at all, so it doesn't matter if it's connected
somewhere

- 2) the pin is user configured as "interrupt" and it's connected to
the interrupt controller (this case is not supported in the driver
right now)

- 3) the pin is user configured as "SLOW" (this case is not supported
in the driver right now). That means it should be connected to a GPIO
pin. This function (SLOW control) will automatically change the PAC
internal sampling frequency to lower the PAC internal power
consumption. For example, the PAC could be configured to a sample rate
of 1024 samples/s (it will consume maximum current). Using the SLOW
control, the chip will internally change to 8 samples/s but the math
internally will "behave" as the 1024 samples/s but at a much lower
power consumption. It's very useful in case the system wants to lower
power consumption (we still need to measure battery power consumption
even if the system is put into a low power state). PAC internal power
consumption is proportional to the number of channels used and also the
sampling frequency.



> > 
> > > The system could work fine without this pin. The driver doesn't
> > > use
> > > interrupt at this time, but it could be extended.
> > 
> > Cheers,
> > Conor.
> 

Thanks,
Marius